Photolytic degradation of methylmercury enhanced by binding to natural organic ligands

Abstract

Methylmercury is a neurotoxin that accumulates in food webs and poses a significant risk to human health. Laboratory experiments suggest that complexation of methylmercury with sulphur-containing ligands can stimulate its degradation. Methylmercury is a neurotoxin that accumulates in food webs and poses a significant risk to human health1. In natural water bodies, methylmercury concentrations remain low due to the degradation of methylmercury into inorganic mercury by sunlight, a process known as photodecomposition. Rates of photodecomposition are relatively rapid in freshwater lakes2,3,4, and slow in marine waters5, but the cause of this difference is not clear. Here, we carry out incubation experiments with artificial freshwater and seawater samples to examine the mechanisms regulating methylmercury photodecomposition. We show that singlet oxygen—a highly reactive form of dissolved oxygen generated by sunlight falling on dissolved organic matter—drives photodecomposition. However, in our experiments the rate of methylmercury degradation depends on the type of methylmercury-binding ligand present in the water. Relatively fast degradation rates (similar to observations in freshwater lakes) were detected when methylmercury species were bound to sulphur-containing ligands such as glutathione and mercaptoacetate. In contrast, methylmercury–chloride complexes, which are the dominant form of methylmercury in marine systems, did not degrade as easily. Our results could help to explain why methylmercury photodecomposition rates are relatively rapid in freshwater lakes and slow in marine waters.