Mercury is one of the more toxic trace elements and also one one of the more elusive for its major form in water, the mercuric ion, is readily reduced to elemental mercury which is highly volatile. Our continuing work on the biogeochemistry of mercury which recently dealt with redox and photoredox processes in collaboration with Marc Amyot's group in Montreal(Lalonde et al. 2004; Amyot et al. 2005) is presently focused on mercury methylation. Since methylmercury is the species accumulated in fish via the food chain, this is a key transformation in determining human exposure to mercury. Yet it has received surprisingly little attention over the past 20 years. The two questions we are trying to answer are: 1) where is methylation occurring in the ocean and by what mechanism? and 2) what controls the rate of methylation by sulfate reducing bacteria in freshwater systems? On the basis of previous field data, we have proposed that methylmercury in the open ocean may originate from the deep sea, perhaps from hydrothermal vents (Kraepiel et al. 2003). We have indeed measured significant concentrations of methylmercury in some hydrothermal samples and shown that mercury methylation can be effected chemically at high pressure and temperature by reaction with yet unidentified trace organic compounds (Eileen Ekstrom’s doctoral Thesis, 2006). In terrestrial systems, we know that mercury methylation is effected by sulfate reducing bacteria. Using specific inhibitors and cobalt limitation as means to modulate the activity of vitamin B12, we have shown that the acetylCoA pathway is responsible for Hg methylation in sulfate reducing bacteria that oxidize their substrate to CO2 (complete oxidizers), but not in others that oxidize their substrate to acetate (incomplete oxidizers) (Ekstrom et al. 2003; Ekstrom and Morel, in review). Our results shed light on the biochemical mechanisms of mercury methylation and show that the availability of trace metals such as cobalt may be important in controlling Hg methylation in anoxic waters and sediments.
