Dimethylmercury Formation Mediated by Inorganic and Organic Reduced Sulfur Surfaces

Dimethylmercury is a volatile and highly toxic form of mercury (Hg)1. It appears to be ubiquitous in marine waters and has been found in deep hypoxic oceanic water, coastal sediments and upwelling waters and in the mixed layer of the Arctic ocean2,3,4,5,6. Reported concentrations of (CH3)2Hg in marine waters range from 0.01–0.4 pM and (CH3)2Hg has been found to constitute a significant fraction (up to 80%) of the methylated Hg pool (CH3Hg + (CH3)2Hg)1,6. The role of (CH3)2Hg in the biogeochemical cycle of mercury, and its bioaccumulative potential, is not well known6,7,8. However, for oceanic systems, and for the marine boundary layer, it has been suggested that degradation of (CH3)2Hg is an important source of CH3Hg5,9,10. Monomethylmercury (CH3HgIIX−I where X is Cl−1, OH−1, R-S−1 etc., here referred to as CH3Hg) is known to bioaccumulate in aquatic food webs to concentrations of concern for human and wildlife health1. Understanding the methylation processes of Hg has thus been a key objective for comprehending the factors influencing its biogeochemical cycle. Formation of CH3Hg and (CH3)2Hg by aquatic organisms was first observed by Jensen and Jernelov in 196911. A large number of bacterial strains have since been tested for their ability to methylate Hg, primarily focusing on CH3Hg formation. A corrinoid type protein and a 2[4Fe-4S] ferredoxin protein encoded by the HgcA and HgcB gene, respectively, was recently identified as essential for CH3Hg production by anaerobic bacteria12. The number of bacterial strains tested for their ability to methylate Hg to (CH3)2Hg is however limited and the main process remains to be identified13,14. In culture studies with Desulfovibrio desulfuricans, Baldi and his coworkers, observed production of (CH3)2Hg in paralell with a white precipitate following high additions of CH3Hg(aq)14. This white precipitate was identified as bismethylmercury sulfide, (CH3Hg)2S(s). Previous work had shown (CH3Hg)2S(s) formation from the reaction between CH3Hg(aq) and H2S, and with time, its degradation to metacinnabar (β-HgS(s)) and (CH3)2Hg15. Baldi and his coworkers thus suggested the production of (CH3)2Hg by bacteria as an effect of sulfidogenic growth. Currently, the known pathways of (CH3)2Hg formation relevant to field conditions include reaction of CH3Hg(aq) with H2S15 or selenoaminoacids16 and methylation with methylcobalamin17. Computational calculations suggest a possible formation pathway from CH3Hg complexed to L-cysteine, however experimental data is lacking18. With a up to 90% of the CH3Hg in marine waters naturally occurring adsorbed to reduced sulfur groups on minerals or bound to thiols on organic matter, surface mediated processes are of interest. We therefore hypothesized that (CH3)2Hg could be formed from CH3Hg adsorbed to inorganic and organic reduced sulfur surfaces.