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Biogeochemical Cycling

Proton Binding Characteristics of Dissolved Organic Matter Extracted from the North Atlantic
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Environmental Science & Technology

Cite this: Environ. Sci. Technol. 2023, 57, 50, 21136–21144
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https://doi.org/10.1021/acs.est.3c01810
Published December 5, 2023

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Abstract

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Marine dissolved organic matter (DOM) presents key thermodynamic properties that are not yet fully constrained. Here, we report the distribution of binding sites occupied by protons (i.e., proton affinity spectra) and parametrize the median intrinsic proton binding affinities (log K̅H) and heterogeneities (m), for DOM samples extracted from the North Atlantic. We estimate that 11.4 ± 0.6% of C atoms in the extracted marine DOM have a functional group with a binding site for ionic species. The log K̅H of the most acidic groups was larger (4.01–4.02 ± 0.02) than that observed in DOM from coastal waters (3.82 ± 0.02), while the chemical binding heterogeneity parameter increased with depth to values (m1= 0.666 ± 0.009) ca. 10% higher than those observed in surface open ocean or coastal samples. On the contrary, the log K̅H for the less acidic groups shows a difference between the surface (10.01 ± 0.08) and deep (9.22 ± 0.35) samples. The latter chemical groups were more heterogeneous for marine than for terrestrial DOM, and m2 decreased with depth to values of 0.28 ± 0.03. Binding heterogeneity reflects aromatic carbon compounds’ persistence and accumulation in diverse, low-abundance chemical forms, while easily degradable low-affinity groups accumulate more uniformly in the deep ocean.

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Synopsis

The thermodynamic binding data presented here for marine DOM are required for a robust description of the current marine carbon cycle and predictions under ongoing climate change.

1. Introduction

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Dissolved organic matter (DOM) comprises the largest pool of organic carbon in seawater (ca. 660 Pg of C) in which it plays important roles. (1,2) The bioavailability and toxicity of trace metals in the ocean depend on their chemical speciation, which is the result of environmental conditions and interactions between trace elements and organic matter. (3) The competition effects between chemical species for binding to marine DOM and the overall binding capacity are also related to the DOM intrinsic acid–base/proton binding properties, (4,5) which define organic alkalinity. (6) Understanding the interdependencies between the intrinsic binding properties of marine DOM and the biogeochemical cycling of trace metals, along with the contribution of organic matter to total alkalinity by interacting with protons, is therefore of high relevance to the contemporary ocean and future climate change scenarios.
The composition of marine DOM can be expected to influence the magnitude and distribution of the intrinsic (i.e., chemical) ion binding affinities. The molecular composition of marine DOM suggests lower diversity and less aromaticity than terrestrial organic matter. (7) Nevertheless, the molecular complexity of marine DOM (8) means that marine DOM will not behave as a well-characterized simple acid molecule (e.g., acetic acid) with discrete values for the proton binding affinities, as most often described by complexometric titrations in seawater. (9) Rather, ion binding affinities occur over a continuum of values (i.e., affinity spectra) and fall into groups comprised of different organic acids with a variable range of binding affinities, as is typically observed for terrestrial organic matter. (10) This variable interval of affinity values is related to the chemical binding heterogeneity, which is a consequence of the compositional heterogeneity that describes DOM at the molecular level. (11) Indeed, a potential correlation between ion binding affinity and molecular composition of DOM is expected, although this topic is beyond the scope of this manuscript.
The chemical binding heterogeneity of open ocean marine DOM reflects its complex nature but is a key parameter not yet constrained. This gap in our current knowledge about the proton binding behavior of marine DOM is relevant to the often observed inconsistencies between measured and calculated parameters of the carbonate system in the ocean (e.g., pH or total alkalinity), which are partially due to the lack of a complete thermodynamic description of the acid–base chemistry of seawater. (12,13)
The binding of trace metals and protons to marine organic ligands is often described using simplified conditional stability constants. (14,9) However, for a better understanding of trace element biogeochemistry, it is essential to untangle the main drivers that influence chemical speciation (e.g., pH, ionic strength and temperature) and determine a complete set of intrinsic binding parameters for marine DOM, which are thermodynamically consistent and independent from the specific conditions of the seawater sample. (15)
Here we present experimental results for open ocean environments, based on the solid-phase extractable fraction of DOM (ca. 38% DOC recovery yield), obtained from proton binding titrations of samples from surface (ca. 2 m depth) and deep (500 m depth) waters of the North Atlantic, for the first time. We determine the intrinsic binding properties of the main chemical groups involved in the binding of cations to oceanic DOM following well-established procedures. We used a combination of the nonideal competitive adsorption (NICA) isotherm, which describes the chemical binding to heterogeneous ligands, and the Donnan electrostatic model for the description of polyelectrolytic effects, i.e., the nonspecific binding. Finally, we critically compare the calculated intrinsic proton binding parameters of North Atlantic DOM with values from the semienclosed Baltic Sea (5) and from generic freshwater organic matter. (16)

2. Materials and Methods

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2.1. Seawater Collection

Samples were collected in the North Atlantic during the Meteor cruise GApr11 (M147 Amazon–GEOTRACES). A water sample (total 470 L) was collected from between 16°25.977′ – 10°07.155′ N and 28°47.427′ – 36°04.983′ W (see Figure S1) on the 24th of April 2018 while the ship was underway via a Teflon bellows pump (Almatec A15) and acid-washed tubing suspended at ca. 2 m depth and ca. 4 m distance from the ship using a towed fish. Water was filtered through 0.8/0.2 μm cartridge filters (AcroPak1000), pumped into water sampling bottles (24 × 12 L, CFree, Ocean Test Equipment) placed inside a trace metal–clean laboratory container, and then acidified with HCl (Romil UHP grade) to a final pH of 2 prior to DOM preconcentration. Deep water samples were collected at ca. 500 m depth from two locations: 09°29.91′ N, 036°47.524′ W and 04°09.524′ N, 042°54.72′ W (see Figure S1), on the 25th and 27th of April 2018, respectively. A total of 460 L was collected from the water sampling bottles deployed on an epoxy-coated aluminum rosette equipped with a Seabird SBE 911 plus CTD. The collected deep seawater was acidified prior to DOM preconcentration as done for the surface water.

2.2. Seawater Analysis

Sampling and methods of analysis for macronutrients (phosphate, silicic acid, nitrate, and nitrite), dissolved organic carbon (DOC), total dissolved nitrogen (TDN), pH, temperature, and salinity at sampling points are described in detail elsewhere. (17)

2.3. Dissolved Organic Matter Preconcentration and Extraction

Dissolved organic matter was preconcentrated from the collected seawater using solid–phase extraction cartridges, Mega Bond–Elut Priority PolLutant (PPL) 5 g, 60 mL from Agilent. The PPL cartridges had previously been soaked in 50 mL of methanol (Fisher Scientific LC–MS grade) for 12 h and then washed by passing 15 mL of HCl (0.1% v/v) through each cartridge before use. The seawater was passed through the PPL cartridges with a slight overpressure provided by filtered nitrogen gas (99.999%, AlphaGaz). One cartridge was used for every 20 (surface) or 29 (depth) L of seawater. Maximum cartridge loading was ca. 1.5 mg of C per gram of PPL. Afterward, the PPL cartridges were stored frozen (−20 °C). For the DOM extraction, the cartridges were defrosted at room temperature, then washed with 15 mL of ultrapure water, and soaked with (2×) 10 mL of acetonitrile for 10 min to elute the DOM. The acetonitrile–DOM solution (20 mL from each cartridge) was collected in a Teflon pot and dried under a stream of ultrapure N2 gas. The extraction efficiencies were determined as the ratios between the DOC content of the DOM extracts and the DOC content in the original seawater samples.

2.4. Dissolved Organic Matter Stock Solutions

The solid DOM extracts were dissolved in NaOH (∼0.02 M, extra pure, 98%, Acros Organics) to final solid-phase extracted dissolved organic matter (SPE-DOM) concentrations of 5.15 and 6.79 g·L–1 for surface and deep samples, respectively, and preserved in the dark at 4 °C. These SPE–DOM stock solutions were used, usually within a week of preparation, to obtain the samples for titration, as described below.

2.5. Potentiometric Titrations

A detailed description of the experimental setup and conditions was provided by Lodeiro et al. (4) Here, DOM concentrations in the titration vessels were 1.03 g of DOM·L–1 (366 mg C·L–1) and 1.18 g of DOM·L–1 (345 mg C·L–1) for surface and deep samples, respectively. The relatively high concentration of DOM used in the titration experiments is motivated by the low percentage of C (29–35%) and low number of titratable groups per C atom (0.112–0.117 mol·molC–1, see Table S3) in our extracted DOM. The ionic strength (I) of the titrated solutions was fixed to values of 0.007, 0.1, 0.7, and 1 M using NaCl (puriss. p.a., ≥ 99.5%, Merck) as an inert background electrolyte. We prepared calibration solutions with the same ionic strengths as the samples and defined the pH and log K̅H on the free proton concentration scale. The conversion to e.g., pH(NBS) can easily be carried out using a suitable ion activity coefficient for H+ in seawater. (18) A typical SPE–DOM titration experiment took about 6–8 h, including an initial solution equilibration step under N2 bubbling of ca. 1 h. These long equilibration times were the result of a strict stabilization criterium of the mV readings of the glass electrode (<0.05 mV/min), the high concentration of the DOM in the titration vessel, and the experimental limitations of the glass electrode at pH values above 8.5–9.5.

2.6. The NICA–Donnan Model

The proton titration data was described by the bimodal NICA isotherm and the Donnan electrostatic model. (19−22) In the absence of metal cations able to compete with protons for the specific binding to the functional groups of DOM (monocomponent system), the bimodal NICA isotherm is formally identical to the weighted sum of two Langmuir–Freundlich isotherms (10)
QH=QmaxH,1(K¯H,1cH,D)m11+(K¯H,1cH,D)m1+QmaxH,2(K¯H,2cH,D)m21+(K¯H,2cH,D)m2
(1)
where QH stands for the amount of bound protons per mol of DOC (mmol·mol C–1), QmaxH,j is the total amount of titratable proton binding sites within each distribution, cH,D is the proton concentration in the Donnan phase, K̅H,j is the median value of the jth intrinsic proton binding affinity distribution, and mj (0 < mj ≤ 1) is related to the width of the affinity distribution function (a measure of the chemical binding heterogeneity). The limiting value of mj = 1 corresponds to a perfectly homogeneous set of sites. The subindexes for j = 1 or 2 represent the two main groups in the affinity distribution of binding sites, hereafter called DOM1 and DOM2. The group with the lowest affinity for protons (DOM1) comprises the most acidic sites, usually associated with carboxylic-like functional groups, whereas the high affinity group DOM2 includes the less acidic sites often associated with phenolic-like chemical groups.
The Donnan model was used to describe the electrostatic contribution to the effective ion binding by DOM. The Donnan model considers that DOM is an electroneutral, permeable gel phase with a homogeneous distribution of fixed charges that originate from dissociation of the binding groups. (20,23) We followed the “master curve approach” and used the gel phase (Donnan) volume (VD) as a variable to fit the obtained charge curves. We used an expression for VD consistent with the nonlinear Poissson-Boltzman (NLPB) equation (PB-VD), which depends on both the ionic strength and macromolecular charge, for the description of the electrostatic binding to marine DOM. (24) In addition, we also provide binding parameters using a standard VD equation, an empirical expression where VD depends only on ionic strength (23) for comparison. Although more refined electrostatic models are available in the recent literature, (25) they rely on detailed molecular information (such as molecular weights and particle radius) that is not currently available, so this topic will be worthy of future research. A detailed description of the Donnan approach models with the PB-VD and standard expression for VD used in this work can be found in Lodeiro et al. (4)
The strategy followed to derive the NICA-Donnan model parameters can be found in the Supporting Information. Briefly, the experimental titration data, E (mV) vs VNaOH (mL), were converted to apparent charge curves, (qDOM = QmaxH,tot – QH) vs pH, using mass and charge balance relationships and the calibration of the electrode in the free proton concentration scale at each ionic strength. The fitting of the NICA-Donnan model to these (pH, QmaxH,tot – QH) curves was carried out by iteratively solving the set of equations of the NICA expression for proton binding and the electrostatic model, as detailed in the Supporting Information of Lodeiro et al. (4) Finally, the optimized model parameters were obtained by minimizing the sum of squared residuals between the experimental and theoretical charges.

3. Results and Discussion

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3.1. Ancillary Parameters

The values observed for salinity, DOC, pH, oxygen, and nutrient concentrations in the surface and deep North Atlantic samples were in line with profiles previously reported for these locations (26) (Tables S1 and S2). We extracted 0.515 g (surface) and 0.340 g (depth) of DOM after preconcentrating 460–470 L of seawater, with an equivalent DOC recovery of 38.4 ± 2.0% for surface and 37.9 ± 4.2% for deep samples. These recovery values are lower than those usually obtained in fresh and brackish waters, (27) and some North Atlantic deep samples, (7,26) but in agreement with other previous yields of DOM from the open ocean and Mediterranean Sea (28−31) using PPL cartridges. Note that the extraction procedure may introduce a bias in the DOM composition, (32) which could affect the number and type of chemical compounds analyzed in our titration experiments.
The organic C/N molar ratios for surface and deep North Atlantic seawater samples were 13.2 ± 3.7 and 17.7 ± 2.9, respectively. These values, and the observed increase of the C/N molar ratio with depth, are in agreement with open ocean ratios previously reported and indicate preferential remineralization of N from DOM as it is transported into deeper waters. (33,34) The extracted DOM samples showed C/N molar ratios higher than those of the bulk seawater. The isolated surface DOM solution had a CSPE/NSPE molar ratio of 17.9 ± 1.4, while for the deep extracted DOM, the ratio increased to 30.5 ± 2.3. These differences have been associated with extraction preferences of nonpolar over polar compounds when using PPL resins. (35,36) For example, it has recently been reported that extracts obtained by SPE with PPL resins seem to be somewhat enriched in N-poor, low molecular weight, and recalcitrant DOM and therefore show less variability than the corresponding bulk DOM. (32) The implications of this for the description of marine DOM binding remain largely unexplored and will be the subject of future work.
The carbon content of the SPE–DOM extracts was slightly higher for the surface (35.5 ± 0.6%) than for the deep (29.2 ± 0.7%) sample. Marine DOM has been reported to be about 50% C by weight, (30,37) as estimated for terrigenous humic matter. (38) We also obtained similar values between 43 ± 1.5 and 53 ± 3.8%, for the mass percentage of C for SPE–DOM samples from the Baltic Sea. (5) However, at present, we have no explanation as to why the carbon content of our extracted DOM was lower in this case.

3.2. North Atlantic DOM Proton Binding Groups

We carried out experimental acid–base titration curves for the surface and deep SPE–DOM at 25 °C at four different ionic strengths. We expressed the DOM charge (qDOM) as the difference between the total amount of binding sites available for protons and the binding sites already occupied by protons (qDOM= QmaxH,tot – QH), i.e., the chemical groups that are free at any point of the titration. Experiments at each ionic strength were done in duplicate, and the combination of those data sets was fitted to the NICA–Donnan model (Figure 1).

Figure 1

Figure 1. NICA–Donnan fits to proton titration data of North Atlantic SPE–DOM at 25 °C and 0.007, 0.1, 0.7, and 1.0 M ionic strength, using the PB–VD model: surface (left panel) and deep (right panel) samples. Symbols: experimental values. Colored lines: model fits at each ionic strength. The uppermost black curve corresponds to the charge “master curve” (QmaxH,tot – QH) vs pHD.

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The total amount of proton binding groups in the North Atlantic SPE–DOM (QmaxH,tot) was not significantly different between the surface (112.7 ± 5.8 mmol·mol C–1) and deep (117.3 ± 5.6 mmol·mol C–1) samples. The total proton binding represents a maximum of binding sites for chemical species that can compete with protons for the DOM binding sites, with a predominantly covalent behavior, (39) e.g., most trace metals in seawater. On average, 11.4 ± 0.6% of the C atoms in our extracted North Atlantic DOM therefore have a functional group with a binding site for ionic species. Considering the DOC concentrations (84.5 and 47.9 μmol·L–1) measured in the surface and deep waters at our tropical North Atlantic sampling sites, the DOM contains about 9.5 and 5.6 μmol·L–1 of acid–base groups, respectively.
Even if we restrict our proton binding curves (Figure 1) to the pH window usually used in alkalinity titrations (from pH 8 to 3), our predicted DOM contribution to organic alkalinity, with the observed chemical binding heterogeneity, is between about 4 and 7 μmol·L–1 for North Atlantic deep and surface waters, respectively.
Currently, organic alkalinity is thought to be responsible for a “missing” 5 μmol·kg–1 contribution to total alkalinity in the open ocean (40) and may represent a more important but poorly understood component of total alkalinity in coastal waters. (41)
The obtained total amount of ion binding groups for open ocean DOM is slightly lower than observed for coastal DOM (135–136 mmol·mol C–1), and much lower than the average value reported for a generic fulvic acid of terrestrial origin (186 mmol·mol C–1) (16) as shown in Figure 2a, which will have an impact in trace metal biogeochemistry and alkalinity.

Figure 2

Figure 2. Total amount of proton binding groups obtained from the fits of the NICA–Donnan model (PB–VD) to the proton binding data shown in Figure 1: QmaxH,tot (a) and QmaxH,i (b), for the low affinity (DOM1, green bars) and high affinity (DOM2, blue bars) distributions. The values of Baltic (Boknis Eck) and Baltic (Kiel Fjord) are from Lodeiro et al. (4,5) Bar heights indicate the mean, and error bars indicate the standard deviation. The FA values indicate QmaxH for a generic terrestrial fulvic acid from Milne et al. (16) (calculated with the standard VD model); the error bar indicates the range of values used for the derivation of the generic value.

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The amount of DOM1 binding groups (QmaxH,1) is higher than that of DOM2 groups (QmaxH,2), with a QmaxH,2/QmaxH,1 ratio of 0.25 ± 0.07 and 0.27 ± 0.06 for surface and deep samples, respectively (Figure 2b). Our ratios are similar to previous values of 0.27–0.37 observed for coastal DOM and a generic fulvic acid. (4,5,16) The fraction of the low affinity groups (DOM1) in the North Atlantic SPE–DOM remains therefore constant with depth (79–80% of the total binding groups), though other authors observed an increase in open ocean DOM samples. (28) This discrepancy could be ascribed to the differences in the experimental techniques used (proton binding vs structural analysis).

