Cobalt limitation of growth and mercury methylation in sulfate-reducing bacteria

Sulfate-reducing bacteria (SRB) have been identified as the primary organisms responsible for monomethylmercury (MeHg) production in aquatic environments, but little is known of the physiology and biochemistry of mercury (Hg) methylation. Corrinoid compounds have been implicated in enzymatic Hg methylation, although recent experiments with a vitamin B₁₂ inhibitor indicated that incomplete-oxidizing SRB likely do not use a corrinoid-enzyme for Hg methylation, whereas experiments with complete-oxidizing SRB were inconclusive due to overall growth limitation. Here we explore the role of corrinoid-containing methyltransferases, which contain a cobalt-reactive center, in Hg methylation. To this end, we performed cobalt-limitation experiments on two SRB strains: Desulfococcus multivorans, a complete-oxidizerthat uses the acetyl-CoA pathway for major carbon metabolism, and Desulfovibrio africanus, an incomplete-oxidizer that does not contain the acetyl-CoA pathway. Cultures of D. multivorans grown with no direct addition of Co or B₁₂ became cobalt-limited and produced 3times less MeHg per cell than control cultures. Differences in growth rate and Hg bioavailability do not account for this large decrease in MeHg production upon Co limitation. In contrast, the growth and Hg methylation rates of D. africanus cultures remained nearly constant regardless of the inorganic cobalt and vitamin B₁₂ concentrations in the medium. These results are consistent with mercury being methylated by different pathways in the two strains: catalyzed by a B₁₂-containing methyltransferase in D. multivorans and a B₁₂-independent methyltransferase in D. africanus. If complete-oxidizing SRB like D. multivorans account for the bulk of MeHg production in coastal sediments as reported, the ambient Co concentration and speciation may control the rate of Hg methylation.