Thiols Drive Mercury Photodegradation

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.