3.3. North Atlantic DOM Intrinsic Proton Binding Parameters

We used the Donnan model to account for the electrostatic contribution to the ion binding by DOM. This model assumes a homogeneous distribution of fixed charges resulting from the dissociation of proton binding functional groups in DOM and considers that DOM behaves as an electroneutral three-dimensional permeable gel that chemical species can penetrate. Experimental measurements of the Donnan volume (VD) are inaccurate, (42) so we used two different fitted values for DOM: 1) VD consistent with Poisson–Boltzmann (PB–VD) and 2) standard VD model. Our fitting criterion of convergence was the merging of the obtained charge curves at different ionic strengths when they were plotted against the local pH value in the Donnan volume, pHD, (−log cH,D), which is termed the “master curve”. (23)
At high ionic strength, the electrostatic contribution to the ion binding is expected to be negligible, and the master curve should overlap with the charge curve obtained at 1 M ionic strength. Nevertheless, application of the standard VD Donnan model resulted in a master curve that deviated significantly from the titration curve obtained at 1 M ionic strength (Figure S3). Application of the PB–VD model (Figure 1) resulted in improved overlap between the master curve and the charge curve at 1 M ionic strength, which is reasonable.
Moreover, with the fitted binding parameters reported here, we could calculate for the first time the intrinsic proton binding affinity spectra for open ocean DOM (Figure 3a).

Figure 3

Figure 3. Proton binding affinity spectra of North Atlantic SPE–DOM at 25 °C calculated from the NICA–Donnan model (PB–VD) parameters of Table S3 for surface (red lines) and deep (blue lines) samples: (a) intrinsic values and (b) effective values calculated at I = 0.7 M and density of protonated sites (occupation of binding sites by protons) at the experimental values of pH 8.08 (shaded red area) and pH 7.56 (shaded blue area) of the surface and deep samples, respectively. All spectra are normalized to one.

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The affinity spectra represent the density of the probability of proton binding affinity (i.e., the fraction of binding sites with a specific value of the microscopic binding constant log k′H) and are therefore a key tool to determine the intrinsic characteristics of the binding sites potentially available for chemical species (e.g., trace metals) in the open ocean. The affinity spectra reflect the complex mixture of marine DOM and indicate its chemical reactivity. Therefore, we provide both the proton affinity distributions (Figure 3) and the median value of these distributions (log K̅H,i) for marine DOM (Figure 4).

Figure 4

Figure 4. Median values of the intrinsic proton binding affinity (log K̅H,i) parameters obtained from the fits of the NICA–Donnan model (PB–VD) to proton binding data shown in Figure 1: low affinity (DOM1, green bars) and high affinity distribution (DOM2, blue bars). Bar heights indicate the mean, and error bars indicate the standard deviation. The values for the Baltic (Kiel Fjord) are from Lodeiro et al. (4)

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Surface and deep intrinsic binding spectra showed significant differences, reflecting the changes in log K̅H,i and mi values with depth (Figure 3a). For the low affinity group of binding sites (DOM1), the values of log K̅H,1 are very similar for surface (4.01 ± 0.02) and deep (4.02 ± 0.02) samples (Figures 3a and 4). On the other hand, and despite the larger uncertainties, the log K̅H,2 values for the high affinity groups (DOM2) show a clear difference between the surface (10.01 ± 0.08) and deep (9.22 ± 0.35) SPE–DOM samples (Figures 3a and 4). Therefore, the affinity spectra showed an overall shift for the DOM2 distribution toward lower affinity values from the surface to depth (Figure 3a).
The chemical binding heterogeneity of DOM (mi) is a key parameter not constrained yet for open ocean samples. This parameter is related to the width of the ion affinity distribution function (affinity spectra, Figure 3), which reflects the chemical binding heterogeneity. It can be hypothesized that this heterogeneity parameter could also serve as a proxy for DOM molecular diversity. We calculated an intrinsic m1 value for North Atlantic SPE–DOM of 0.601 ± 0.005 for the DOM1 groups in the surface and of 0.666 ± 0.009 in the deep samples (Figure 5). In contrast, the intrinsic m2 value of the high affinity DOM2 groups in the deep sample (0.28 ± 0.03) was lower (i.e., more heterogeneous) than that observed in the surface sample (0.38 ± 0.03). This agrees with the wider affinity distribution function for DOM1 groups and the narrower distribution for the DOM2 group of the surface compared to the deep sample (Figure 3a).

Figure 5

Figure 5. Chemical binding heterogeneity (mi) parameters obtained from the fits of the NICA–Donnan model (PB–VD) to proton binding data shown in Figure 1 for the low affinity (DOM1, green bars) and high affinity (DOM2, blue bars) distributions. Bar heights indicate the mean, and error bars indicate the standard deviation. The values for the Baltic (Kiel Fjord) are from Lodeiro et al. (4)

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Current intrinsic and emergent recalcitrant concepts of marine DOM persistence (43) are reflected in the obtained heterogeneity values. The greater heterogeneity of the DOM2 compared to the DOM1 group distribution in open ocean SPE–DOM indicates that the low affinity binding sites behaved closer to what would be expected for a simple carboxylic chemical group. On the contrary, the high heterogeneity (low m2 values) of DOM2 probably reflects the characteristics of aromatic carbon compounds (e.g., phenols), which are difficult to degrade and thus accumulate in a larger variety of chemical species of lower abundance than the easier degradable/mineralized DOM1 groups that accumulate in the deep ocean as more homogeneous chemical compounds.
We compared our open ocean data with the only available intrinsic proton binding parameters for a coastal DOM (4) (Figures 4 and 5). Both data sets were obtained with the NICA-Donnan model, using an expression for VD consistent with the NLPB equation (PB-VD). The DOM1 group of the North Atlantic DOM presents log K̅H,1 values higher than those observed in coastal waters (3.82 ± 0.02). This DOM1 distribution of our deep open ocean DOM is ca. 10% more homogeneous than the surface sample, which is very similar to that previously reported for a coastal surface DOM (Figure 5). Direct comparison of the open ocean and coastal DOM high affinity groups for protons is not possible, since the intrinsic data of DOM2 for the Baltic Sea (Kiel Fjord) were estimated at I = 0.7 M using the generic intrinsic NICA parameters for fulvic acid.
Intrinsic binding affinities depend on the electrostatic model, and therefore comparison of NICA-Donnan constants derived using the PB-VD and standard VD models is not straightforward. The Donnan model with the PB-VD equation has not been used so far to account for the electrostatic contribution to the binding by DOM in natural waters; hence, the comparison of our intrinsic ion binding affinity values with, e.g., generic fulvic or humic acids of terrestrial origin or effective ion binding constants for marine humic substances is not possible. Therefore, to compare intrinsic binding data for terrestrial and marine DOM, we included the data fitted to a standard VD Donnan model in the Supporting Information. Nevertheless, the use of the PB-VD model is recommended over that of the standard VD expression. Among the advantages of the former, we highlight its direct connection with the NLPB treatment for different geometries, its independency of the kind of macromolecular ligand (e.g., DOM), homogeneous or heterogeneous, and the explicit consideration of the DOM charge in the calculation of VD. (24)

3.4. North Atlantic DOM Effective Proton Binding Parameters at 0.7 M Ionic Strength

In addition to the DOM intrinsic proton binding affinities, we also calculate the effective binding parameters at I = 0.7 M for the sampling site in the North Atlantic (Figure 6, Table S3). The effective parameters combine both the chemical and electrostatic contributions to the binding. We anticipate a minor electrostatic input to the ion binding at the high ionic strength (ca. 0.7 M) of open ocean waters. Therefore, the calculated effective DOM binding parameters at I = 0.7 M are very similar to the intrinsic ones (Table S3).

Figure 6

Figure 6. Effective binding parameters estimated at I = 0.7 M, using the intrinsic NICA-Donnan model with PB-VD, and the proton binding data shown in Figure 1 for the DOM1 (green bars) and DOM2 (blue bars) distributions: log K̅H,i (a) and mi (b). The values Baltic (Boknis Eck) and Baltic (Kiel Fjord) are from Lodeiro et al. (4,5) Bar heights indicate the mean, and error bars indicate the standard deviation.

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Surface and deep samples presented similar values for the effective proton binding affinity of DOM1 groups, although significant differences were observed for the high affinity group values. The marine DOM1 group at 0.7 M ionic strength had slightly higher effective binding affinities than those previously calculated for stratified coastal DOM at the same ionic strength and at 0.32 M (salinity 16, Kiel Baltic Fjord), while for the DOM2 groups, the increase was only observed for the marine surface sample (Figure 6a).
Moreover, the effective m values at I = 0.7 M for DOM1 were lower than the calculated intrinsic ones (Table S3). Regarding surface and deep samples, we observed the same tendency for the effective values with m1 increasing and m2 decreasing with depth (Figure 6b). Therefore, the DOM1 groups of marine DOM became less heterogeneous with depth, while the opposite was observed for the DOM2 groups.
Compared to coastal DOM, the effective m1 value calculated at 0.7 M ionic strength for the deep Atlantic sample is similar to a Baltic mixed but higher than a stratified sample as described in Lodeiro et al. (5) The surface Atlantic DOM has an effective m1 value between that observed for coastal and deep marine DOM (Figure 6b). On the contrary, the DOM2 groups of North Atlantic SPE–DOM are markedly more heterogeneous than coastal SPE–DOM at I = 0.7 M.
We also calculated the distribution of sites occupied by protons from the effective affinity spectrum of marine DOM (I = 0.7 M) at the measured pH values of surface (8.08) and deep (7.56) seawater samples (Figure 3b). These distributions reflect the affinity of the sites actually involved in proton exchange under the relevant environmental conditions. It is straightforward to prove that at a given pH value the sites with proton affinity log k′H = pH are half occupied. Note that in both cases (surface and deep samples) the DOM2 groups are mostly protonated at the experimental pH, compared to the DOM1 sites, which are essentially unoccupied. This is a result of the relative values of pH and the central values (log K̅H,i values) and widths (mi) of each mode of groups. (22) Consequently, it is expected that the DOM1 distribution will have little relevance to proton competition with trace metal ions for binding by DOM in marine environments. When comparing the fractional occupation of sites between surface and deep samples, we observe that, in the latter case, the density distribution of protonated sites became broader and shifts toward lower affinity values. This is a result of the lower pH values (7.56 vs 8.08), log K̅H,2 (9.5 vs 10.4), and m2 (0.32 vs 0.42) of deep DOM compared to surface DOM. This might indicate that metal-proton exchange in marine DOM is more relevant for aged organic matter.
Our data show that the average affinity and binding heterogeneity of the DOM2 groups could underpin the chemical reactivity of marine DOM and shed some light on explaining its stabilization and persistence in seawater. Moreover, as we also determined intrinsic parameters, it would be possible to extrapolate beyond the sample conditions, though the electrostatic model used is critical for the binding description and a correct extrapolation. (24)
Nevertheless, the information obtained from the binding affinity spectra of protons does not provide a straightforward perception of how the ion–DOM binding behavior will be in natural conditions, and an evolution of the spectra from the monocomponent system calculated here to the multicomponent seawater mixture can be expected. (22)
Further efforts are needed to solve the current lack of knowledge on whether and how the thermodynamic properties of marine DOM affect the interactions with major seawater components (e.g., Ca and Mg) and trace metals. For example, the determination of the conditional affinity spectrum (CAS) for the ionic species present in seawater under a given constraint (pH, concentration of competing ions, etc.) will enable us to determine the effect of all interfering cations on the binding affinity distribution of a given one. The study of the CAS seen by a trace metal ion binding to marine DOM can allow the assessment of its effective binding affinity and heterogeneity as a function of the concentration of competing species (e.g., H+). (44) We hypothesize that, at the natural pH of seawater, the influence of proton ions is probably the most important contribution for ions competing for the high affinity sites of DOM. In addition to the pH, temperature and pressure effects on metal binding by marine DOM also need to be constrained to study the implications of expected changes under ongoing climate change.

Data Availability

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The data sets generated and/or analyzed during the current study are available in the attached spreadsheet.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.est.3c01810.

  • 1. Sampling map (Figure S1, page S2); 2. Seawater analysis (page S3): North Atlantic seawater main characteristics (Table S1) and North Atlantic seawater micro- and macronutrient concentrations (Table S2); 3. Strategy for the derivation of NICA–Donnan (PB–VD) model parameters (page S4); 4. Intrinsic and effective proton binding parameters calculated using the NICA–Donnan (PB–VD) model (pages S5–S6): Optimized NICA–Donnan (PB–VD) parameter values (Table S3), NICA–Donnan (PB–VD) fits (Figure S2); 5. Potentiometric titration data fitted to a standard VD Donnan model (pages S7–S10): NICA-Donnan fits (Figures S3), proton binding affinity (log K̅H,i) (Figure S4) and heterogeneity (mi) parameters (Figure S5); 6. Glossary of terms (pages S11–S12) (PDF)

  • Complete experimental data set (XLSX)

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Author Information

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  • Corresponding Author
  • Authors
    • Carlos Rey-Castro - Department of Chemistry, Physics, Environmental and Soil Sciences, University of Lleida − AGROTECNIO-CERCA Center, Rovira Roure 191, 25198 Lleida, Spain;  Orcidhttps://orcid.org/0000-0002-9286-4206
    • Calin David - Department of Chemistry, Physics, Environmental and Soil Sciences, University of Lleida − AGROTECNIO-CERCA Center, Rovira Roure 191, 25198 Lleida, Spain;  Orcidhttps://orcid.org/0000-0002-7304-1378
    • Matthew P. Humphreys - Department of Ocean Systems (OCS), NIOZ Royal Netherlands Institute for Sea Research, P.O. Box 59, 1790 AB Den Burg (Texel), The Netherlands;  Orcidhttps://orcid.org/0000-0002-9371-7128
    • Martha Gledhill - GEOMAR Helmholtz Centre for Ocean Research Kiel, Wischhofstraße 1-3, 24148 Kiel, Germany;  Orcidhttps://orcid.org/0000-0003-3859-2112
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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We thank the captain and crew of R/V Meteor, chief scientists A. Koschinsky and M. Frank, and A. Mutzberg for the nutrient analysis. The authors gratefully acknowledge support from the Deutsche Forschungsgemeinschaft (Project GL 807/2) and for funding R/V Meteor cruise M147, the German Helmholtz Association, and Grants PID2019-107033GB-C21, PID2020-117910GB-C21, and PID2022-140312NB-C21funded by MCIN/AEI/10.13039/501100011033 and FEDER funds. P.L. also acknowledges current support from the Ministerio de Ciencia, Innovación y Universidades of Spain and University of Lleida (Beatriz Galindo Senior award number BG20/00104).

References

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This article references 44 other publications.

