Science News - September 8, 2004

Neglected forms of phosphorus play important role

During the past 30 years, most studies of phosphorus—the nutrient that fuels nuisance algal blooms in lakes—have focused exclusively on forms of soluble, reactive phosphorus thought to be most important to biological systems. But a more detailed look reveals that other forms of soluble phosphorus, which are hard to measure, constitute 50–100% of the phosphorus pool in some ecosystems and are more readily bioavailable than previously believed, according to research presented in August at the Ecological Society of America (ESA) meeting in Portland. The new work is challenging conventional wisdom about nutrient limitation in coastal estuaries, the open ocean, and threatened ecosystems, such as Florida’s Everglades, experts say.

Mining of phosphate rock for fertilizer has accelerated the global phosphorus cycle, leading to a 75% increase in phosphorus storage in land and freshwater ecosystems, says P. V. Sundareshwar, a biogeochemist at the South Dakota School of Mines and Technology and lead author on the new research. Previous studies have looked at total phosphorus and one of its components, soluble, reactive phosphorus, but have not focused on the dozen or more phosphorus-containing compounds that make up what has been labeled “soluble, unreactive phosphorus”. Termed “unreactive” because they don’t react in standard techniques used to measure phosphorus, such as the molybdate blue reaction, these forms of soluble phosphorus include polyphosphates, inositol phosphates, and phosphorus sorbed to mineral or organic compounds.

These compounds have been neglected because many have been thought to be biologically unavailable and some are difficult to measure, Sundareshwar says. But new applications of techniques such as ³¹P nuclear magnetic resonance (NMR) spectroscopy are allowing a finer characterization of this fraction of the phosphorus pool, he says.

Sundareshwar has used ³¹P NMR analysis to show for the first time that pyrophosphate (P₂O₇^4–) can constitute more than 50% of the phosphorus in some coastal estuarine sediments. He demonstrated that soil microorganisms readily used the pyrophosphate and that its accumulation in coastal zones was directly related to human activities, such as industrial use and fertilizer runoff. This finding is important because in some coastal zones, phosphorus availability can limit the growth of the bacterial community.

“Our results suggest that the full extent of bioavailable phosphorus accumulation in estuaries is unknown because of the presence of pyrophosphate,” Sundareshwar says. Although plants in estuaries are limited by nitrogen, phosphorus is important because it can lead to bacterial overgrowth and zones of low oxygen, even in the absence of nitrogen-driven algal blooms, he says. This means that resource managers must abandon their current focus on nitrogen alone and work to curb both nitrogen and phosphorus inputs, he adds.

NMR analysis of phosphorus is helping to determine why the treatment-engineered wetlands—designed to cut phosphorus in agricultural runoff down to safe levels before it enters the Everglades—aren’t meeting their target of 10 parts per million (ppm) total phosphorus, says Curt Richardson, director of the Duke University Wetland Center and a coauthor of the research presented at the ESA meeting. It turns out that the bacteria, algae, and plants in the wetlands convert the soluble, reactive phosphorus from fertilizers into dissolved organic phosphorus, which has been overlooked because it was considered an unreactive form of phosphorus, he says. Although it is biologically available, it is not taken up right away and stays suspended in the treated water as it flows into the Everglades. “Because it is not particulate, it doesn’t drop out of the water column onto the sediment. And because it is not orthophosphate, it is not taken up right away so it just flows out of the treatment wetlands,” he says. “The wetland designers didn’t count on the dissolved organic phosphorus not coming out and they will be lucky if they can get the total phosphorus levels down to 15–20 ppm,” he says.

On the other side of the globe, changes in ocean mixing over the past decade due to strong El Niño weather patterns have led to a 70% decline in soluble, reactive phosphorus concentrations in the ocean off Hawaii, says Dave Karl, biological oceanographer at the University of Hawaii. Characterization of soluble unreactive phosphorus, which now makes up more than 90% of the phosphorus in the ocean, has helped researchers track how algae are now using it more than the disappearing soluble, reactive phosphorus. This has caused the algae community to shift toward a higher proportion of the less-edible blue-green algae, he says.

New applications for ³¹P NMR analysis of soluble unreactive phosphorus, including its use as an indicator of ecosystem health and restoration, are in the pipeline, Richardson adds. —JANET PELLEY