The kinetics of Hg (Hg(II)i) association, methylation, and methylmercury (MeHg) demethylation were examined for a suite of Desulfovibrio (DSV) species with and without MeHg production capability. To do so, we employed a detailed method for assessing MeHg production in cultures, including careful control of medium chemistry, cell density and growth phase, plus mass balance of Hg(II)i and MeHg during the assays. We tested the hypothesis that differences in Hg(II)i sorption and/or uptake rates drive observed differences in methylation rates among DSV species. Hg(II)i associated rapidly and with high affinity to both methylating and non-methylating species. MeHg production by Hg-methylating strains was rapid, plateauing after ∼3 h. All MeHg produced was rapidly exported. We also tested the idea that all DSV are capable of Hg(II)i methylation, but that rapid demethylation masks its production, but found this was not the case. Therefore, the underlying reason why MeHg production capability is not universal in the DSV is not differences in Hg affinity for cells, nor differences in the ability of strains to degrade MeHg. However, Hg methylation rates varied substantially between Hg methylating DSV even in these controlled experiments and after normalization to cell density. Thus, biological differences may drive cross-species differences in Hg methylation rates. As part of this study, we identified 4 new Hg-methylators (D. aespoeensis, D. alkalitolerans, D. psychrotolerans, and D. sulfodismutans) and 4 non-methylating species (D. alcoholovorans, D. tunisiensis, D. carbinoliphilus, and D. piger) in our ongoing effort to generate a library of strains for Hg-methylation genomics.