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    Adsorption on heterogeneous surfaces may be influenced not only by the heterogeneity but also by lateral interactions. For multicomponent solns., competition increases the complexity even further. In practice, such complex systems frequently occur and to study the behavior of these systems, it is rather useful to have anal. equations available which can describe the adsorption. Existing anal. mono- and multicomponent binding equations for heterogeneous surfaces are reviewed briefly. These equations are based on a high degree of ideality with respect to both the local isotherm and the affinity distribution for the different components. A new more generally valid anal. competitive binding equation is derived by using a nonideal local isotherm equation. In this local isotherm, all component-specific nonideality is incorporated; i.e., it includes both lateral interactions and component-specific heterogeneity contributions. Under these conditions, the remaining adsorbate-surface interactions can be characterized by an "intrinsic" affinity distribution that applies for all components. The features of the new equation are discussed and illustrated on the basis of some model calcns. The model is tested for Cd2+ ion binding to fulvic acid at various pH (using exptl. data reported by R. Saar and J. Weber, 1979). These data cannot be modeled satisfactorily with the classical equation for multicomponent adsorption on heterogeneous surfaces. The authors' equation gives excellent results.
  20. 20
    Kinniburgh, D. G.; Milne, C. J.; Benedetti, M. F.; Pinheiro, J. P.; Filius, J.; Koopal, L. K.; Van Riemsdijk, W. H. Metal Ion Binding by Humic Acid: Application of the NICA-Donnan Model. Environ. Sci. Technol. 1996, 30 (5), 1687– 1698,  DOI: 10.1021/es950695h
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    Metal Ion Binding by Humic Acid: Application of the NICA-Donnan Model
    Kinniburgh, David G.; Milne, Christopher J.; Benedetti, Marc F.; Pinheiro, Jose P.; Filius, Jeroen; Koopal, Luuk K.; Van Riemsdijk, Willem H.
    Environmental Science and Technology (1996), 30 (5), 1687-98CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    A Donnan-type model for nonspecific binding of electrolyte ions has been combined with the nonideal competitive adsorption (NICA) model for specific binding to produce a model for ion binding to humic substances. The model considers site heterogeneity, nonideality, multicomponent competition, and electrostatic interactions. The NICA-Donnan model was fitted to data for H+, Ca, Cd, Cu, and Pb binding by a purified peat humic acid. The model fits were good and covered a wide range of pH and free metal concns. The parameters from these single metal data sets were then used to predict the competitive effect of Ca on Cd and Cu binding at various pHs. These predictions agreed well with the exptl. data although there were some small but systematic differences. The new NICA-Donnan model also predicted reasonably well the increase in Cd and Cu binding on changing from a 0.1 M KNO3 background electrolyte to 0.01 M KNO3. A shortcoming of the model is that in some cases it significantly underestimated the H+/M2+ exchange ratio, esp. at high pH and for Cu binding.
  21. 21
    Kinniburgh, D. G.; van Riemsdijk, W. H.; Koopal, L. K.; Borkovec, M.; Benedetti, M. F.; Avena, M. J. Ion Binding to Natural Organic Matter: Competition, Heterogeneity, Stoichiometry and Thermodynamic Consistency. Colloids Surf. Physicochem. Eng. Asp. 1999, 151 (1–2), 147– 166,  DOI: 10.1016/S0927-7757(98)00637-2
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    Rey-Castro, C.; Mongin, S.; Huidobro, C.; David, C.; Salvador, J.; Garcés, J. L.; Galceran, J.; Mas, F.; Puy, J. Effective Affinity Distribution for the Binding of Metal Ions to a Generic Fulvic Acid in Natural Waters. Environ. Sci. Technol. 2009, 43 (19), 7184– 7191,  DOI: 10.1021/es803006p
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    22
    Effective Affinity Distribution for the Binding of Metal Ions to a Generic Fulvic Acid in Natural Waters
    Rey-Castro, Carlos; Mongin, Sandrine; Huidobro, Cesar; David, Calin; Salvador, Jose; Garces, Josep Lluis; Galceran, Josep; Mas, Francesc; Puy, Jaume
    Environmental Science & Technology (2009), 43 (19), 7184-7191CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    The effective distribution of affinities (Conditional Affinity Spectrum, CAS) seen by a metal ion binding to a humic substance under natural water conditions is derived and discussed within the NICA-Donnan model. Anal. expressions for the av. affinity of these distributions in general multi-ion mixts. are reported here for the first time. These expressions enable a simple evaluation of the effect of all interfering cations on the affinity distribution of a given one. We illustrate this methodol. by plotting the affinity spectra of a generic fulvic acid for 14 different cations in the presence of major inorg. ions and trace metals at pH and concn. values representative of a river water. The distribution of occupied sites and their av. affinity at the typical freshwater conditions are also reported for each ion. The CAS allows us to distinguish three groups of cations: (a) Al, H, Pb, Hg, and Cr, which are preferentially bound to the phenolic sites of the fulvic ligand; (b) Ca, Mg, Cd, Fe(II), and Mn, which display a greater effective affinity for carboxylic sites, in contrast to what would be expected from their individual complexation parameters; and (c) Fe(III), Cu, Zn, and Ni, for which phenolic and carboxylic distributions are overlapped.
  23. 23
    Benedetti, M. F.; vanRiemsdik, W. H.; Koopal, L. K. Humic Substances Considered as a Heterogeneous Donnan Gel Phase. Environ. Sci. Technol. 1996, 30 (6), 1805– 1813,  DOI: 10.1021/es950012y
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    23
    Humic Substances Considered as a Heterogeneous Donnan Gel Phase
    Benedetti, M. F.; Van Riemsdijk, W. H.; Koopal, L. K.
    Environmental Science and Technology (1996), 30 (6), 1805-13CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    A Donnan model in combination with a site-binding model is applied to interpret proton binding to humic substances as a function of pH at different salt levels. The Donnan model accounts for the salt effect on the charging behavior; it can only be successfully applied if it is assumed that the sp. vol. of humic substances is strongly dependent on the ionic strength. Empirical relationships have been derived that relate the vol. of the Donnan phase with the ionic strength. For fulvic acid, the Donnan approach is phys. not very realistic. The approach presumably mimics the behavior of the diffuse double layer around the relatively small fulvic acid mols. For humic acids, however, the approach is phys. realistic. The Donnan approach is a relatively simple and numerically efficient technique to account for the effects of ionic strength on the charging behavior. The ionic compn. of the gel phase follows directly from the technique. The insight obtained on the swelling behavior of the humic acids may be of use for interpreting binding of org. pollutants in natural systems.
  24. 24
    Companys, E.; Garces, J. L.; Salvador, J.; Galceran, J.; Puy, J.; Mas, F. Electrostatic and Specific Binding to Macromolecular Ligands - A General Analytical Expression for the Donnan Volume. Colloids Surf. -Physicochem. Eng. Asp. 2007, 306 (1–3), 2– 13,  DOI: 10.1016/j.colsurfa.2007.01.016
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    Pinheiro, J. P.; Rotureau, E.; Duval, J. F. L. Addressing the Electrostatic Component of Protons Binding to Aquatic Nanoparticles beyond the Non-Ideal Competitive Adsorption (NICA)-Donnan Level: Theory and Application to Analysis of Proton Titration Data for Humic Matter. J. Colloid Interface Sci. 2021, 583, 642– 651,  DOI: 10.1016/j.jcis.2020.09.059
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    25
    Addressing the electrostatic component of protons binding to aquatic nanoparticles beyond the Non-Ideal Competitive Adsorption (NICA)-Donnan level: Theory and application to analysis of proton titration data for humic matter
    Pinheiro, Jose Paulo; Rotureau, Elise; Duval, Jerome F. L.
    Journal of Colloid and Interface Science (2021), 583 (), 642-651CODEN: JCISA5; ISSN:0021-9797. (Elsevier B.V.)
    Charge descriptors of aquatic nanoparticles (NPs) are evaluated from proton titrn. curves measured at different salt concns. and routinely analyzed by the Non-Ideal Competitive Adsorption-Donnan (NICAD) model. This model, however, suffers from approxns. regarding particle electrostatics, which may bias particle charge estn. Implementation of Poisson-Boltzmann (PB) theory within consistent treatment of NPs protolytic data is expected to address NICAD shortcomings. An alternative to NICAD is elaborated on the basis of nonlinearized PB equation for soft particle electrostatics to properly unravel the electrostatic and chem. components of proton binding to NPs. A numerical package is developed for automated anal. of proton titrn. curves and proton affinity spectra at different salt concns. The performance of the method is illustrated for humic matter nanoparticles with different charge and size, and compared to that of NICAD. Unlike NICAD, PB-based treatment successfully reproduces particle charge dependence on pH for practical salt concns. from the thin to thick elec. double layer limit. Donnan representation in NICAD leads to moderate to dramatic misestimations of proton affinity and binding heterogeneity depending on particle size to Debye layer thickness ratio. Interpretation of NPs protolytic properties with PB theory further avoids adjustment of the 'particle Donnan vol.' empirically introduced in NICAD.
  26. 26
    Wagner, S.; Brandes, J.; Spencer, R. G. M.; Ma, K.; Rosengard, S. Z.; Moura, J. M. S.; Stubbins, A. Isotopic Composition of Oceanic Dissolved Black Carbon Reveals Non-Riverine Source. Nat. Commun. 2019, 10 (1), 5064,  DOI: 10.1038/s41467-019-13111-7
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    Isotopic composition of oceanic dissolved black carbon reveals non-riverine source
    Wagner Sasha; Brandes Jay; Ma Kun; Spencer Robert G M; Rosengard Sarah Z; Moura Jose Mauro S; Stubbins Aron
    Nature communications (2019), 10 (1), 5064 ISSN:.
    A portion of the charcoal and soot produced during combustion processes on land (e.g., wildfire, burning of fossil fuels) enters aquatic systems as dissolved black carbon (DBC). In terms of mass flux, rivers are the main identified source of DBC to the oceans. Since DBC is believed to be representative of the refractory carbon pool, constraining sources of marine DBC is key to understanding the long-term persistence of carbon in our global oceans. Here, we use compound-specific stable carbon isotopes (δ(13)C) to reveal that DBC in the oceans is ~6‰ enriched in (13)C compared to DBC exported by major rivers. This isotopic discrepancy indicates most riverine DBC is sequestered and/or rapidly degraded before it reaches the open ocean. Thus, we suggest that oceanic DBC does not predominantly originate from rivers and instead may be derived from another source with an isotopic signature similar to that of marine phytoplankton.
  27. 27
    Li, Y.; Harir, M.; Lucio, M.; Kanawati, B.; Smirnov, K.; Flerus, R.; Koch, B. P.; Schmitt-Kopplin, P.; Hertkorn, N. Proposed Guidelines for Solid Phase Extraction of Suwannee River Dissolved Organic Matter. Anal. Chem. 2016, 88 (13), 6680– 6688,  DOI: 10.1021/acs.analchem.5b04501
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    Proposed Guidelines for Solid Phase Extraction of Suwannee River Dissolved Organic Matter
    Li, Yan; Harir, Mourad; Lucio, Marianna; Kanawati, Basem; Smirnov, Kirill; Flerus, Ruth; Koch, Boris P.; Schmitt-Kopplin, Philippe; Hertkorn, Norbert
    Analytical Chemistry (Washington, DC, United States) (2016), 88 (13), 6680-6688CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
    This paper proposes improved guidelines for dissolved org. matter (DOM) isolation by solid phase extn. (SPE) with a styrene-divinylbenzene copolymer (PPL) sorbent, which has become an established method for the isolation of DOM from natural waters, because of its ease of application and appreciable carbon recovery. Suwannee River water was selected to systematically study the effects of crit. SPE variables such as loading mass, concn., flow rate, and up-scaling on the extn. selectivity of the PPL sorbent. High-field Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS) and proton NMR (1H NMR) spectroscopy were performed to interpret the DOM chem. space of eluates, as well as permeates and wash liqs. with mol. resoln. Up to 89% dissolved org. carbon (DOC) recovery was obtained with a DOC/PPL mass ratio of 1:800 at a DOC concn. of 20 mg/L. With the application of larger loading vols., low proportions of highly oxygenated compds. were retained on the PPL sorbent. The effects of the flow rate on the extn. selectivity of the sorbent were marginal. Up-scaling had a limited effect on the extn. selectivity with the exception of increased self-esterification with a methanol solvent, resulting in Me ester groups. Furthermore, the SPE/PPL ext. exhibited highly authentic characteristics in comparison with original water and reverse osmosis samples. These findings will be useful for reproducibly isolating DOM with representative mol. compns. from various sources and concns. and minimizing potential inconsistencies among interlab. comparative studies.
  28. 28
    Hertkorn, N.; Harir, M.; Koch, B. P.; Michalke, B.; Schmitt-Kopplin, P. High-Field NMR Spectroscopy and FTICR Mass Spectrometry: Powerful Discovery Tools for the Molecular Level Characterization of Marine Dissolved Organic Matter. Biogeosciences 2013, 10 (3), 1583– 1624,  DOI: 10.5194/bg-10-1583-2013
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    High-field NMR spectroscopy and FTICR mass spectrometry: powerful discovery tools for the molecular level characterization of marine dissolved organic matter
    Hertkorn, N.; Harir, M.; Koch, B. P.; Michalke, B.; Schmitt-Kopplin, P.
    Biogeosciences (2013), 10 (3), 1583-1624CODEN: BIOGGR; ISSN:1726-4189. (Copernicus Publications)
    High-performance, non-target, high-resoln. org. structural spectroscopy was applied to solid phase extd. marine dissolved org. matter (SPE-DOM) isolated from four different depths in the open South Atlantic Ocean off the Angola coast (3° E, 18° S; Angola Basin) and provided mol. level information with extraordinary coverage and resoln. Sampling was performed at depths of 5m (Angola Current; near-surface photic zone), 48m (Angola Current; fluorescence max.), 200m (still above Antarctic Intermediate Water, AAIW; upper mesopelagic zone) and 5446m (North Atlantic Deep Water, NADW; abyssopelagic, ∼30m above seafloor) and produced SPE-DOM with near 40% carbon yield and beneficial NMR (NMR) relaxation properties, a crucial prerequisite for the acquisition of NMR spectra with excellent resoln. 1H and 13C NMR spectra of all four marine SPE-DOM showed smooth bulk envelopes, reflecting intrinsic averaging from massive signal overlap, with a few percent of visibly resolved signatures and variable abundances for all major chem. environments. The abundance of singly oxygenated aliphatics and acetate derivs. in 1H NMR spectra declined from surface to deep marine SPE-DOM, whereas C-based aliphatics and carboxyl-rich alicyclic mols. (CRAM) increased in abundance. Surface SPE-DOMcontained fewer Me esters than all other samples, likely a consequence of direct exposure to sunlight. Integration of 13C NMR spectra revealed continual increase of carboxylic acids and ketones from surface to depth, reflecting a progressive oxygenation, with concomitant decline of carbohydrate-related substructures.
  29. 29
    Martínez-Pérez, A. M.; Osterholz, H.; Nieto-Cid, M.; Álvarez, M.; Dittmar, T.; Álvarez-Salgado, X. A. Molecular Composition of Dissolved Organic Matter in the Mediterranean Sea. Limnol. Oceanogr. 2017, 62 (6), 2699– 2712,  DOI: 10.1002/lno.10600
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    Molecular composition of dissolved organic matter in the Mediterranean Sea
    Martinez-Perez, Alba Maria; Osterholz, Helena; Nieto-Cid, Mar; Alvarez, Marta; Dittmar, Thorsten; Alvarez-Salgado, Xose Anton
    Limnology and Oceanography (2017), 62 (6), 2699-2712CODEN: LIOCAH; ISSN:0024-3590. (John Wiley & Sons, Inc.)
    The mol. compn. of marine dissolved org. matter (DOM) is still poorly understood, particularly in the Mediterranean Sea. In this work, DOM from the open Mediterranean Sea and the adjacent Northeast Atlantic Ocean was isolated by solid-phase extn. (SPE-DOM) and molecularly characterized using Fourier-transform ion cyclotron resonance mass spectrometry. We assessed the gradual reworking of the SPE-DOM transported by the shallow overturning circulation of the Mediterranean Sea by following the increase in mol. wt. (+20 Da), oxygenation (+5%), degrdn. index (Ideg +22%), and the proportional decrease of unsatd. aliph. compds. (+34%) along the Levantine Intermediate Water. This reworked SPE-DOM that leaves the Mediterranean Sea through the Strait of Gibraltar strongly contrasts with the fresh material transported by the inflow of Atlantic water (Ideg -25%). In the deep eastern and western overturning cells, the mol. compn. of the deep waters varied according to their area and/or time of formation. SPE-DOM of the waters formed in the Aegean Sea during the Eastern Mediterranean Transient (EMT) was more processed than the DOM in pre-EMT waters formed in the Adriatic Sea (mol. wt. and the proportion of unsatd. aliph. compds. were increased by 5 Da and 9%, resp.). Furthermore, pre-EMT waters contain more reworked SPE-DOM (Ideg +7%) than post-EMT waters formed also in the Adriatic Sea. In summary, our study shows that the Mediterranean Sea constitutes a lab. basin where degrdn. processes and diagenetic transformations of DOM can be obsd. on close spatial and temporal scales.
  30. 30
    Broek, T. A. B.; Walker, B. D.; Guilderson, T. P.; McCarthy, M. D. Coupled Ultrafiltration and Solid Phase Extraction Approach for the Targeted Study of Semi-Labile High Molecular Weight and Refractory Low Molecular Weight Dissolved Organic Matter. Mar. Chem. 2017, 194, 146– 157,  DOI: 10.1016/j.marchem.2017.06.007
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    Coupled ultrafiltration and solid phase extraction approach for the targeted study of semi-labile high molecular weight and refractory low molecular weight dissolved organic matter
    Broek, Taylor A. B.; Walker, Brett D.; Guilderson, Thomas P.; McCarthy, Matthew D.
    Marine Chemistry (2017), 194 (), 146-157CODEN: MRCHBD; ISSN:0304-4203. (Elsevier B.V.)
    Only a small fraction of dissolved org. matter (DOM) can be characterized at the mol. level by direct seawater anal. Thus, the study of DOM requires isolation of extremely dil. orgs. from orders of magnitude greater concns. of inorg. salts. Traditional isolation approaches have sought to isolate representative DOM fractions, however, currently available isolation methods all have selective chem. or phys. biases. Recent work has indicated that DOM exists in a functional continuum of mol. size and 14C age. High mol. wt. (HMW) DOM is primarily composed of younger, semi-labile material, while much older, low mol. wt. (LMW) DOM dominates the refractory background pool. Here we describe a new large vol. DOM isolation approach that selectively isolates HMW and LMW DOM fractions with distinct 14C ages, a proxy for reactivity. The method uses ultrafiltration (UF) to isolate HMW DOM (UDOM), and then solid phase extn. (SPE) to isolate LMW DOM permeating the UF system. We first assess two SPE sorbents (Agilent Bond Elute PPL and Diaion HP-20) for DOM chem. and isotopic selectivity. Second, we evaluate our UF/SPE approach in the context of DOM recovery, elemental (C/N) and isotopic (δ13C, δ15N, Δ14C) compn. of 8 HMW and LMW sample pairs, isolated from the North Central Pacific Ocean. Radiocarbon (Δ14C) anal. shows major differences in the Δ14C value of HMW (Δ14C = - 37 to - 380‰) and LMW (Δ14C = - 343 to - 578‰) DOM fractions. We also observe elemental and stable isotopic offsets between HMW and LMW DOM at all depths. HMW UDOM (C/N = 11.5 to 13.1, δ13C = - 22.5 to - 21.1‰, δ15N = 6.2 to 7.1‰) has significantly lower C/N ratios and higher δ13C and δ15N values than LMW SPE-DOM (C/N = 24.2 to 28.5, δ13C = - 23.3 to - 22.2‰, δ15N = 3.1 to 4.0‰), with the exception of surface δ13C, which is equiv. in both size fractions. Together, these results indicate that our combined UF/SPE method successfully isolates sep. young (semi-labile, HMW) and old (refractory, LMW) DOM fractions, each with distinct chem. and isotopic compn. Ultimately, by limiting the influence of DOM reactivity mixts., our method provides an alternative approach for understanding DOM sources and cycling.
  31. 31
    Bercovici, S. K.; Koch, B. P.; Lechtenfeld, O. J.; McCallister, S. L.; Schmitt-Kopplin, P.; Hansell, D. A. Aging and Molecular Changes of Dissolved Organic Matter Between Two Deep Oceanic End-Members. Glob. Biogeochem. Cycles 2018, 32 (10), 1449– 1456,  DOI: 10.1029/2017GB005854
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    Aging and Molecular Changes of Dissolved Organic Matter Between Two Deep Oceanic End-Members
    Bercovici, S. K.; Koch, B. P.; Lechtenfeld, O. J.; McCallister, S. L.; Schmitt-Kopplin, P.; Hansell, D. A.
    Global Biogeochemical Cycles (2018), 32 (10), 1449-1456CODEN: GBCYEP; ISSN:1944-9224. (Wiley-Blackwell)
    The global ocean contains a massive reservoir of dissolved org. carbon (DOC), rivaling the atm.'s pool of CO2. The most recalcitrant fractions have mean radiocarbon ages of ∼4,000 years in the Atlantic to ∼6,000 years in the Pacific. Knowing the radiocarbon signatures of DOC and the mol. compn. of dissolved org. matter (DOM) is crucial to develop understanding of the persistence and lifetime of the DOC pool. In this research, we collected samples from the deep North Pacific in August 2013 (aboard the RV Melville) to couple the Δ14C content of solid-phase-extd. DOM (Δ14C-SPE-DOM) with its mol. compn. in the ocean's oldest deep waters. We find that deep waters in this region held a mean Δ14C-SPE-DOM value of -554 ± 9‰ (∼6,400 14C years), substantially more depleted than that in the deep Atlantic, which held a mean Δ14C-SPE-DOM value of -445 ± 5‰. While we find a more degraded mol. compn. of DOM in the deep Pacific than the deep Atlantic, the mol. formulas within the Island of Stability (Lechtenfeld et al., 2014, ), are largely retained. These results imply that a fraction of deep DOM is resistant to removal and present in both the deep Atlantic and Pacific Oceans.
  32. 32
    Jerusalén-Lleó, E.; Nieto-Cid, M.; Fuentes-Santos, I.; Dittmar, T.; Álvarez-Salgado, X. A. Solid Phase Extraction of Ocean Dissolved Organic Matter with PPL Cartridges: Efficiency and Selectivity. Front. Mar. Sci. 2023, 10, 1159762  DOI: 10.3389/fmars.2023.1159762
