the methylation process of dimethylmercury mercury

It is well known that monomethylmercury (CH3HgIIX−I, where Concentration 1. Therefore, understanding the methylation process of mercury is a key goal in understanding the factors that influence its biogeochemical cycling. The formation of dimethylmercury CH3Hg and (CH3)2Hg in 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, focusing primarily on CH3Hg formation. The corrin-like and 2[4Fe-4S]ferredoxin proteins encoded by the HgcA and HgcB genes, respectively, were recently determined to be critical for CH3Hg production by anaerobic bacteria. However, the number of bacterial strains tested for their ability to methylate Hg to (CH3)2Hg is limited and the primary process remains to be determined

In studies on the culture of Desulfovibrio desulfovibrio, Baldi and colleagues observed that (CH3)2Hg was produced simultaneously with a white precipitate after the addition of large amounts of CH3Hg(aq)14. The white precipitate was identified as dimethylmercuric sulfide, (CH3Hg)2S(s). Previous work has shown that the reaction between CH3Hg(aq) and H2S forms (CH3Hg)2S(s), which degrades over time to cinnabar (β-HgS(s)) and (CH3)2Hg15. Therefore, Baldi and colleagues believe that bacterial production of (CH3)2Hg is the result of sulfide growth. Currently, known pathways for (CH3)2Hg formation relevant to field conditions include reaction of CH3Hg(aq) with H2S15 or selenoamino acid 16 and methylation with methylcobalamin 17. Calculations suggest a possible formation pathway for CH3Hg complexed with L-cysteine, but experimental data are lacking18. Surface-mediated processes are of interest since up to 90% of naturally occurring CH3Hg in seawater is adsorbed onto reduced sulfur groups on minerals or combined with thiols on organic matter. Therefore, we hypothesize that (CH3)2Hg can be formed from CH3Hg adsorbed on the surface of inorganic and organic reduced sulfur.

To test whether CH3Hg can form (CH3)2Hg on reduced sulfur surfaces, we first adsorbed CH3Hg onto disordered Mackinawite (FeSm(s)) in degassed purified water under a low oxygen atmosphere and quantified the formed (CH3 )2Hg amount. In experiments up to 1 hour, we detected 0.37±0.08 (0–20min), 0.21±0.07 (20–40min) and 0.16±0.07 (40–60min) pmol of (CH3)2Hg formed from 2.3nmol CH3Hg (Supplementary Table S1). Control experiments using water and filtered FeSm(s) slurry (0.02 μm) did not produce detectable levels of (CH3)2Hg, supporting its formation from CH3Hg adsorbed on FeSm(s) particles.

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