Dimethylmercury Formation Mediated by Inorganic and Organic Reduced Sulfur Surfaces

Underlying formation pathways of dimethylmercury ((CH(3))(2)Hg) in the ocean are unknown. Early work proposed reactions of inorganic Hg (Hg(II)) with methyl cobalamin or of dissolved monomethylmercury (CH(3)Hg) with hydrogen sulfide as possible bacterial mediated or abiotic pathways. A significant fraction (up to 90%) of CH(3)Hg in natural waters is however adsorbed to reduced sulfur groups on mineral or organic surfaces. We show that binding of CH(3)Hg to such reactive sites facilitates the formation of (CH(3))(2)Hg by degradation of the adsorbed CH(3)Hg. We demonstrate that the reaction can be mediated by different sulfide minerals, as well as by dithiols suggesting that e.g. reduced sulfur groups on mineral particles or on protein surfaces could mediate the reaction. The observed fraction of CH(3)Hg methylated on sulfide mineral surfaces exceeded previously observed methylation rates of CH(3)Hg to (CH(3))(2)Hg in seawaters and we suggest the pathway demonstrated here could account for much of the (CH(3))(2)Hg found in the ocean.