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    Chaichana, S.; Jickells, T.; Johnson, M. Interannual Variability in the Summer Dissolved Organic Matter Inventory of the North Sea: Implications for the Continental Shelf Pump. Biogeosciences 2019, 16 (5), 1073– 1096,  DOI: 10.5194/bg-16-1073-2019
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    Interannual variability in the summer dissolved organic matter inventory of the North Sea: implications for the continental shelf pump
    Chaichana, Saisiri; Jickells, Tim; Johnson, Martin
    Biogeosciences (2019), 16 (5), 1073-1096CODEN: BIOGGR; ISSN:1726-4189. (Copernicus Publications)
    We present the distribution and C:N stoichiometry of dissolved org. matter (DOM) in the North Sea in two summers (August 2011 and August 2012), with supporting data from the intervening winter (Jan. 2012). These data demonstrate local variability superimposed on a general pattern of decreasing DOM with increasing distance from land, suggesting concns. of DOM are controlled on large spatial scales by mixing between the open North Atlantic and either riverine sources or high DOM productivity in nearshore coastal waters driven by riverine nutrient discharge. Given the large size and long residence time of water in the North Sea, we find concns. are commonly modified from simple conservative mixing between two endmembers. We observe differences in dissolved org. carbon (DOC) and dissolved org. nitrogen (DON) concns. and land-ocean gradients between the two summers, leading to an estd. 10-20 Tg difference in the DOC inventory between the two years, which is of the same order of magnitude as the annual uptake of atm. CO2 by the North Sea system, and thus significant for the carbon budget of the North Sea. This difference is not consistent with addnl. terrestrial loading and is more likely to be due to balancing of mixing and in situ prodn. and loss processes across the North Sea. Differences were particularly pronounced in the bottom layer of the seasonally stratifying northern North Sea, with higher DOC and C:N ratio in 2011 than in 2012. Using other data, we consider the extent to which these differences in the concns. and C:N ratio of DOM could be due to changes in the biogeochem. or phys. circulation in the two years, or a combination of both. The evidence we have is consistent with a flushing event in winter 2011/12 exchanging DOM-rich, high C:N shelf waters, which may have accumulated over more than 1 yr, with deep North Atlantic waters with lower DOC and marginally higher DON. We discuss the implications of these observations for the shelf sea carbon pump and the export of carbon-rich org. matter off the shelf and hypothesise that intermittent flushing of temperate shelf systems may be a key mechanism in the maintenance of the continental shelf pump, via the accumulation and subsequent export of carbon-rich DOM.
  34. 34
    Letscher, R. T.; Moore, J. K. Preferential Remineralization of Dissolved Organic Phosphorus and Non-Redfield DOM Dynamics in the Global Ocean: Impacts on Marine Productivity, Nitrogen Fixation, and Carbon Export. Glob. Biogeochem. Cycles 2015, 29 (3), 325– 340,  DOI: 10.1002/2014GB004904
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    Preferential remineralization of dissolved organic phosphorus and non-Redfield DOM dynamics in the global ocean: Impacts on marine productivity, nitrogen fixation, and carbon export
    Letscher, Robert T.; Moore, J. Keith
    Global Biogeochemical Cycles (2015), 29 (3), 325-340CODEN: GBCYEP; ISSN:1944-9224. (Wiley-Blackwell)
    Selective removal of nitrogen (N) and phosphorus (P) from the marine dissolved org. matter (DOM) pool has been reported in several regional studies. Because DOM is an important advective/mixing pathway of carbon (C) export from the ocean surface layer and its non-Redfieldian stoichiometry would affect ests. of marine export prodn. per unit N and P, we investigated the stoichiometry of marine DOM and its remineralization globally using a compiled DOM data set. Marine DOM is enriched in C and N compared to Redfield stoichiometry, averaging 317:39:1 and 810:48:1 for C:N:P within the degradable and total bulk pools, resp. Dissolved org. phosphorus (DOP) is found to be preferentially remineralized about twice as rapidly with respect to the enriched C:N stoichiometry of marine DOM. Biogeochem. simulations with the Biogeochem. Elemental Cycling model using Redfield and variable DOM stoichiometry corroborate the need for non-Redfield dynamics to match the obsd. DOM stoichiometry. From our model simulations, preferential DOP remineralization is found to increase the strength of the biol. pump by ∼9% vs. the case of Redfield DOM cycling. Global net primary productivity increases ∼10% including an increase in marine nitrogen fixation of ∼26% when preferential DOP remineralization and direct utilization of DOP by phytoplankton are included. The largest increases in marine nitrogen fixation, net primary productivity, and carbon export are obsd. within the western subtropical gyres, suggesting the lateral transfer of P in the form of DOP from the productive eastern and poleward gyre margins may be important for sustaining these processes downstream in the subtropical gyres.
  35. 35
    Dittmar, T.; Koch, B.; Hertkorn, N.; Kattner, G. A Simple and Efficient Method for the Solid-Phase Extraction of Dissolved Organic Matter (SPE-DOM) from Seawater. Limnol. Oceanogr. Methods 2008, 6 (6), 230– 235,  DOI: 10.4319/lom.2008.6.230
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    A simple and efficient method for the solid-phase extraction of dissolved organic matter (SPE-DOM) from seawater
    Dittmar, Thorsten; Koch, Boris; Hertkorn, Norbert; Kattner, Gerhard
    Limnology and Oceanography: Methods (2008), 6 (June), 230-235CODEN: LOMIBY; ISSN:1541-5856. (American Society of Limnology and Oceanography)
    A simple protocol is presented for the solid-phase extn. of dissolved org. matter (SPE-DOM) from seawater using com. prepacked cartridges. The method does not require major instrumentation and can be performed in the field. Modified styrene divinyl benzene polymer type sorbents (Varian PPL and ENV) and sorbents of a silica structure bonded with different hydrocarbon chains (Varian C8, C18, C18OH, and C18EWP) were considered. Except for C18OH, which heavily contaminated the samples, none of the sorbents leached significant amts. of dissolved org. carbon (DOC) or nitrogen (DON). Samples from the North Brazil shelf with strong mixing gradients of terrigenous and marine DOM were used to compare the various sorbents. PPL was the most efficient-on av., 62% of DOC was recovered as salt-free exts. C18 was found to be most efficient among the silica-based sorbents, but it showed only two-thirds of the extn. efficiency of PPL. As indicated by [1H]NMR, C/N, and δ13C analyses, PPL extd. a more representative proportion of DOM than C18. Therefore, PPL was used for comparative studies in the Gulf of Mexico and Antarctica. From brackish marsh and river waters, 65% and 62% of total DOC, resp., could be extd. For purely marine DOM in Antarctica and the deep sea, the extn. efficiency was lower (43% on av.). The efficiency of the new method to isolate marine DOM is better than or similar to highly laborious methods. A further advantage is the complete desalination of the sample. The isolation of a major DOM fraction, which is salt-free, offers many possibilities to further characterize DOM by advanced anal. techniques.
  36. 36
    Ksionzek, K. B.; Zhang, J.; Ludwichowski, K.-U.; Wilhelms-Dick, D.; Trimborn, S.; Jendrossek, T.; Kattner, G.; Koch, B. P. Stoichiometry, Polarity, and Organometallics in Solid-Phase Extracted Dissolved Organic Matter of the Elbe-Weser Estuary. PLoS One 2018, 13 (9), e0203260  DOI: 10.1371/journal.pone.0203260
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    37
    Stoichiometry, polarity, and organometallics in solid-phase extracted dissolved organic matter of the Elbe-Weser estuary
    Ksionzek, Kerstin B.; Zhang, Jing; Ludwichowski, Kai-Uwe; Wilhelms-Dick, Dorothee; Trimborn, Scarlett; Jendrossek, Thomas; Kattner, Gerhard; Koch, Boris P.
    PLoS One (2018), 13 (9), e0203260/1-e0203260/24CODEN: POLNCL; ISSN:1932-6203. (Public Library of Science)
    Dissolved org. matter (DOM) is ubiquitous in natural waters and plays a central role in the biogeochem. in riverine, estuarine and marine environments. This study quantifies and characterizes solid-phase extractable DOM and trace element complexation at different salinities in the Weser and Elbe River, northern Germany, and the North Sea. Dissolved org. carbon (DOC), total dissolved nitrogen (TDN), Co and Cu concns. were analyzed in original water samples. Solid-phase extd. (SPE) water samples were analyzed for DOC (DOCSP E), dissolved org. nitrogen (DONSP E), sulfur (DOSSP E) and trace metal (51V, 52Cr, 59Co, 60Ni, 63Cu, 75As) concns. Addnl., different pre-treatment conditions (acidification vs. non-acidification prior to SPE) were tested. In agreement with previous studies, acidification led to generally higher recoveries for DOM and trace metals. Overall, higher DOM and trace metal concns. and subsequently higher complexation of trace metals with carbon and sulfur-contg. org. complexes were found in riverine compared to marine samples. With increasing salinity, the concns. of DOM decreased due to estuarine mixing. However, the slightly lower relative decrease of both, DOCSP E and DONSP E (∼77%) compared to DOSSP E (∼86%) suggests slightly faster removal processes for DOSSP E.
  37. 37
    Green, N. W.; Perdue, E. M.; Aiken, G. R.; Butler, K. D.; Chen, H.; Dittmar, T.; Niggemann, J.; Stubbins, A. An Intercomparison of Three Methods for the Large-Scale Isolation of Oceanic Dissolved Organic Matter. Mar. Chem. 2014, 161, 14– 19,  DOI: 10.1016/j.marchem.2014.01.012
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    An intercomparison of three methods for the large-scale isolation of oceanic dissolved organic matter
    Green, Nelson W.; Perdue, E. Michael; Aiken, George R.; Butler, Kenna D.; Chen, Hongmei; Dittmar, Thorsten; Niggemann, Jutta; Stubbins, Aron
    Marine Chemistry (2014), 161 (), 14-19CODEN: MRCHBD; ISSN:0304-4203. (Elsevier B.V.)
    Dissolved org. matter (DOM) was isolated from large vols. of deep (674 m) and surface (21 m) ocean water via reverse osmosis/electrodialysis (RO/ED) and two solid-phase extn. (SPE) methods (XAD-8/4 and PPL) at the Natural Energy Lab. of Hawaii Authority (NELHA). By applying the three methods to common water samples, the efficiencies of XAD, PPL and RO/ED DOM isolation were compared. XAD recovered 42% of dissolved org. carbon (DOC) from deep water (25% with XAD-8; 17% with XAD-4) and 30% from surface water (16% with XAD-8; 14% with XAD-4). PPL recovered 61 ± 3% of DOC from deep water and 61% from surface water. RO/ED recovered 82 ± 3% of DOC from deep water, 14 ± 3% of which was recovered in a sodium hydroxide rinse, and 75 ± 5% of DOC from surface water, with 12 ± 2% in the sodium hydroxide rinse. The highest recoveries of all were achieved by the sequential isolation of DOC, first with PPL and then via RO/ED. This combined technique recovered 98% of DOC from a deep water sample and 101% of DOC from a surface water sample. In total, 1.9, 10.3 and 1.6 g-C of DOC were collected via XAD, PPL and RO/ED, resp. Rates of DOC recovery using the XAD, PPL and RO/ED methods were 10, 33 and 10 mg-C h- 1, resp. Based upon C/N ratios, XAD isolates were heavily C-enriched compared with water column DOM, whereas RO/ED and PPL → RO/ED isolate C/N values were most representative of the original DOM. All techniques are suitable for the isolation of large amts. of DOM with purities suitable for most advanced anal. techniques. Coupling PPL and RO/ED techniques may provide substantial progress in the search for a method to quant. isolate oceanic DOC, bringing the entirety of the DOM pool within the marine chemist's anal. window.
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    Lodeiro, P.; Martínez-Cabanas, M.; Herrero, R.; Barriada, J. L.; Vilariño, T.; Rodríguez-Barro, P.; Sastre de Vicente, M. E. The Proton Binding Properties of Biosorbents. Environ. Chem. Lett. 2019, 17 (3), 1281– 1298,  DOI: 10.1007/s10311-019-00883-z
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    The proton binding properties of biosorbents
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    Environmental Chemistry Letters (2019), 17 (3), 1281-1298CODEN: ECLNBJ; ISSN:1610-3653. (Springer)
    A review. A broad variety of materials of biol. origin have been successfully used in recent decades for the removal of pollutants from waters. These biosorbents include natural polymers that play a key role for adsorption. It is therefore crit. to understand the physicochem. properties of the chem. groups of these biopolymers. The acid-base properties of biomass are affected by pH, ionic strength and medium compn. Nevertheless, these parameters are not always considered during biosorption studies. According to the literature, less than 3% of biosorption reports include studies on proton binding. Moreover, in 60% of these papers, there is key exptl. information missing such as the calibration of the electrodes employed for potentiometric titrns. We consider therefore that there is an important need for reviewing the role of proton binding in biosorption studies. This review outlines the major advances on data interpretation and modeling of proton binding on biosorbents. In addn., we discuss issues concerning the acid-base properties of biosorbents.
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    Fong, M. B.; Dickson, A. G. Insights from GO-SHIP Hydrography Data into the Thermodynamic Consistency of CO2 System Measurements in Seawater. Mar. Chem. 2019, 211, 52– 63,  DOI: 10.1016/j.marchem.2019.03.006
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    Insights from GO-SHIP hydrography data into the thermodynamic consistency of CO2 system measurements in seawater
    Fong, Michael B.; Dickson, Andrew G.
    Marine Chemistry (2019), 211 (), 52-63CODEN: MRCHBD; ISSN:0304-4203. (Elsevier B.V.)
    Due to advances in technol., routine seawater pH measurements of excellent repeatability are becoming increasingly common for studying the ocean CO2 system. However, the accuracy of pH measurements has come into question due to a widespread observation, from a large no. of carefully calibrated state-of-the-art CO2 measurements on various cruises, of there being a significant pH-dependent discrepancy between pH that was measured spectrophotometrically and pH calcd. from concurrent measurements of total dissolved inorg. carbon (CT) and total alky. (AT), using a thermodn. model of seawater acid-base systems. From an anal. of four recent GO-SHIP repeat hydrog. datasets, we show that a combination of small systematic errors in the dissocn. consts. of carbonic acid (K1 and K2), the total boron-salinity ratio, and in CT and AT measurements are likely responsible for some, but not all of the obsd. pH-dependent discrepancy. The residual discrepancy can only be fully accounted for if there exists a small, but meaningful amt. (~ 4μ mol kg-1) of an unidentified and typically neglected contribution to measured AT, likely from org. bases, that is widespread in the open ocean. A combination of these errors could achieve consistency between measured and calcd. pH, without requiring that any of the shipboard measurements be significantly in error. Future research should focus on establishing the existence of org. alky. in the open ocean and constraining the uncertainty in both CO2 measurements and in the consts. used in CO2 calcns.
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    Kerr, D. E.; Turner, C.; Keogh, J.; Grey, A.; Brown, P. J.; Kelleher, B. P. OrgAlkCalc: Estimation of Organic Alkalinity Quantities and Acid-Base Properties with Proof of Concept in Dublin Bay. Mar. Chem. 2023, 251, 104234,  DOI: 10.1016/j.marchem.2023.104234
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    OrgAlkCalc: Estimation of organic alkalinity quantities and acid-base properties with proof of concept in Dublin Bay
    Kerr, Daniel E.; Turner, Charles; Grey, Anthony; Keogh, Jill; Brown, Peter J.; Kelleher, Brian P.
    Marine Chemistry (2023), 251 (), 104234CODEN: MRCHBD; ISSN:0304-4203. (Elsevier B.V.)
    The presence and influence of org. species is generally omitted in total alky. (TA) anal. This has direct implications to calcd. carbonate system parameters and to key descriptors of ocean acidification, esp. in coastal waters where org. alky. (OrgAlk) can contribute significantly to TA. As titratable charge groups of OrgAlk can act as unknown seawater acid-base systems, the inclusion of the total concn. and apparent dissocn. consts. of OrgAlk in carbonate calcns. involving TA is required to minimise uncertainty in computed speciation. Here we present OrgAlkCalc, an open-source Python based program that can be used in conjunction with simply modified Global Ocean Acidification Observing Network (GOA-ON) TA titrn. app. to measure TA and OrgAlk, as well as return estns. of assocd. acid-base properties. The modified titrn. app. and OrgAlkCalc were tested using samples collected from the transitional waters of Dublin Bay, Ireland over a 8 mo period (n = 100). TA values ranged from 2257 to 4692μmol.kg-1 and indicated that freshwater inputs pose a significant source of carbonate alky. to Dublin Bay. OrgAlk values ranged from 46 to 234μmol.kg-1 and were generally obsd. to be higher in more saline waters, with elevated levels in the Autumn/Winter period. The dissocn. consts. of two distinct OrgAlk charge groups were identified, with pK values in agreement with previously reported values for humic substances. The majority of OrgAlk charge group concns. were assocd. with carboxyl-like charge groups.
  42. 42
    Avena, M. J.; Vermeer, A. W. P.; Koopal, L. K. Volume and Structure of Humic Acids Studied by Viscometry pH and Electrolyte Concentration Effects. Colloids Surf., A 1999, 151, 213– 224,  DOI: 10.1016/S0927-7757(98)00504-4
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    Volume and structure of humic acids studied by viscometry pH and electrolyte concentration effects
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    Viscometry was used to evaluate the effects of pH and supporting electrolyte concn. on the intrinsic viscosities of eight humic acids and one fulvic acid. Two synthetic poly(acrylic acid) (PAA) samples of different mol. wt. were also studied for comparison. Humic and fulvic acid mols. behave as flexible entities that can swell or shrink in response to changes in pH and ionic strength. An increase in the soln. pH leads to the development of neg. charges in the mols. with the consequent electrostatic repulsion between ionized groups and mol. swelling. Increasing the ionic strength increases the screening of charges and leads to mol. shrinkage. The pH dependence decreases with increasing electrolyte concn. and at 10-1 M electrolyte the intrinsic viscosity is almost pH independent. The general behavior of PAAs is similar to that of the humics, though the effects of pH and electrolyte concn. are much larger for the PAAs. The degree of hydration of the humics differs for different samples. There are compact samples with low water content and swelling properties whereas other humics are more hydrated and flexible. All the studied humics have an internal structure that limits the expansion of the mols. when the electrolyte concn. is decreased. The latter is in accordance with the low values of the Mark-Houwink coeff., α, of humics.
  43. 43
    Dittmar, T.; Lennartz, S. T.; Buck-Wiese, H.; Hansell, D. A.; Santinelli, C.; Vanni, C.; Blasius, B.; Hehemann, J. H. Enigmatic Persistence of Dissolved Organic Matter in the Ocean. Nat. Rev. Earth Environ. 2021, 2 (8), 570– 583,  DOI: 10.1038/s43017-021-00183-7
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    Enigmatic persistence of dissolved organic matter in the ocean
    Dittmar, Thorsten; Lennartz, Sinikka T.; Buck-Wiese, Hagen; Hansell, Dennis A.; Santinelli, Chiara; Vanni, Chiara; Blasius, Bernd; Hehemann, Jan-Hendrik
    Nature Reviews Earth & Environment (2021), 2 (8), 570-583CODEN: NREECQ; ISSN:2662-138X. (Springer International Publishing AG)
    Marine dissolved org. matter (DOM) contains more carbon than the combined stocks of Earth's biota. Organisms in the ocean continuously release a myriad of mols. that become food for microheterotrophs, but, for unknown reasons, a residual fraction persists as DOM for millennia. In this Perspective, we discuss and compare two concepts that could explain this persistence. The long-standing 'intrinsic recalcitrance paradigm attributes DOM stability to inherent mol. properties. In the 'emergent recalcitrance' concept, DOM is continuously transformed by marine microheterotrophs, with recalcitrance emerging on an ecosystems level. Both concepts are consistent with observations in the modern ocean, but they imply very different responses of the DOM pool to climate-related changes. To better understand DOM persistence, we propose a new overarching research strategy - the ecol. of mols. - that integrates the concepts of intrinsic and emergent recalcitrance with the ecol. and environmental context.
  44. 44
    David, C.; Mongin, S.; Rey-Castro, C.; Galceran, J.; Companys, E.; Garcés, J. L.; Salvador, J.; Puy, J.; Cecilia, J.; Lodeiro, P.; Mas, F. Competition Effects in Cation Binding to Humic Acid: Conditional Affinity Spectra for Fixed Total Metal Concentration Conditions. Geochim. Cosmochim. Acta 2010, 74 (18), 5216– 5227,  DOI: 10.1016/j.gca.2010.06.023
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    Competition effects in cation binding to humic acid: Conditional affinity spectra for fixed total metal concentration conditions
    David, Calin; Mongin, Sandrine; Rey-Castro, Carlos; Galceran, Josep; Companys, Encarnacio; Garces, Jose Luis; Salvador, Jose; Puy, Jaume; Cecilia, Joan; Lodeiro, Pablo; Mas, Francesc
    Geochimica et Cosmochimica Acta (2010), 74 (18), 5216-5227CODEN: GCACAK; ISSN:0016-7037. (Elsevier B.V.)
    Information on the Pb and Cd binding to a purified Aldrich humic acid (HA) is obtained from the influence of different fixed total metal concns. on the acid-base titrns. of this ligand. NICA (Non-Ideal Competitive Adsorption) isotherm has been used for a global quant. description of the binding, which has then been interpreted by plotting the Conditional Affinity Spectra of the H+ binding at fixed total metal concns. (CAScTM). This new physicochem. tool allows the interpretation of binding results in terms of distributions of proton binding energies. A large increase in the acidity of the phenolic sites as the total metal concn. increases, esp. in presence of Pb, is revealed from the shift of the CAScTM towards lower affinities. The variance of the CAScTM distribution, which can be used as a direct measure of the heterogeneity, also shows a significant dependence on the total metal concn. A discussion of the factors that influence the heterogeneity of the HA under the conditions of each expt. is provided, so that the smoothed pattern exhibited by the titrn. curves can be justified.

