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Environment

Thiols Drive Mercury Photodegradation

Chloride outcompetes organic thiols in seawater, allowing methylmercury to bioaccumulate in seafood

by David Pittman
July 12, 2010 | APPEARED IN VOLUME 88, ISSUE 28

Mercury accumulates in seafood at high levels even though the amount that is found in seawater is small. But why has been unclear, until now. Duke University’s Tong Zhang and Heileen Hsu-Kim have found that singlet oxygen (1O2) formed by sunlight falling on dissolved organic matter drives methylmercury (CH3Hg+) photodecomposition (Nat. Geosci., DOI: 10.1038/ngeo892). However, degradation of CH3Hg+, a neurotoxin that accumulates in food webs and poses a risk to human health, depends on the type of binding ligand present in the water. For example, in freshwater, CH3Hg+ degradation occurred at low CH3Hg+ concentration and only when humic acid was present, the researchers showed. They suggest that complexes formed between CH3Hg+ and thiols in humic acid are necessary for the breakdown to occur. But in seawater, where chloride dominates over thiols, degradation rates were an order of magnitude slower.

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