The potential pathways for the formation of dimethylmercury

The potential pathways for the formation of dimethylmercury ((CH3)2Hg) in the ocean are unknown. Earlier work suggested that the reaction of inorganic mercury (HgII) with methylcobalamin or dissolved monomethylmercury (CH3Hg) with hydrogen sulfide may be bacterially mediated or abiotic. However, a significant fraction (up to 90%) of CH3Hg in natural water is adsorbed onto reduced sulfur groups on mineral or dimethylmercury organic surfaces. We show that the binding of CH3Hg to such reactive sites promotes the formation of (CH3)2Hg by degrading the adsorbed CH3Hg. We demonstrate that this reaction can be mediated by different sulfide minerals as well as dithiols, suggesting that, for example, reduced sulfur groups on mineral particles or protein surfaces can mediate this reaction. The observed CH3Hg methylation ratio at the surface of sulfide minerals exceeds the CH3Hg to (CH3)2Hg methylation ratio previously observed in seawater, and we believe that the pathway demonstrated here can account for most of the (CH3)Hg found in the ocean. 2Hg.

Dimethylmercury is a volatile and highly toxic form of mercury (Hg)1. It appears to be ubiquitous in seawater and is found in mixed layers of deep anoxic seawater, coastal sediments and upwelling waters, and the Arctic Ocean2,3,4,5,6. Concentrations of (CH3)2Hg in seawater have been reported to range from 0.01–0.4 pM, and (CH3)2Hg has been found to constitute a significant fraction (up to 80%) of the methylated mercury pool (CH3Hg + (CH3)2Hg)1 , 6. The role of (CH3)2Hg in the biogeochemical cycle of mercury and its potential for bioaccumulation is unclear6,7,8. However, degradation of (CH3)2Hg is an important source of CH3Hg for marine systems and ocean boundary layers

It is well known that monomethylmercury (CH3HgIIX−I, where X is Cl−1, OH−1, R-S−1, etc., referred to here as CH3Hg) bioaccumulates in aquatic food webs to levels that affect human and wildlife health. Concentration 1. Therefore, understanding the methylation process of Hg is a key goal in understanding the factors that influence its biogeochemical cycle. The 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, focusing mainly on CH3Hg formation. It was recently identified that the corrinoid-type protein and the 2[4Fe-4S]ferredoxin protein, encoded by the HgcA and HgcB genes, respectively, are essential 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 main process remains to be determined

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