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  2. C. S. Hassler, R. Simó, S. E. Fawcett, M. J. Ellwood, S. L. Jaccard. Marine biogenic humic substances control iron biogeochemistry across the Southern Ocean. Nature Communications 2025, 16 (1) https://doi.org/10.1038/s41467-025-57491-5
  3. David R. Turner, Peter L. Croot, Andrew G. Dickson, Martha Gledhill. Physicochemical controls on seawater. 2025, 1-51. https://doi.org/10.1016/B978-0-323-99762-1.00035-8
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  • Abstract

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    Figure 1

    Figure 1. NICA–Donnan fits to proton titration data of North Atlantic SPE–DOM at 25 °C and 0.007, 0.1, 0.7, and 1.0 M ionic strength, using the PB–VD model: surface (left panel) and deep (right panel) samples. Symbols: experimental values. Colored lines: model fits at each ionic strength. The uppermost black curve corresponds to the charge “master curve” (QmaxH,tot – QH) vs pHD.

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    Figure 2

    Figure 2. Total amount of proton binding groups obtained from the fits of the NICA–Donnan model (PB–VD) to the proton binding data shown in Figure 1: QmaxH,tot (a) and QmaxH,i (b), for the low affinity (DOM1, green bars) and high affinity (DOM2, blue bars) distributions. The values of Baltic (Boknis Eck) and Baltic (Kiel Fjord) are from Lodeiro et al. (4,5) Bar heights indicate the mean, and error bars indicate the standard deviation. The FA values indicate QmaxH for a generic terrestrial fulvic acid from Milne et al. (16) (calculated with the standard VD model); the error bar indicates the range of values used for the derivation of the generic value.

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    Figure 3

    Figure 3. Proton binding affinity spectra of North Atlantic SPE–DOM at 25 °C calculated from the NICA–Donnan model (PB–VD) parameters of Table S3 for surface (red lines) and deep (blue lines) samples: (a) intrinsic values and (b) effective values calculated at I = 0.7 M and density of protonated sites (occupation of binding sites by protons) at the experimental values of pH 8.08 (shaded red area) and pH 7.56 (shaded blue area) of the surface and deep samples, respectively. All spectra are normalized to one.

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    Figure 4

    Figure 4. Median values of the intrinsic proton binding affinity (log K̅H,i) parameters obtained from the fits of the NICA–Donnan model (PB–VD) to proton binding data shown in Figure 1: low affinity (DOM1, green bars) and high affinity distribution (DOM2, blue bars). Bar heights indicate the mean, and error bars indicate the standard deviation. The values for the Baltic (Kiel Fjord) are from Lodeiro et al. (4)

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    Figure 5

    Figure 5. Chemical binding heterogeneity (mi) parameters obtained from the fits of the NICA–Donnan model (PB–VD) to proton binding data shown in Figure 1 for the low affinity (DOM1, green bars) and high affinity (DOM2, blue bars) distributions. Bar heights indicate the mean, and error bars indicate the standard deviation. The values for the Baltic (Kiel Fjord) are from Lodeiro et al. (4)

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    Figure 6

    Figure 6. Effective binding parameters estimated at I = 0.7 M, using the intrinsic NICA-Donnan model with PB-VD, and the proton binding data shown in Figure 1 for the DOM1 (green bars) and DOM2 (blue bars) distributions: log K̅H,i (a) and mi (b). The values Baltic (Boknis Eck) and Baltic (Kiel Fjord) are from Lodeiro et al. (4,5) Bar heights indicate the mean, and error bars indicate the standard deviation.

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  • References

    This article references 44 other publications.

