Arsenic Mobilization by the Dissimilatory Fe(III)-Reducing Bacterium Shewanella alga BrY

The mobility of arsenic commonly increases as reducing conditions are established within sediments or flooded soils. Although the reduction of arsenic increases its solubility at circumneutral pH, hydrous ferric oxides (HFO) strongly sorb both As(V) (arsenate) and As(III) (arsenite), the two primary inorganic species. Thus, in the presence of excess HFO, reductive dissolution of iron may be the dominant mechanism by which As is released into solution. In this paper, we report that the dissimilatory iron-reducing bacterium Shewanella alga strain BrY promoted As mobilization from a crystalline ferric arsenate as well as from sorption sites within whole sediments. S. alga cells released arsenate from the mineral scorodite (FeAsO₄·2H₂O) as a result of dissimilatory (i.e., respiratory) reduction of Fe(III) to Fe(II). Solid-phase analysis with SEM-EDS and XAFS (X-ray absorption fine structure) spectroscopy revealed that the valence states of Fe and As in the solid-phase product were identical to those in solution, i.e., Fe(II) and As(V). Additionally, As(V) sorbed to sediments from Lake Coeur d'Alene, ID, a mining-impacted environment enriched in both Fe and As, was solubilized by the activity of S. alga BrY. In neither experiment was As(III) detected. We conclude that arsenic mobility can be enhanced by the activity of dissimilatory iron-reducing bacteria in the absence of arsenic reduction.