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Chemistry Is Key to Mercury Levels in Saltwater Fish

By Eddy Ball
August 2010

Heileen Hsu-Kim, Ph.D.
Hsu-Kim, shown in her lab, conceived the study, supervised the research, and carried out speciation calculations. (Photo courtesy of Duke University Photography)

Tong Zhang
Tong Zhang carried out all experiments and data analysis. The experiments were part of the work on her thesis topic, " Role of sulfur-coordination on rates of mercury methylation and demethylation." (Photo courtesy of Heileen Hsu-Kim and Duke University)

Saltwater fish, especially large ones higher up in the food chain, are known to contain high levels of methylmercury even though seawater contains very low levels as compared to fresh water. For that reason, the U.S. Food and Drug Administration advisory( ) Exit NIEHS on mercury in fish cautions against any consumption of Shark, Swordfish, King Mackerel, or Tilefish by women who may become pregnant, pregnant women, nursing mothers, and young children.

A new study by NIEHS-funded researchers at Duke University reports on the chemistry of methylmercury (MeHg) degradation in the environment and could explain how this organometal persists in seawater.

Published online in the journal Nature Geoscience, the findings ( Exit NIEHS point to the benefits of more research into how the differences between the rates of photodecomposition of MeHg in fresh and saltwater environments affect accumulation of MeHg in the tissues of fish and shellfish - and ultimately the humans who may endanger their health by consuming them.

Sunlight and dissolved organic matter speed up degradation

"The most common way nature turns methylmercury into a less toxic form is through sunlight," said principal investigator Heileen Hsu-Kim, Ph.D.( Exit NIEHS, in a June 27 press release issued by Duke University. "When it [MeHg] is attached to dissolved organic matter, like decayed plants or animal matter, sunlight more readily breaks down the methylmercury.

"However, in seawater, the methylmercury remains tightly bonded to the chloride, where sunlight does not degrade it as easily," Hsu-Kim explained. "In this form, methylmercury can then be ingested by marine animals."

In the experiments that led to publication of their study, Hsu-Kim and first author Tong Zhang, a doctoral student in Hsu-Kim's research group( Exit NIEHS, tested the effects of photoreactive intermediates on the decomposition of MeHg by sunlight. Sunlight falling on dissolved organic matter generates a highly reactive form of dissolved oxygen that drives the process of photolytic degradation.

The researchers measured decomposition rates of different MeHg-ligand complexes in saltwater and freshwater. They calculated and compared the degradation rates of MeHg linked to sulfur-containing ligands, such as glutathione. mercaptoacetate, and humic acid that are found in freshwater, to the rates of MeHg-chloride complexes that predominate in saltwater.

According to the researchers, the rates of decomposition in freshwater with high enough concentrations of humic acid were relatively rapid, while MeHg photodecomposition rates in saltwater were considerably slower, by at least an order of magnitude. These differences help explain why the lower concentrations of MeHg present in saltwater actually pose a greater risk to health because of their persistence in the marine environment.

Public health implications of understanding photodecomposition

The authors point to the public health implications of mercury concentrations in saltwater fish. "The exposure rate of mercury in the U.S. is quite high," Hsu-Kim explained. "A recent epidemiological survey found that up to 8 percent of women had mercury levels higher than national guidelines. Since humans are on the top of the food chain, any mercury in our food accumulates in our body."

The researchers are hopeful that this kind of research may lead to secondary prevention measures to limit exposure to MeHg. "With this new understanding of how photodemethylation occurs, we can improve efforts to predict mercury cycling in the aquatic environment and prevent bioaccumulation of MeHg in food webs," they concluded.

The research by Hsu-Kim and Zhang is the latest of nearly 70 publications associated with the NIEHS grant to principal investigator Richard Di Giulio, Ph.D., for the Center for Comparative Biology of Vulnerable Populations at Duke. The grant is administered by NIEHS Program Administrator Les Reinlib, Ph.D.

"The Duke Center works with federally-supported investigators to try to understand why certain people develop disease when challenged with environmental agents and others remain healthy," Reinlib said. "This study extends the Center's focus on biological, physiological, and social aspects of vulnerability that may alter the effect of environmental toxins on human health."

Citation: Zhang T, Hsu-Kim H( Exit NIEHS. 2010. Photolytic degradation of methylmercury enhanced by binding to natural organic ligands. Nat Geosci 3(7):473-476.

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