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      Influence of pH and Dissolved Organic Matter on Iron Speciation and Apparent Iron Solubility in the Peruvian Shelf and Slope Region
      Zhu, Kechen; Hopwood, Mark J.; Groenenberg, Jan E.; Engel, Anja; Achterberg, Eric P.; Gledhill, Martha
      Environmental Science & Technology (2021), 55 (13), 9372-9383CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      The chem. speciation of Fe (Fe) in oceans is influenced by ambient pH, dissolved O, and the concns. and strengths of the binding sites of dissolved org. matter (DOM). Here, we derived new nonideal competitive adsorption (NICA) consts. for Fe(III) binding to marine DOM via pH-Fe titrns. We used the consts. to calc. Fe(III) speciation and derive the apparent Fe(III) soly. (SFe(III)app) in the ambient H2O column across the Peruvian shelf and slope region. We define SFe(III)app as the sum of aq. inorg. Fe(III) species and Fe(III) bound to DOM at a free Fe (Fe3+) concn. equal to the limiting soly. of Fe hydroxide (Fe(OH)3(s)). A ∼2-fold increase in SFe(III)app in the O min. zone (OMZ) compared to surface waters is predicted. The increase results from a 1 order of magnitude decrease in H+ concn. which impacts both Fe(III) hydroxide soly. and org. complexation. A correlation matrix suggests that changes in pH have a larger impact on SFe(III)app and Fe(III) speciation than DOM in this region. Using Fe(II) measurements, we calcd. ambient DFe(III) and compared the value with the predicted SFe(III)app. The underlying distribution of ambient DFe(III) largely reflected the predicted SFe(III)app, indicating that decreased pH as a result of OMZ intensification and ocean acidification may increase SFe(III)app with potential impacts on surface DFe inventories.
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      Lodeiro, P.; Rey-Castro, C.; David, C.; Achterberg, E. P.; Puy, J.; Gledhill, M. Acid-Base Properties of Dissolved Organic Matter Extracted from the Marine Environment. Sci. Total Environ. 2020, 729, 138437  DOI: 10.1016/j.scitotenv.2020.138437
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      Acid-base properties of dissolved organic matter extracted from the marine environment
      Lodeiro, Pablo; Rey-Castro, Carlos; David, Calin; Achterberg, Eric P.; Puy, Jaume; Gledhill, Martha
      Science of the Total Environment (2020), 729 (), 138437CODEN: STENDL; ISSN:0048-9697. (Elsevier B.V.)
      Marine dissolved org. matter (DOM) plays a key role in the current and future global carbon cycle, which supports life on Earth. Trace metals such as iron, an essential micronutrient, compete with protons and major ions for the binding to DOM. These competitive effects and the DOM binding capacity are related to the DOM acid-base properties, which also influence DOM transport and reactivity in marine waters. Here we present the results of a complete set of acid-base titrn. expts. of a pre-concd. marine DOM sample in the range 0.01 ≤ I ≤ 0.7 M and 3 ≤ pH ≤ 10. We characterize the obtained proton binding curves using a combination of the non-ideal competitive adsorption (NICA) isotherm and Donnan electrostatic model. Within the main chem. groups of marine DOM, the carboxylic distribution was accurately characterized from the obtained data (QmaxH, 1=2.52 mol·kg-1, log ‾kH,1 = 3.26, m1 = 0.69 and b = 0.70). This carboxylic mode was found to be less acidic and more homogeneous than a generic fulvic acid, but the differences are consistent with the reported variability of fulvic acids of freshwater and terrestrial origin. We find that changes in temp. (down to 5.5°C), and the presence of calcium or magnesium (at 0.01 M) resulted in no significant modification of the proton ion binding curves obtained at 25°C and 0.7 or 0.1 M ionic strength, resp. We demonstrate the relevance of proton binding parameters for the modeling of the system iron/marine DOM throughout a wide range of salinity and acidity conditions in the context of different future ocean scenarios.
    5. 5
      Lodeiro, P.; Rey-Castro, C.; David, C.; Puy, J.; Achterberg, E. P.; Gledhill, M. Seasonal Variations in Proton Binding Characteristics of Dissolved Organic Matter Isolated from the Southwest Baltic Sea. Environ. Sci. Technol. 2021, 55 (23), 16215– 16223,  DOI: 10.1021/acs.est.1c04773
      5
      Seasonal Variations in Proton Binding Characteristics of Dissolved Organic Matter Isolated from the Southwest Baltic Sea
      Lodeiro, Pablo; Rey-Castro, Carlos; David, Calin; Puy, Jaume; Achterberg, Eric P.; Gledhill, Martha
      Environmental Science & Technology (2021), 55 (23), 16215-16223CODEN: ESTHAG; ISSN:1520-5851. (American Chemical Society)
      The physicochem. characteristics of dissolved org. matter (DOM) strongly influence its interactions with inorg. species such as protons and trace elements in natural waters. We collected water samples at Boknis Eck, a time series station in the Baltic Sea with a low exposure to freshwater inputs, to investigate how seasonal fluctuations impact the proton binding properties of the isolated DOM. We used potentiometric titrns. to assess the binding properties of solid-phase extd. DOM (SPE-DOM) over a seasonal cycle. We report and critically analyze the first NICA parameters ests. of carboxylic-like and phenolic-like sites for brackish water SPE-DOM. The total amt. of functional groups (QmaxH,tot) showed no seasonal fluctuations and an av. value of 136 ± 5.2 mmol·mol C-1. The av. proton affinity (logKH) and binding site heterogeneity (m) showed a relatively minor variability for samples obtained between Apr. and Sept., when the water remained stratified. These results contribute to a better understanding of the ion binding characteristics of DOM in natural brackish waters.
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      Middelburg, J. J.; Soetaert, K.; Hagens, M. Ocean Alkalinity, Buffering and Biogeochemical Processes. Rev. Geophys. 2020, 58 (3), e2019RG000681  DOI: 10.1029/2019RG000681
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      Seidel, M.; Vemulapalli, S. P. B.; Mathieu, D.; Dittmar, T. Marine Dissolved Organic Matter Shares Thousands of Molecular Formulae Yet Differs Structurally across Major Water Masses. Environ. Sci. Technol. 2022, 56 (6), 3758– 3769,  DOI: 10.1021/acs.est.1c04566
      7
      Marine Dissolved Organic Matter Shares Thousands of Molecular Formulae Yet Differs Structurally across Major Water Masses
      Seidel, Michael; Vemulapalli, Sahithya Phani Babu; Mathieu, Daniel; Dittmar, Thorsten
      Environmental Science & Technology (2022), 56 (6), 3758-3769CODEN: ESTHAG; ISSN:1520-5851. (American Chemical Society)
      Most oceanic dissolved org. matter (DOM) is still not fully molecularly characterized. We combined high-field NMR and ultra-high-resoln. mass spectrometry (Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS)) for the structural and mol. formula-level characterization of solid-phase extd. (SPE) DOM from surface, mesopelagic, and bathypelagic Atlantic and Pacific Ocean samples. Using a MicroCryoProbe, unprecedented low amts. of SPE-DOM (∼1 mg carbon) were sufficient for two-dimensional NMR anal. Low proportions of olefinic and arom. relative to aliph. and carboxylated structures (NMR) at the sea surface were likely related to photochem. transformations. This was consistent with lower mol. masses and higher degrees of satn. and oxygenation (FT-ICR-MS) compared to those of the deep sea. Carbohydrate structures in the mesopelagic North Pacific Ocean suggest export and release from sinking particles. In our sample set, the universal mol. DOM compn., as captured by FT-ICR-MS, appears to be structurally more diverse when analyzed by NMR, suggesting DOM variability across oceanic provinces to be more pronounced than previously assumed. As a proof of concept, our study takes advantage of new complementary approaches resolving thousands of structural and mol. DOM features while applying reasonable instrument times, allowing for the anal. of large oceanic data sets to increase our understanding of marine DOM biogeochem.
    8. 8
      Zark, M.; Christoffers, J.; Dittmar, T. Molecular Properties of Deep-Sea Dissolved Organic Matter Are Predictable by the Central Limit Theorem: Evidence from Tandem FT-ICR-MS. Mar. Chem. 2017, 191, 9– 15,  DOI: 10.1016/j.marchem.2017.02.005
      8
      Molecular properties of deep-sea dissolved organic matter are predictable by the central limit theorem: Evidence from tandem FT-ICR-MS
      Zark, Maren; Christoffers, Jens; Dittmar, Thorsten
      Marine Chemistry (2017), 191 (), 9-15CODEN: MRCHBD; ISSN:0304-4203. (Elsevier B.V.)
      The reason behind the millennial stability of marine dissolved org. matter (DOM) is subject of controversial discussion. It may be derived by either the occurrence of mainly stable chem. structures or concns. of individual DOM mols. too low for efficient microbial growth. One of the major challenges in solving this enigma is that, to date, full structural elucidation of DOM remains impossible. Ultrahigh-resoln. mass spectrometry can resolve the compn. of DOM on a mol. formula level, but the mol. diversity of the isomers behind each formula is unknown. The objective of our study was to fill this gap of knowledge. Mol. fragmentation expts. were performed via Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) on single substances and mol. formulas isolated from deep-sea DOM. We estd. carboxyl (COOH) content with help of two novel measures, one derived from mol. formula information alone and one from fragment ion intensities. For the carboxyl content of single substances, they were poor predictors and fragment ion intensities were highly sensitive to structural properties. For natural DOM, on the contrary, both parameters were highly significantly correlated, despite obvious structural differences between the mol. formulas considered in this study. By using a model approach based on the central limit theorem, we were able to show that the obsd. fragment ion intensities of DOM may be explained by intrinsic averaging. These results are clear exptl. evidence that many isomers exist per mol. formula. Model calcns. showed that the observable mol. properties of DOM apparently emerged as avs. of multiple isomers according to the central limit theorem. Structural differences between isomers that would reduce the accuracy of our measures for carboxyl content lost their effect due to averaging. Our model calcns. based on the central limit theorem indicated that there are at least 100,000 different compds. in DOM each present in seawater at picomolar concns.
    9. 9
      Gledhill, M.; Gerringa, L. J. A. The Effect of Metal Concentration on the Parameters Derived from Complexometric Titrations of Trace Elements in Seawater─A Model Study. Front. Mar. Sci. 2017, 4, 1– 15,  DOI: 10.3389/fmars.2017.00254
      There is no corresponding record for this reference.
    10. 10
      Koopal, L.; Tan, W.; Avena, M. Equilibrium Mono- and Multicomponent Adsorption Models: From Homogeneous Ideal to Heterogeneous Non-Ideal Binding. Adv. Colloid Interface Sci. 2020, 280, 102138  DOI: 10.1016/j.cis.2020.102138
      10
      Equilibrium mono- and multicomponent adsorption models: From homogeneous ideal to heterogeneous non-ideal binding
      Koopal, Luuk; Tan, Wenfeng; Avena, Marcelo
      Advances in Colloid and Interface Science (2020), 280 (), 102138CODEN: ACISB9; ISSN:0001-8686. (Elsevier B.V.)
      Aq. sorption processes play an important role in, for example, pollutant binding to natural nanoparticles, colloid stability, sepn. and enrichment of components and remediation processes. In this article, which is a tribute to Hans Lyklema, models of localized (ad)sorption of mols. and ions from aq. soln. on homogeneous and heterogeneous nanoparticles are presented. The discussed models range from ideal monocomponent sorption on homogeneous (Langmuir) and heterogeneous sites, to multicomponent ideal sorption on homogeneous and heterogeneous sites, multicomponent multisite ion complexation with charge distribution (CD-MUSIC) and non-ideal competitive adsorption on heterogeneous sites (NICA). Attention is also paid to lateral interaction, site-induced aggregation, binding stoichiometry and multilayer formation. Elec. double layer models are discussed in relation to ion binding on impermeable and permeable nanoparticles. Insight in models that can describe sorption of mols. and ions on nanoparticles leads to awareness of the limitations of using simple models for complex systems and is needed for the selection and application of an appropriate model for a given system. This is relevant for all practical sorption processes and for a better understanding of the role of natural nanoparticles in the binding of nutrients and pollutants.
    11. 11
      Catalá, T. S.; Shorte, S.; Dittmar, T. Marine Dissolved Organic Matter: A Vast and Unexplored Molecular Space. Appl. Microbiol. Biotechnol. 2021, 105 (19), 7225– 7239,  DOI: 10.1007/s00253-021-11489-3
      11
      Marine dissolved organic matter: a vast and unexplored molecular space
      Catala, Teresa S.; Shorte, Spencer; Dittmar, Thorsten
      Applied Microbiology and Biotechnology (2021), 105 (19), 7225-7239CODEN: AMBIDG; ISSN:0175-7598. (Springer)
      Abstr.: Marine dissolved org. matter (DOM) comprises a vast and unexplored mol. space. Most of it resided in the oceans for thousands of years. It is among the most diverse mol. mixts. known, consisting of millions of individual compds. More than 1 Eg of this material exists on the planet. As such, it comprises a formidable source of natural products promising significant potential for new biotechnol. purposes. Great emphasis has been placed on understanding the role of DOM in biogeochem. cycles and climate attenuation, its lifespan, interaction with microorganisms, as well as its mol. compn. Yet, probing DOM bioactivities is in its infancy, largely because it is tech. challenging due to the chem. complexity of the material. It is of considerable interest to develop technologies capable to better discern DOM bioactivities. Modern screening technologies are opening new avenues allowing accelerated identification of bioactivities for small mols. from natural products. These methods diminish a priori the need for laborious chem. fractionation. We examine here the application of untargeted metabolomics and multiplexed high-throughput mol.-phenotypic screening techniques that are providing first insights on previously undetectable DOM bioactivities. Key points: • Marine DOM is a vast, unexplored biotechnol. resource. • Untargeted bioscreening approaches are emerging for natural product screening. • Perspectives for developing bioscreening platforms for marine DOM are discussed.
    12. 12
      Kerr, D. E.; Brown, P. J.; Grey, A.; Kelleher, B. P. The Influence of Organic Alkalinity on the Carbonate System in Coastal Waters. Mar. Chem. 2021, 237, 104050  DOI: 10.1016/j.marchem.2021.104050
      12
      The influence of organic alkalinity on the carbonate system in coastal waters
      Kerr, Daniel E.; Brown, Peter J.; Grey, Anthony; Kelleher, Brian P.
      Marine Chemistry (2021), 237 (), 104050CODEN: MRCHBD; ISSN:0304-4203. (Elsevier B.V.)
      A review. Total alky. (TA) is one of the four main carbonate system variables and is a conventionally measured parameter used to characterize marine water carbonate chem. It is an important indicator of a waterbody's buffering capacity and a measure of its ability to resist acidification, a matter of growing concern in the marine environment. Although TA is primarily assocd. with the inorg. components of seawater such as bicarbonate, there is a growing consensus that dissolved org. matter (DOM) can significantly contribute to TA in coastal waters. This org. fraction of TA (OrgAlk) is typically deemed negligible and is not accounted for in conventional TA expressions. However, omission of OrgAlk can lead to the propagation of errors in subsequent carbonate system calcns. and to misinterpretation of key carbonate chem. descriptors such as calcium carbonate satn. states. Here we provide an overview of OrgAlk contributions to TA and investigate the implications of its omission in carbonate system studies conducted in coastal waters. We examine the prevalence of OrgAlk across both coastal and pelagic waters using publicly available carbonate system data products, such as GLODAP and GOMECC. Current measures to account for, incorporate and characterize the contribution of OrgAlk to TA are also critically examd.
    13. 13
      Sharp, J. D.; Byrne, R. H. Interpreting Measurements of Total Alkalinity in Marine and Estuarine Waters in the Presence of Proton-Binding Organic Matter. Deep-Sea Res. Part Oceanogr. Res. Pap. 2020, 165, 103338,  DOI: 10.1016/j.dsr.2020.103338
      13
      Interpreting measurements of total alkalinity in marine and estuarine waters in the presence of proton-binding organic matter
      Sharp, Jonathan D.; Byrne, Robert H.
      Deep Sea Research, Part I: Oceanographic Research Papers (2020), 165 (), 103338CODEN: DRORE7; ISSN:0967-0637. (Elsevier Ltd.)
      In this work, a numerical model is used to illustrate (a) how proton-binding dissolved org. mols. influence the reported results of total alky. titrns. in marine and estuarine settings and (b) how errors in interpretations of reported AT values can then propagate through carbonate system calcns., thus distorting biogeochem. interpretations of calcd. parameters. We examine five distinct approaches for alky. measurement by titrn. Ideally, the difference between the measured (reported) AT and the conventional (thermodn.) definition of inorg. alky. (Ainorg) would be zero. However, in the presence of titratable org. matter, our model results show consistent non-zero differences that vary with the chem. properties of the org. matter. The differences between reported AT and Ainorg also display previously undescribed variation among measurement approaches, most significantly when pKorg is between approx. 3 and 6 (typical of carboxylic acid groups). For calcd. carbonate system parameters relevant to in situ conditions (e.g., pH, pCO2, calcium carbonate mineral satn. states), errors resulting from the presence of proton-binding orgs. are largest when calcns. are based on the input pair of directly measured dissolved inorg. carbon (CT) and directly measured AT, and can vary in magnitude depending on the titrn. approach that is used to obtain AT.
    14. 14
      Ardiningsih, I.; Krisch, S.; Lodeiro, P.; Reichart, G.-J.; Achterberg, E. P.; Gledhill, M.; Middag, R.; Gerringa, L. J. A. A. Natural Fe-Binding Organic Ligands in Fram Strait and over the Northeast Greenland Shelf. Mar. Chem. 2020, 224, 103815  DOI: 10.1016/j.marchem.2020.103815
      14
      Natural Fe-binding organic ligands in Fram Strait and over the northeast Greenland shelf
      Ardiningsih, Indah; Krisch, Stephan; Lodeiro, Pablo; Reichart, Gert-Jan; Achterberg, Eric P.; Gledhill, Martha; Middag, Rob; Gerringa, Loes J. A.
      Marine Chemistry (2020), 224 (), 103815CODEN: MRCHBD; ISSN:0304-4203. (Elsevier B.V.)
      There is a paucity of data on Fe-binding ligands in the Arctic Ocean. Here we investigate the distribution and chem. properties of natural Fe-binding ligands in Fram Strait and over the northeast Greenland shelf, shedding light on their potential sources and transport. Our results indicate that the main sources of org. ligands to surface waters of Fram Strait included primary productivity and supply from the Arctic Ocean. We calcd. the mean total Fe-binding ligand concn., [Lt], in Polar Surface Water from the western Fram Strait to be 1.65 ± 0.4 nM eq. Fe. This value is in between reported values for the Arctic and North Atlantic Oceans, confirming reports of north to south decreases in [Lt] from the Arctic Ocean. The differences between ligand sources in different biogeochem. provinces, resulted in distinctive ligand properties and distributions that are reflected in [Lt], binding strength (log K'Fe'L) and competing strength (log αFe'L) of ligands. However, org. ligands near the 79 N Glacier terminus and in the Westwind Trough were weaker, and therefore less reactive than org. ligands in the Norske Trough and in Fram Strait. Ongoing changes in the Arctic and sub-Arctic Oceans will influence both terrestrially derived and in-situ produced Fe-binding ligands, and therefore will have consequences for Fe soly. and availability to microbial populations and Fe cycling in Fram Strait.
    15. 15
      Ye, Y.; Völker, C.; Gledhill, M. Exploring the Iron-Binding Potential of the Ocean Using a Combined pH and DOC Parameterization. Glob. Biogeochem. Cycles 2020, 34 (6), 1– 16,  DOI: 10.1029/2019GB006425
      There is no corresponding record for this reference.
    16. 16
      Milne, C. J.; Kinniburgh, D. G.; Tipping, E. Generic NICA-Donnan Model Parameters for Proton Binding by Humic Substances. Environ. Sci. Technol. 2001, 35 (10), 2049– 2059,  DOI: 10.1021/es000123j
      16
      Generic NICA-Donnan Model Parameters for Proton Binding by Humic Substances
      Milne, Christopher J.; Kinniburgh, David G.; Tipping, Edward
      Environmental Science and Technology (2001), 35 (10), 2049-2059CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      Forty-nine datasets consisting of literature and exptl. data for proton binding by fulvic and humic acids have been analyzed using the NICA-Donnan model. The model successfully described the behavior of the individual datasets with a high degree of accuracy and highlighted the differences in site d. and binding affinity between fulvic acids (FA) and humic acids (HA) while demonstrating their strong similarities. The data have also been used to derive generic model descriptions of proton binding by FA and HA that can be used for modeling in the absence of specific parameter sets for the particular humic substance of interest. These generic parameters can provide ests. of the amt. of proton binding by a wide variety of humic substances to within approx. ±20% under any given conditions. The max. site d. for protons was 7.74 and 5.70 equiv kg-1 for a generic FA and HA, resp. The recommended generic NICA-Donnan parameter values for FA are b = 0.57, Qmax1,H = 5.88, log ~KH1 = 2.34, mH1 = 0.38, Qmax2,H = 1.86, log ~KH2 = 8.6, and mH2 = 0.53; for HA the values are b = 0.49, Qmax1,H = 3.15, log ~KH1 = 2.93, mH1 = 0.50, Qmax2,H = 2.55, log ~KH2 = 8.0, and mH2 = 0.26.
    17. 17
      Hollister, A. P.; Whitby, H.; Seidel, M.; Lodeiro, P.; Gledhill, M.; Koschinsky, A. Dissolved Concentrations and Organic Speciation of Copper in the Amazon River Estuary and Mixing Plume. Mar. Chem. 2021, 234 (June), 104005  DOI: 10.1016/j.marchem.2021.104005
      17
      Dissolved concentrations and organic speciation of copper in Amazon river estuary and mixing plume
      Hollister, Adrienne Patricia; Whitby, Hannah; Seidel, Michael; Lodeiro, Pablo; Gledhill, Martha; Koschinsky, Andrea
      Marine Chemistry (2021), 234 (), 104005CODEN: MRCHBD; ISSN:0304-4203. (Elsevier B.V.)
      The Amazon is Earth's largest river by vol. output, making it an important source of trace metals and dissolved org. matter (DOM) to the Atlantic Ocean. Despite major recent anthropogenic disruptions to the Amazon catchment area, data for trace metals such as copper (Cu) in the Amazon River estuary and assocd. mixing plume are still rare. Furthermore, there is currently no existing data in this region for Cu-binding ligands, which govern the amt. of bioavailable Cu. To understand trace metal mixing and transport processes, the GEOTRACES process study GApr11 (cruise M147 with RV Meteor) was conducted in 2018 in the Amazon and Para River estuaries and mixing plume in the tropical North Atlantic Ocean during high river discharge. Size-fractionated surface samples were collected along the full salinity gradient for concns. of Cu, apparent Cu-binding org. ligands (LCu) and corresponding conditional stability consts. (K'CuL,Cu2+cond), electroactive humic substances (eHS), solid phase extractable org. Cu (SPE-Cu), dissolved org. carbon (DOC), chlorophyll α (Chl α) and macronutrients. Dissolved (<0.2μm) and sol. (<0.015μm) Cu correlated neg. with salinity and largely followed values expected from conservative mixing. Cu was primarily in the sol. fraction, with the exception of a minor fraction of large colloidal Cu at low salinity (S ≤ 10). Org. ligands (log K'CuL,Cu2+cond = 12.6-15.6) were present in excess of Cu and likely played a role in solubilizing Cu and preventing Cu being affected by colloidal flocculation. Cu-assocd. DOM (measured as LCu, eHS and SPE-Cu) correlated neg. with salinity and appeared to be primarily governed by river input and mixing with seawater. However, an increase in the colloidal fraction for LCu and eHS obsd. at S ∼ 6-10 was attributed to possible addnl. autochthonous (phytoplankton) ligand prodn. In all dissolved samples, org. complexation kept free Cu below levels potentially toxic for phytoplankton (<1 pmol L-1). Despite increasing anthropogenic activity over the past century, we find Cu concn. remained similar to the 1970s, suggesting that the large overall river flow may so far minimize the impact of Cu pollution.
    18. 18
      Millero, F. J. The Physical Chemistry of Natural Waters; Wiley - Interscience Series in Geochemistry: 2000.
      There is no corresponding record for this reference.
    19. 19
      Koopal, L. K.; van Riemsdijk, W. H.; de Wit, J. C. M.; Benedetti, M. F. Analytical Isotherm Equations for Multicomponent Adsorption to Heterogeneous Surfaces. J. Colloid Interface Sci. 1994, 166 (1), 51– 60,  DOI: 10.1006/jcis.1994.1270
      19
      Analytical isotherm equations for multicomponent adsorption to heterogeneous surfaces
      Koopal, L. K.; van Riemsdijk, W. H.; De Wit, J. C. M.; Benedetti, M. F.
      Journal of Colloid and Interface Science (1994), 166 (1), 51-60CODEN: JCISA5; ISSN:0021-9797.
      Adsorption on heterogeneous surfaces may be influenced not only by the heterogeneity but also by lateral interactions. For multicomponent solns., competition increases the complexity even further. In practice, such complex systems frequently occur and to study the behavior of these systems, it is rather useful to have anal. equations available which can describe the adsorption. Existing anal. mono- and multicomponent binding equations for heterogeneous surfaces are reviewed briefly. These equations are based on a high degree of ideality with respect to both the local isotherm and the affinity distribution for the different components. A new more generally valid anal. competitive binding equation is derived by using a nonideal local isotherm equation. In this local isotherm, all component-specific nonideality is incorporated; i.e., it includes both lateral interactions and component-specific heterogeneity contributions. Under these conditions, the remaining adsorbate-surface interactions can be characterized by an "intrinsic" affinity distribution that applies for all components. The features of the new equation are discussed and illustrated on the basis of some model calcns. The model is tested for Cd2+ ion binding to fulvic acid at various pH (using exptl. data reported by R. Saar and J. Weber, 1979). These data cannot be modeled satisfactorily with the classical equation for multicomponent adsorption on heterogeneous surfaces. The authors' equation gives excellent results.
    20. 20
      Kinniburgh, D. G.; Milne, C. J.; Benedetti, M. F.; Pinheiro, J. P.; Filius, J.; Koopal, L. K.; Van Riemsdijk, W. H. Metal Ion Binding by Humic Acid: Application of the NICA-Donnan Model. Environ. Sci. Technol. 1996, 30 (5), 1687– 1698,  DOI: 10.1021/es950695h
      20
      Metal Ion Binding by Humic Acid: Application of the NICA-Donnan Model
      Kinniburgh, David G.; Milne, Christopher J.; Benedetti, Marc F.; Pinheiro, Jose P.; Filius, Jeroen; Koopal, Luuk K.; Van Riemsdijk, Willem H.
      Environmental Science and Technology (1996), 30 (5), 1687-98CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      A Donnan-type model for nonspecific binding of electrolyte ions has been combined with the nonideal competitive adsorption (NICA) model for specific binding to produce a model for ion binding to humic substances. The model considers site heterogeneity, nonideality, multicomponent competition, and electrostatic interactions. The NICA-Donnan model was fitted to data for H+, Ca, Cd, Cu, and Pb binding by a purified peat humic acid. The model fits were good and covered a wide range of pH and free metal concns. The parameters from these single metal data sets were then used to predict the competitive effect of Ca on Cd and Cu binding at various pHs. These predictions agreed well with the exptl. data although there were some small but systematic differences. The new NICA-Donnan model also predicted reasonably well the increase in Cd and Cu binding on changing from a 0.1 M KNO3 background electrolyte to 0.01 M KNO3. A shortcoming of the model is that in some cases it significantly underestimated the H+/M2+ exchange ratio, esp. at high pH and for Cu binding.
    21. 21
      Kinniburgh, D. G.; van Riemsdijk, W. H.; Koopal, L. K.; Borkovec, M.; Benedetti, M. F.; Avena, M. J. Ion Binding to Natural Organic Matter: Competition, Heterogeneity, Stoichiometry and Thermodynamic Consistency. Colloids Surf. Physicochem. Eng. Asp. 1999, 151 (1–2), 147– 166,  DOI: 10.1016/S0927-7757(98)00637-2
      There is no corresponding record for this reference.
    22. 22
      Rey-Castro, C.; Mongin, S.; Huidobro, C.; David, C.; Salvador, J.; Garcés, J. L.; Galceran, J.; Mas, F.; Puy, J. Effective Affinity Distribution for the Binding of Metal Ions to a Generic Fulvic Acid in Natural Waters. Environ. Sci. Technol. 2009, 43 (19), 7184– 7191,  DOI: 10.1021/es803006p
      22
      Effective Affinity Distribution for the Binding of Metal Ions to a Generic Fulvic Acid in Natural Waters
      Rey-Castro, Carlos; Mongin, Sandrine; Huidobro, Cesar; David, Calin; Salvador, Jose; Garces, Josep Lluis; Galceran, Josep; Mas, Francesc; Puy, Jaume
      Environmental Science & Technology (2009), 43 (19), 7184-7191CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      The effective distribution of affinities (Conditional Affinity Spectrum, CAS) seen by a metal ion binding to a humic substance under natural water conditions is derived and discussed within the NICA-Donnan model. Anal. expressions for the av. affinity of these distributions in general multi-ion mixts. are reported here for the first time. These expressions enable a simple evaluation of the effect of all interfering cations on the affinity distribution of a given one. We illustrate this methodol. by plotting the affinity spectra of a generic fulvic acid for 14 different cations in the presence of major inorg. ions and trace metals at pH and concn. values representative of a river water. The distribution of occupied sites and their av. affinity at the typical freshwater conditions are also reported for each ion. The CAS allows us to distinguish three groups of cations: (a) Al, H, Pb, Hg, and Cr, which are preferentially bound to the phenolic sites of the fulvic ligand; (b) Ca, Mg, Cd, Fe(II), and Mn, which display a greater effective affinity for carboxylic sites, in contrast to what would be expected from their individual complexation parameters; and (c) Fe(III), Cu, Zn, and Ni, for which phenolic and carboxylic distributions are overlapped.
    23. 23
      Benedetti, M. F.; vanRiemsdik, W. H.; Koopal, L. K. Humic Substances Considered as a Heterogeneous Donnan Gel Phase. Environ. Sci. Technol. 1996, 30 (6), 1805– 1813,  DOI: 10.1021/es950012y
      23
      Humic Substances Considered as a Heterogeneous Donnan Gel Phase
      Benedetti, M. F.; Van Riemsdijk, W. H.; Koopal, L. K.
      Environmental Science and Technology (1996), 30 (6), 1805-13CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      A Donnan model in combination with a site-binding model is applied to interpret proton binding to humic substances as a function of pH at different salt levels. The Donnan model accounts for the salt effect on the charging behavior; it can only be successfully applied if it is assumed that the sp. vol. of humic substances is strongly dependent on the ionic strength. Empirical relationships have been derived that relate the vol. of the Donnan phase with the ionic strength. For fulvic acid, the Donnan approach is phys. not very realistic. The approach presumably mimics the behavior of the diffuse double layer around the relatively small fulvic acid mols. For humic acids, however, the approach is phys. realistic. The Donnan approach is a relatively simple and numerically efficient technique to account for the effects of ionic strength on the charging behavior. The ionic compn. of the gel phase follows directly from the technique. The insight obtained on the swelling behavior of the humic acids may be of use for interpreting binding of org. pollutants in natural systems.
    24. 24
      Companys, E.; Garces, J. L.; Salvador, J.; Galceran, J.; Puy, J.; Mas, F. Electrostatic and Specific Binding to Macromolecular Ligands - A General Analytical Expression for the Donnan Volume. Colloids Surf. -Physicochem. Eng. Asp. 2007, 306 (1–3), 2– 13,  DOI: 10.1016/j.colsurfa.2007.01.016
      There is no corresponding record for this reference.
    25. 25
      Pinheiro, J. P.; Rotureau, E.; Duval, J. F. L. Addressing the Electrostatic Component of Protons Binding to Aquatic Nanoparticles beyond the Non-Ideal Competitive Adsorption (NICA)-Donnan Level: Theory and Application to Analysis of Proton Titration Data for Humic Matter. J. Colloid Interface Sci. 2021, 583, 642– 651,  DOI: 10.1016/j.jcis.2020.09.059
      25
      Addressing the electrostatic component of protons binding to aquatic nanoparticles beyond the Non-Ideal Competitive Adsorption (NICA)-Donnan level: Theory and application to analysis of proton titration data for humic matter
      Pinheiro, Jose Paulo; Rotureau, Elise; Duval, Jerome F. L.
      Journal of Colloid and Interface Science (2021), 583 (), 642-651CODEN: JCISA5; ISSN:0021-9797. (Elsevier B.V.)
      Charge descriptors of aquatic nanoparticles (NPs) are evaluated from proton titrn. curves measured at different salt concns. and routinely analyzed by the Non-Ideal Competitive Adsorption-Donnan (NICAD) model. This model, however, suffers from approxns. regarding particle electrostatics, which may bias particle charge estn. Implementation of Poisson-Boltzmann (PB) theory within consistent treatment of NPs protolytic data is expected to address NICAD shortcomings. An alternative to NICAD is elaborated on the basis of nonlinearized PB equation for soft particle electrostatics to properly unravel the electrostatic and chem. components of proton binding to NPs. A numerical package is developed for automated anal. of proton titrn. curves and proton affinity spectra at different salt concns. The performance of the method is illustrated for humic matter nanoparticles with different charge and size, and compared to that of NICAD. Unlike NICAD, PB-based treatment successfully reproduces particle charge dependence on pH for practical salt concns. from the thin to thick elec. double layer limit. Donnan representation in NICAD leads to moderate to dramatic misestimations of proton affinity and binding heterogeneity depending on particle size to Debye layer thickness ratio. Interpretation of NPs protolytic properties with PB theory further avoids adjustment of the 'particle Donnan vol.' empirically introduced in NICAD.
    26. 26
      Wagner, S.; Brandes, J.; Spencer, R. G. M.; Ma, K.; Rosengard, S. Z.; Moura, J. M. S.; Stubbins, A. Isotopic Composition of Oceanic Dissolved Black Carbon Reveals Non-Riverine Source. Nat. Commun. 2019, 10 (1), 5064,  DOI: 10.1038/s41467-019-13111-7
      26
      Isotopic composition of oceanic dissolved black carbon reveals non-riverine source
      Wagner Sasha; Brandes Jay; Ma Kun; Spencer Robert G M; Rosengard Sarah Z; Moura Jose Mauro S; Stubbins Aron
      Nature communications (2019), 10 (1), 5064 ISSN:.
      A portion of the charcoal and soot produced during combustion processes on land (e.g., wildfire, burning of fossil fuels) enters aquatic systems as dissolved black carbon (DBC). In terms of mass flux, rivers are the main identified source of DBC to the oceans. Since DBC is believed to be representative of the refractory carbon pool, constraining sources of marine DBC is key to understanding the long-term persistence of carbon in our global oceans. Here, we use compound-specific stable carbon isotopes (δ(13)C) to reveal that DBC in the oceans is ~6‰ enriched in (13)C compared to DBC exported by major rivers. This isotopic discrepancy indicates most riverine DBC is sequestered and/or rapidly degraded before it reaches the open ocean. Thus, we suggest that oceanic DBC does not predominantly originate from rivers and instead may be derived from another source with an isotopic signature similar to that of marine phytoplankton.
    27. 27
      Li, Y.; Harir, M.; Lucio, M.; Kanawati, B.; Smirnov, K.; Flerus, R.; Koch, B. P.; Schmitt-Kopplin, P.; Hertkorn, N. Proposed Guidelines for Solid Phase Extraction of Suwannee River Dissolved Organic Matter. Anal. Chem. 2016, 88 (13), 6680– 6688,  DOI: 10.1021/acs.analchem.5b04501
      27
      Proposed Guidelines for Solid Phase Extraction of Suwannee River Dissolved Organic Matter
      Li, Yan; Harir, Mourad; Lucio, Marianna; Kanawati, Basem; Smirnov, Kirill; Flerus, Ruth; Koch, Boris P.; Schmitt-Kopplin, Philippe; Hertkorn, Norbert
      Analytical Chemistry (Washington, DC, United States) (2016), 88 (13), 6680-6688CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
      This paper proposes improved guidelines for dissolved org. matter (DOM) isolation by solid phase extn. (SPE) with a styrene-divinylbenzene copolymer (PPL) sorbent, which has become an established method for the isolation of DOM from natural waters, because of its ease of application and appreciable carbon recovery. Suwannee River water was selected to systematically study the effects of crit. SPE variables such as loading mass, concn., flow rate, and up-scaling on the extn. selectivity of the PPL sorbent. High-field Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS) and proton NMR (1H NMR) spectroscopy were performed to interpret the DOM chem. space of eluates, as well as permeates and wash liqs. with mol. resoln. Up to 89% dissolved org. carbon (DOC) recovery was obtained with a DOC/PPL mass ratio of 1:800 at a DOC concn. of 20 mg/L. With the application of larger loading vols., low proportions of highly oxygenated compds. were retained on the PPL sorbent. The effects of the flow rate on the extn. selectivity of the sorbent were marginal. Up-scaling had a limited effect on the extn. selectivity with the exception of increased self-esterification with a methanol solvent, resulting in Me ester groups. Furthermore, the SPE/PPL ext. exhibited highly authentic characteristics in comparison with original water and reverse osmosis samples. These findings will be useful for reproducibly isolating DOM with representative mol. compns. from various sources and concns. and minimizing potential inconsistencies among interlab. comparative studies.
    28. 28
      Hertkorn, N.; Harir, M.; Koch, B. P.; Michalke, B.; Schmitt-Kopplin, P. High-Field NMR Spectroscopy and FTICR Mass Spectrometry: Powerful Discovery Tools for the Molecular Level Characterization of Marine Dissolved Organic Matter. Biogeosciences 2013, 10 (3), 1583– 1624,  DOI: 10.5194/bg-10-1583-2013
      29
      High-field NMR spectroscopy and FTICR mass spectrometry: powerful discovery tools for the molecular level characterization of marine dissolved organic matter
      Hertkorn, N.; Harir, M.; Koch, B. P.; Michalke, B.; Schmitt-Kopplin, P.
      Biogeosciences (2013), 10 (3), 1583-1624CODEN: BIOGGR; ISSN:1726-4189. (Copernicus Publications)
      High-performance, non-target, high-resoln. org. structural spectroscopy was applied to solid phase extd. marine dissolved org. matter (SPE-DOM) isolated from four different depths in the open South Atlantic Ocean off the Angola coast (3° E, 18° S; Angola Basin) and provided mol. level information with extraordinary coverage and resoln. Sampling was performed at depths of 5m (Angola Current; near-surface photic zone), 48m (Angola Current; fluorescence max.), 200m (still above Antarctic Intermediate Water, AAIW; upper mesopelagic zone) and 5446m (North Atlantic Deep Water, NADW; abyssopelagic, ∼30m above seafloor) and produced SPE-DOM with near 40% carbon yield and beneficial NMR (NMR) relaxation properties, a crucial prerequisite for the acquisition of NMR spectra with excellent resoln. 1H and 13C NMR spectra of all four marine SPE-DOM showed smooth bulk envelopes, reflecting intrinsic averaging from massive signal overlap, with a few percent of visibly resolved signatures and variable abundances for all major chem. environments. The abundance of singly oxygenated aliphatics and acetate derivs. in 1H NMR spectra declined from surface to deep marine SPE-DOM, whereas C-based aliphatics and carboxyl-rich alicyclic mols. (CRAM) increased in abundance. Surface SPE-DOMcontained fewer Me esters than all other samples, likely a consequence of direct exposure to sunlight. Integration of 13C NMR spectra revealed continual increase of carboxylic acids and ketones from surface to depth, reflecting a progressive oxygenation, with concomitant decline of carbohydrate-related substructures.
    29. 29
      Martínez-Pérez, A. M.; Osterholz, H.; Nieto-Cid, M.; Álvarez, M.; Dittmar, T.; Álvarez-Salgado, X. A. Molecular Composition of Dissolved Organic Matter in the Mediterranean Sea. Limnol. Oceanogr. 2017, 62 (6), 2699– 2712,  DOI: 10.1002/lno.10600
      30
      Molecular composition of dissolved organic matter in the Mediterranean Sea
      Martinez-Perez, Alba Maria; Osterholz, Helena; Nieto-Cid, Mar; Alvarez, Marta; Dittmar, Thorsten; Alvarez-Salgado, Xose Anton
      Limnology and Oceanography (2017), 62 (6), 2699-2712CODEN: LIOCAH; ISSN:0024-3590. (John Wiley & Sons, Inc.)
      The mol. compn. of marine dissolved org. matter (DOM) is still poorly understood, particularly in the Mediterranean Sea. In this work, DOM from the open Mediterranean Sea and the adjacent Northeast Atlantic Ocean was isolated by solid-phase extn. (SPE-DOM) and molecularly characterized using Fourier-transform ion cyclotron resonance mass spectrometry. We assessed the gradual reworking of the SPE-DOM transported by the shallow overturning circulation of the Mediterranean Sea by following the increase in mol. wt. (+20 Da), oxygenation (+5%), degrdn. index (Ideg +22%), and the proportional decrease of unsatd. aliph. compds. (+34%) along the Levantine Intermediate Water. This reworked SPE-DOM that leaves the Mediterranean Sea through the Strait of Gibraltar strongly contrasts with the fresh material transported by the inflow of Atlantic water (Ideg -25%). In the deep eastern and western overturning cells, the mol. compn. of the deep waters varied according to their area and/or time of formation. SPE-DOM of the waters formed in the Aegean Sea during the Eastern Mediterranean Transient (EMT) was more processed than the DOM in pre-EMT waters formed in the Adriatic Sea (mol. wt. and the proportion of unsatd. aliph. compds. were increased by 5 Da and 9%, resp.). Furthermore, pre-EMT waters contain more reworked SPE-DOM (Ideg +7%) than post-EMT waters formed also in the Adriatic Sea. In summary, our study shows that the Mediterranean Sea constitutes a lab. basin where degrdn. processes and diagenetic transformations of DOM can be obsd. on close spatial and temporal scales.
    30. 30
      Broek, T. A. B.; Walker, B. D.; Guilderson, T. P.; McCarthy, M. D. Coupled Ultrafiltration and Solid Phase Extraction Approach for the Targeted Study of Semi-Labile High Molecular Weight and Refractory Low Molecular Weight Dissolved Organic Matter. Mar. Chem. 2017, 194, 146– 157,  DOI: 10.1016/j.marchem.2017.06.007
      31
      Coupled ultrafiltration and solid phase extraction approach for the targeted study of semi-labile high molecular weight and refractory low molecular weight dissolved organic matter
      Broek, Taylor A. B.; Walker, Brett D.; Guilderson, Thomas P.; McCarthy, Matthew D.
      Marine Chemistry (2017), 194 (), 146-157CODEN: MRCHBD; ISSN:0304-4203. (Elsevier B.V.)
      Only a small fraction of dissolved org. matter (DOM) can be characterized at the mol. level by direct seawater anal. Thus, the study of DOM requires isolation of extremely dil. orgs. from orders of magnitude greater concns. of inorg. salts. Traditional isolation approaches have sought to isolate representative DOM fractions, however, currently available isolation methods all have selective chem. or phys. biases. Recent work has indicated that DOM exists in a functional continuum of mol. size and 14C age. High mol. wt. (HMW) DOM is primarily composed of younger, semi-labile material, while much older, low mol. wt. (LMW) DOM dominates the refractory background pool. Here we describe a new large vol. DOM isolation approach that selectively isolates HMW and LMW DOM fractions with distinct 14C ages, a proxy for reactivity. The method uses ultrafiltration (UF) to isolate HMW DOM (UDOM), and then solid phase extn. (SPE) to isolate LMW DOM permeating the UF system. We first assess two SPE sorbents (Agilent Bond Elute PPL and Diaion HP-20) for DOM chem. and isotopic selectivity. Second, we evaluate our UF/SPE approach in the context of DOM recovery, elemental (C/N) and isotopic (δ13C, δ15N, Δ14C) compn. of 8 HMW and LMW sample pairs, isolated from the North Central Pacific Ocean. Radiocarbon (Δ14C) anal. shows major differences in the Δ14C value of HMW (Δ14C = - 37 to - 380‰) and LMW (Δ14C = - 343 to - 578‰) DOM fractions. We also observe elemental and stable isotopic offsets between HMW and LMW DOM at all depths. HMW UDOM (C/N = 11.5 to 13.1, δ13C = - 22.5 to - 21.1‰, δ15N = 6.2 to 7.1‰) has significantly lower C/N ratios and higher δ13C and δ15N values than LMW SPE-DOM (C/N = 24.2 to 28.5, δ13C = - 23.3 to - 22.2‰, δ15N = 3.1 to 4.0‰), with the exception of surface δ13C, which is equiv. in both size fractions. Together, these results indicate that our combined UF/SPE method successfully isolates sep. young (semi-labile, HMW) and old (refractory, LMW) DOM fractions, each with distinct chem. and isotopic compn. Ultimately, by limiting the influence of DOM reactivity mixts., our method provides an alternative approach for understanding DOM sources and cycling.
    31. 31
      Bercovici, S. K.; Koch, B. P.; Lechtenfeld, O. J.; McCallister, S. L.; Schmitt-Kopplin, P.; Hansell, D. A. Aging and Molecular Changes of Dissolved Organic Matter Between Two Deep Oceanic End-Members. Glob. Biogeochem. Cycles 2018, 32 (10), 1449– 1456,  DOI: 10.1029/2017GB005854
      32
      Aging and Molecular Changes of Dissolved Organic Matter Between Two Deep Oceanic End-Members
      Bercovici, S. K.; Koch, B. P.; Lechtenfeld, O. J.; McCallister, S. L.; Schmitt-Kopplin, P.; Hansell, D. A.
      Global Biogeochemical Cycles (2018), 32 (10), 1449-1456CODEN: GBCYEP; ISSN:1944-9224. (Wiley-Blackwell)
      The global ocean contains a massive reservoir of dissolved org. carbon (DOC), rivaling the atm.'s pool of CO2. The most recalcitrant fractions have mean radiocarbon ages of ∼4,000 years in the Atlantic to ∼6,000 years in the Pacific. Knowing the radiocarbon signatures of DOC and the mol. compn. of dissolved org. matter (DOM) is crucial to develop understanding of the persistence and lifetime of the DOC pool. In this research, we collected samples from the deep North Pacific in August 2013 (aboard the RV Melville) to couple the Δ14C content of solid-phase-extd. DOM (Δ14C-SPE-DOM) with its mol. compn. in the ocean's oldest deep waters. We find that deep waters in this region held a mean Δ14C-SPE-DOM value of -554 ± 9‰ (∼6,400 14C years), substantially more depleted than that in the deep Atlantic, which held a mean Δ14C-SPE-DOM value of -445 ± 5‰. While we find a more degraded mol. compn. of DOM in the deep Pacific than the deep Atlantic, the mol. formulas within the Island of Stability (Lechtenfeld et al., 2014, ), are largely retained. These results imply that a fraction of deep DOM is resistant to removal and present in both the deep Atlantic and Pacific Oceans.
    32. 32
      Jerusalén-Lleó, E.; Nieto-Cid, M.; Fuentes-Santos, I.; Dittmar, T.; Álvarez-Salgado, X. A. Solid Phase Extraction of Ocean Dissolved Organic Matter with PPL Cartridges: Efficiency and Selectivity. Front. Mar. Sci. 2023, 10, 1159762  DOI: 10.3389/fmars.2023.1159762
      There is no corresponding record for this reference.
    33. 33
      Chaichana, S.; Jickells, T.; Johnson, M. Interannual Variability in the Summer Dissolved Organic Matter Inventory of the North Sea: Implications for the Continental Shelf Pump. Biogeosciences 2019, 16 (5), 1073– 1096,  DOI: 10.5194/bg-16-1073-2019
      34
      Interannual variability in the summer dissolved organic matter inventory of the North Sea: implications for the continental shelf pump
      Chaichana, Saisiri; Jickells, Tim; Johnson, Martin
      Biogeosciences (2019), 16 (5), 1073-1096CODEN: BIOGGR; ISSN:1726-4189. (Copernicus Publications)
      We present the distribution and C:N stoichiometry of dissolved org. matter (DOM) in the North Sea in two summers (August 2011 and August 2012), with supporting data from the intervening winter (Jan. 2012). These data demonstrate local variability superimposed on a general pattern of decreasing DOM with increasing distance from land, suggesting concns. of DOM are controlled on large spatial scales by mixing between the open North Atlantic and either riverine sources or high DOM productivity in nearshore coastal waters driven by riverine nutrient discharge. Given the large size and long residence time of water in the North Sea, we find concns. are commonly modified from simple conservative mixing between two endmembers. We observe differences in dissolved org. carbon (DOC) and dissolved org. nitrogen (DON) concns. and land-ocean gradients between the two summers, leading to an estd. 10-20 Tg difference in the DOC inventory between the two years, which is of the same order of magnitude as the annual uptake of atm. CO2 by the North Sea system, and thus significant for the carbon budget of the North Sea. This difference is not consistent with addnl. terrestrial loading and is more likely to be due to balancing of mixing and in situ prodn. and loss processes across the North Sea. Differences were particularly pronounced in the bottom layer of the seasonally stratifying northern North Sea, with higher DOC and C:N ratio in 2011 than in 2012. Using other data, we consider the extent to which these differences in the concns. and C:N ratio of DOM could be due to changes in the biogeochem. or phys. circulation in the two years, or a combination of both. The evidence we have is consistent with a flushing event in winter 2011/12 exchanging DOM-rich, high C:N shelf waters, which may have accumulated over more than 1 yr, with deep North Atlantic waters with lower DOC and marginally higher DON. We discuss the implications of these observations for the shelf sea carbon pump and the export of carbon-rich org. matter off the shelf and hypothesise that intermittent flushing of temperate shelf systems may be a key mechanism in the maintenance of the continental shelf pump, via the accumulation and subsequent export of carbon-rich DOM.
    34. 34
      Letscher, R. T.; Moore, J. K. Preferential Remineralization of Dissolved Organic Phosphorus and Non-Redfield DOM Dynamics in the Global Ocean: Impacts on Marine Productivity, Nitrogen Fixation, and Carbon Export. Glob. Biogeochem. Cycles 2015, 29 (3), 325– 340,  DOI: 10.1002/2014GB004904
      35
      Preferential remineralization of dissolved organic phosphorus and non-Redfield DOM dynamics in the global ocean: Impacts on marine productivity, nitrogen fixation, and carbon export
      Letscher, Robert T.; Moore, J. Keith
      Global Biogeochemical Cycles (2015), 29 (3), 325-340CODEN: GBCYEP; ISSN:1944-9224. (Wiley-Blackwell)
      Selective removal of nitrogen (N) and phosphorus (P) from the marine dissolved org. matter (DOM) pool has been reported in several regional studies. Because DOM is an important advective/mixing pathway of carbon (C) export from the ocean surface layer and its non-Redfieldian stoichiometry would affect ests. of marine export prodn. per unit N and P, we investigated the stoichiometry of marine DOM and its remineralization globally using a compiled DOM data set. Marine DOM is enriched in C and N compared to Redfield stoichiometry, averaging 317:39:1 and 810:48:1 for C:N:P within the degradable and total bulk pools, resp. Dissolved org. phosphorus (DOP) is found to be preferentially remineralized about twice as rapidly with respect to the enriched C:N stoichiometry of marine DOM. Biogeochem. simulations with the Biogeochem. Elemental Cycling model using Redfield and variable DOM stoichiometry corroborate the need for non-Redfield dynamics to match the obsd. DOM stoichiometry. From our model simulations, preferential DOP remineralization is found to increase the strength of the biol. pump by ∼9% vs. the case of Redfield DOM cycling. Global net primary productivity increases ∼10% including an increase in marine nitrogen fixation of ∼26% when preferential DOP remineralization and direct utilization of DOP by phytoplankton are included. The largest increases in marine nitrogen fixation, net primary productivity, and carbon export are obsd. within the western subtropical gyres, suggesting the lateral transfer of P in the form of DOP from the productive eastern and poleward gyre margins may be important for sustaining these processes downstream in the subtropical gyres.
    35. 35
      Dittmar, T.; Koch, B.; Hertkorn, N.; Kattner, G. A Simple and Efficient Method for the Solid-Phase Extraction of Dissolved Organic Matter (SPE-DOM) from Seawater. Limnol. Oceanogr. Methods 2008, 6 (6), 230– 235,  DOI: 10.4319/lom.2008.6.230
      36
      A simple and efficient method for the solid-phase extraction of dissolved organic matter (SPE-DOM) from seawater
      Dittmar, Thorsten; Koch, Boris; Hertkorn, Norbert; Kattner, Gerhard
      Limnology and Oceanography: Methods (2008), 6 (June), 230-235CODEN: LOMIBY; ISSN:1541-5856. (American Society of Limnology and Oceanography)
      A simple protocol is presented for the solid-phase extn. of dissolved org. matter (SPE-DOM) from seawater using com. prepacked cartridges. The method does not require major instrumentation and can be performed in the field. Modified styrene divinyl benzene polymer type sorbents (Varian PPL and ENV) and sorbents of a silica structure bonded with different hydrocarbon chains (Varian C8, C18, C18OH, and C18EWP) were considered. Except for C18OH, which heavily contaminated the samples, none of the sorbents leached significant amts. of dissolved org. carbon (DOC) or nitrogen (DON). Samples from the North Brazil shelf with strong mixing gradients of terrigenous and marine DOM were used to compare the various sorbents. PPL was the most efficient-on av., 62% of DOC was recovered as salt-free exts. C18 was found to be most efficient among the silica-based sorbents, but it showed only two-thirds of the extn. efficiency of PPL. As indicated by [1H]NMR, C/N, and δ13C analyses, PPL extd. a more representative proportion of DOM than C18. Therefore, PPL was used for comparative studies in the Gulf of Mexico and Antarctica. From brackish marsh and river waters, 65% and 62% of total DOC, resp., could be extd. For purely marine DOM in Antarctica and the deep sea, the extn. efficiency was lower (43% on av.). The efficiency of the new method to isolate marine DOM is better than or similar to highly laborious methods. A further advantage is the complete desalination of the sample. The isolation of a major DOM fraction, which is salt-free, offers many possibilities to further characterize DOM by advanced anal. techniques.
    36. 36
      Ksionzek, K. B.; Zhang, J.; Ludwichowski, K.-U.; Wilhelms-Dick, D.; Trimborn, S.; Jendrossek, T.; Kattner, G.; Koch, B. P. Stoichiometry, Polarity, and Organometallics in Solid-Phase Extracted Dissolved Organic Matter of the Elbe-Weser Estuary. PLoS One 2018, 13 (9), e0203260  DOI: 10.1371/journal.pone.0203260
      37
      Stoichiometry, polarity, and organometallics in solid-phase extracted dissolved organic matter of the Elbe-Weser estuary
      Ksionzek, Kerstin B.; Zhang, Jing; Ludwichowski, Kai-Uwe; Wilhelms-Dick, Dorothee; Trimborn, Scarlett; Jendrossek, Thomas; Kattner, Gerhard; Koch, Boris P.
      PLoS One (2018), 13 (9), e0203260/1-e0203260/24CODEN: POLNCL; ISSN:1932-6203. (Public Library of Science)
      Dissolved org. matter (DOM) is ubiquitous in natural waters and plays a central role in the biogeochem. in riverine, estuarine and marine environments. This study quantifies and characterizes solid-phase extractable DOM and trace element complexation at different salinities in the Weser and Elbe River, northern Germany, and the North Sea. Dissolved org. carbon (DOC), total dissolved nitrogen (TDN), Co and Cu concns. were analyzed in original water samples. Solid-phase extd. (SPE) water samples were analyzed for DOC (DOCSP E), dissolved org. nitrogen (DONSP E), sulfur (DOSSP E) and trace metal (51V, 52Cr, 59Co, 60Ni, 63Cu, 75As) concns. Addnl., different pre-treatment conditions (acidification vs. non-acidification prior to SPE) were tested. In agreement with previous studies, acidification led to generally higher recoveries for DOM and trace metals. Overall, higher DOM and trace metal concns. and subsequently higher complexation of trace metals with carbon and sulfur-contg. org. complexes were found in riverine compared to marine samples. With increasing salinity, the concns. of DOM decreased due to estuarine mixing. However, the slightly lower relative decrease of both, DOCSP E and DONSP E (∼77%) compared to DOSSP E (∼86%) suggests slightly faster removal processes for DOSSP E.
    37. 37
      Green, N. W.; Perdue, E. M.; Aiken, G. R.; Butler, K. D.; Chen, H.; Dittmar, T.; Niggemann, J.; Stubbins, A. An Intercomparison of Three Methods for the Large-Scale Isolation of Oceanic Dissolved Organic Matter. Mar. Chem. 2014, 161, 14– 19,  DOI: 10.1016/j.marchem.2014.01.012
      38
      An intercomparison of three methods for the large-scale isolation of oceanic dissolved organic matter
      Green, Nelson W.; Perdue, E. Michael; Aiken, George R.; Butler, Kenna D.; Chen, Hongmei; Dittmar, Thorsten; Niggemann, Jutta; Stubbins, Aron
      Marine Chemistry (2014), 161 (), 14-19CODEN: MRCHBD; ISSN:0304-4203. (Elsevier B.V.)
      Dissolved org. matter (DOM) was isolated from large vols. of deep (674 m) and surface (21 m) ocean water via reverse osmosis/electrodialysis (RO/ED) and two solid-phase extn. (SPE) methods (XAD-8/4 and PPL) at the Natural Energy Lab. of Hawaii Authority (NELHA). By applying the three methods to common water samples, the efficiencies of XAD, PPL and RO/ED DOM isolation were compared. XAD recovered 42% of dissolved org. carbon (DOC) from deep water (25% with XAD-8; 17% with XAD-4) and 30% from surface water (16% with XAD-8; 14% with XAD-4). PPL recovered 61 ± 3% of DOC from deep water and 61% from surface water. RO/ED recovered 82 ± 3% of DOC from deep water, 14 ± 3% of which was recovered in a sodium hydroxide rinse, and 75 ± 5% of DOC from surface water, with 12 ± 2% in the sodium hydroxide rinse. The highest recoveries of all were achieved by the sequential isolation of DOC, first with PPL and then via RO/ED. This combined technique recovered 98% of DOC from a deep water sample and 101% of DOC from a surface water sample. In total, 1.9, 10.3 and 1.6 g-C of DOC were collected via XAD, PPL and RO/ED, resp. Rates of DOC recovery using the XAD, PPL and RO/ED methods were 10, 33 and 10 mg-C h- 1, resp. Based upon C/N ratios, XAD isolates were heavily C-enriched compared with water column DOM, whereas RO/ED and PPL → RO/ED isolate C/N values were most representative of the original DOM. All techniques are suitable for the isolation of large amts. of DOM with purities suitable for most advanced anal. techniques. Coupling PPL and RO/ED techniques may provide substantial progress in the search for a method to quant. isolate oceanic DOC, bringing the entirety of the DOM pool within the marine chemist's anal. window.
    38. 38
      Tipping, E. Cation Binding by Humic Substances; Cambridge University Press: Cambridge, 2002; DOI: 10.1017/CBO9780511535598 .
      There is no corresponding record for this reference.
    39. 39
      Lodeiro, P.; Martínez-Cabanas, M.; Herrero, R.; Barriada, J. L.; Vilariño, T.; Rodríguez-Barro, P.; Sastre de Vicente, M. E. The Proton Binding Properties of Biosorbents. Environ. Chem. Lett. 2019, 17 (3), 1281– 1298,  DOI: 10.1007/s10311-019-00883-z
      40
      The proton binding properties of biosorbents
      Lodeiro, Pablo; Martinez-Cabanas, Maria; Herrero, Roberto; Barriada, Jose L.; Vilarino, Teresa; Rodriguez-Barro, Pilar; Sastre de Vicente, Manuel E.
      Environmental Chemistry Letters (2019), 17 (3), 1281-1298CODEN: ECLNBJ; ISSN:1610-3653. (Springer)
      A review. A broad variety of materials of biol. origin have been successfully used in recent decades for the removal of pollutants from waters. These biosorbents include natural polymers that play a key role for adsorption. It is therefore crit. to understand the physicochem. properties of the chem. groups of these biopolymers. The acid-base properties of biomass are affected by pH, ionic strength and medium compn. Nevertheless, these parameters are not always considered during biosorption studies. According to the literature, less than 3% of biosorption reports include studies on proton binding. Moreover, in 60% of these papers, there is key exptl. information missing such as the calibration of the electrodes employed for potentiometric titrns. We consider therefore that there is an important need for reviewing the role of proton binding in biosorption studies. This review outlines the major advances on data interpretation and modeling of proton binding on biosorbents. In addn., we discuss issues concerning the acid-base properties of biosorbents.
    40. 40
      Fong, M. B.; Dickson, A. G. Insights from GO-SHIP Hydrography Data into the Thermodynamic Consistency of CO2 System Measurements in Seawater. Mar. Chem. 2019, 211, 52– 63,  DOI: 10.1016/j.marchem.2019.03.006
      41
      Insights from GO-SHIP hydrography data into the thermodynamic consistency of CO2 system measurements in seawater
      Fong, Michael B.; Dickson, Andrew G.
      Marine Chemistry (2019), 211 (), 52-63CODEN: MRCHBD; ISSN:0304-4203. (Elsevier B.V.)
      Due to advances in technol., routine seawater pH measurements of excellent repeatability are becoming increasingly common for studying the ocean CO2 system. However, the accuracy of pH measurements has come into question due to a widespread observation, from a large no. of carefully calibrated state-of-the-art CO2 measurements on various cruises, of there being a significant pH-dependent discrepancy between pH that was measured spectrophotometrically and pH calcd. from concurrent measurements of total dissolved inorg. carbon (CT) and total alky. (AT), using a thermodn. model of seawater acid-base systems. From an anal. of four recent GO-SHIP repeat hydrog. datasets, we show that a combination of small systematic errors in the dissocn. consts. of carbonic acid (K1 and K2), the total boron-salinity ratio, and in CT and AT measurements are likely responsible for some, but not all of the obsd. pH-dependent discrepancy. The residual discrepancy can only be fully accounted for if there exists a small, but meaningful amt. (~ 4μ mol kg-1) of an unidentified and typically neglected contribution to measured AT, likely from org. bases, that is widespread in the open ocean. A combination of these errors could achieve consistency between measured and calcd. pH, without requiring that any of the shipboard measurements be significantly in error. Future research should focus on establishing the existence of org. alky. in the open ocean and constraining the uncertainty in both CO2 measurements and in the consts. used in CO2 calcns.
    41. 41
      Kerr, D. E.; Turner, C.; Keogh, J.; Grey, A.; Brown, P. J.; Kelleher, B. P. OrgAlkCalc: Estimation of Organic Alkalinity Quantities and Acid-Base Properties with Proof of Concept in Dublin Bay. Mar. Chem. 2023, 251, 104234,  DOI: 10.1016/j.marchem.2023.104234
      42
      OrgAlkCalc: Estimation of organic alkalinity quantities and acid-base properties with proof of concept in Dublin Bay
      Kerr, Daniel E.; Turner, Charles; Grey, Anthony; Keogh, Jill; Brown, Peter J.; Kelleher, Brian P.
      Marine Chemistry (2023), 251 (), 104234CODEN: MRCHBD; ISSN:0304-4203. (Elsevier B.V.)
      The presence and influence of org. species is generally omitted in total alky. (TA) anal. This has direct implications to calcd. carbonate system parameters and to key descriptors of ocean acidification, esp. in coastal waters where org. alky. (OrgAlk) can contribute significantly to TA. As titratable charge groups of OrgAlk can act as unknown seawater acid-base systems, the inclusion of the total concn. and apparent dissocn. consts. of OrgAlk in carbonate calcns. involving TA is required to minimise uncertainty in computed speciation. Here we present OrgAlkCalc, an open-source Python based program that can be used in conjunction with simply modified Global Ocean Acidification Observing Network (GOA-ON) TA titrn. app. to measure TA and OrgAlk, as well as return estns. of assocd. acid-base properties. The modified titrn. app. and OrgAlkCalc were tested using samples collected from the transitional waters of Dublin Bay, Ireland over a 8 mo period (n = 100). TA values ranged from 2257 to 4692μmol.kg-1 and indicated that freshwater inputs pose a significant source of carbonate alky. to Dublin Bay. OrgAlk values ranged from 46 to 234μmol.kg-1 and were generally obsd. to be higher in more saline waters, with elevated levels in the Autumn/Winter period. The dissocn. consts. of two distinct OrgAlk charge groups were identified, with pK values in agreement with previously reported values for humic substances. The majority of OrgAlk charge group concns. were assocd. with carboxyl-like charge groups.
    42. 42
      Avena, M. J.; Vermeer, A. W. P.; Koopal, L. K. Volume and Structure of Humic Acids Studied by Viscometry pH and Electrolyte Concentration Effects. Colloids Surf., A 1999, 151, 213– 224,  DOI: 10.1016/S0927-7757(98)00504-4
      43
      Volume and structure of humic acids studied by viscometry pH and electrolyte concentration effects
      Avena, M. J.; Vermeer, A. W. P.; Koopal, L. K.
      Colloids and Surfaces, A: Physicochemical and Engineering Aspects (1999), 151 (1-2), 213-224CODEN: CPEAEH; ISSN:0927-7757. (Elsevier Science B.V.)
      Viscometry was used to evaluate the effects of pH and supporting electrolyte concn. on the intrinsic viscosities of eight humic acids and one fulvic acid. Two synthetic poly(acrylic acid) (PAA) samples of different mol. wt. were also studied for comparison. Humic and fulvic acid mols. behave as flexible entities that can swell or shrink in response to changes in pH and ionic strength. An increase in the soln. pH leads to the development of neg. charges in the mols. with the consequent electrostatic repulsion between ionized groups and mol. swelling. Increasing the ionic strength increases the screening of charges and leads to mol. shrinkage. The pH dependence decreases with increasing electrolyte concn. and at 10-1 M electrolyte the intrinsic viscosity is almost pH independent. The general behavior of PAAs is similar to that of the humics, though the effects of pH and electrolyte concn. are much larger for the PAAs. The degree of hydration of the humics differs for different samples. There are compact samples with low water content and swelling properties whereas other humics are more hydrated and flexible. All the studied humics have an internal structure that limits the expansion of the mols. when the electrolyte concn. is decreased. The latter is in accordance with the low values of the Mark-Houwink coeff., α, of humics.
    43. 43
      Dittmar, T.; Lennartz, S. T.; Buck-Wiese, H.; Hansell, D. A.; Santinelli, C.; Vanni, C.; Blasius, B.; Hehemann, J. H. Enigmatic Persistence of Dissolved Organic Matter in the Ocean. Nat. Rev. Earth Environ. 2021, 2 (8), 570– 583,  DOI: 10.1038/s43017-021-00183-7
      44
      Enigmatic persistence of dissolved organic matter in the ocean
      Dittmar, Thorsten; Lennartz, Sinikka T.; Buck-Wiese, Hagen; Hansell, Dennis A.; Santinelli, Chiara; Vanni, Chiara; Blasius, Bernd; Hehemann, Jan-Hendrik
      Nature Reviews Earth & Environment (2021), 2 (8), 570-583CODEN: NREECQ; ISSN:2662-138X. (Springer International Publishing AG)
      Marine dissolved org. matter (DOM) contains more carbon than the combined stocks of Earth's biota. Organisms in the ocean continuously release a myriad of mols. that become food for microheterotrophs, but, for unknown reasons, a residual fraction persists as DOM for millennia. In this Perspective, we discuss and compare two concepts that could explain this persistence. The long-standing 'intrinsic recalcitrance paradigm attributes DOM stability to inherent mol. properties. In the 'emergent recalcitrance' concept, DOM is continuously transformed by marine microheterotrophs, with recalcitrance emerging on an ecosystems level. Both concepts are consistent with observations in the modern ocean, but they imply very different responses of the DOM pool to climate-related changes. To better understand DOM persistence, we propose a new overarching research strategy - the ecol. of mols. - that integrates the concepts of intrinsic and emergent recalcitrance with the ecol. and environmental context.
    44. 44
      David, C.; Mongin, S.; Rey-Castro, C.; Galceran, J.; Companys, E.; Garcés, J. L.; Salvador, J.; Puy, J.; Cecilia, J.; Lodeiro, P.; Mas, F. Competition Effects in Cation Binding to Humic Acid: Conditional Affinity Spectra for Fixed Total Metal Concentration Conditions. Geochim. Cosmochim. Acta 2010, 74 (18), 5216– 5227,  DOI: 10.1016/j.gca.2010.06.023
      45
      Competition effects in cation binding to humic acid: Conditional affinity spectra for fixed total metal concentration conditions
      David, Calin; Mongin, Sandrine; Rey-Castro, Carlos; Galceran, Josep; Companys, Encarnacio; Garces, Jose Luis; Salvador, Jose; Puy, Jaume; Cecilia, Joan; Lodeiro, Pablo; Mas, Francesc
      Geochimica et Cosmochimica Acta (2010), 74 (18), 5216-5227CODEN: GCACAK; ISSN:0016-7037. (Elsevier B.V.)
      Information on the Pb and Cd binding to a purified Aldrich humic acid (HA) is obtained from the influence of different fixed total metal concns. on the acid-base titrns. of this ligand. NICA (Non-Ideal Competitive Adsorption) isotherm has been used for a global quant. description of the binding, which has then been interpreted by plotting the Conditional Affinity Spectra of the H+ binding at fixed total metal concns. (CAScTM). This new physicochem. tool allows the interpretation of binding results in terms of distributions of proton binding energies. A large increase in the acidity of the phenolic sites as the total metal concn. increases, esp. in presence of Pb, is revealed from the shift of the CAScTM towards lower affinities. The variance of the CAScTM distribution, which can be used as a direct measure of the heterogeneity, also shows a significant dependence on the total metal concn. A discussion of the factors that influence the heterogeneity of the HA under the conditions of each expt. is provided, so that the smoothed pattern exhibited by the titrn. curves can be justified.

  • Supporting Information

    Supporting Information

    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.est.3c01810.

    • 1. Sampling map (Figure S1, page S2); 2. Seawater analysis (page S3): North Atlantic seawater main characteristics (Table S1) and North Atlantic seawater micro- and macronutrient concentrations (Table S2); 3. Strategy for the derivation of NICA–Donnan (PB–VD) model parameters (page S4); 4. Intrinsic and effective proton binding parameters calculated using the NICA–Donnan (PB–VD) model (pages S5–S6): Optimized NICA–Donnan (PB–VD) parameter values (Table S3), NICA–Donnan (PB–VD) fits (Figure S2); 5. Potentiometric titration data fitted to a standard VD Donnan model (pages S7–S10): NICA-Donnan fits (Figures S3), proton binding affinity (log K̅H,i) (Figure S4) and heterogeneity (mi) parameters (Figure S5); 6. Glossary of terms (pages S11–S12) (PDF)

    • Complete experimental data set (XLSX)


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