Sulfate-reducing bacteria methylate mercury at variable rates in pure culture and in marine sediments

Differences in methylmercury (CH3Hg) production normalized to the sulfate r eduction rate (SRR) in various species of sulfate-reducing bacteria (SRB) w ere quantified in pure cultures and in marine sediment slurries in order to determine if SRB strains which differ phylogenetically methylate mercury ( Hg) at similar rates. Cultures representing five genera of the SRB (Desulfo vibrio desulfuricans, Desulfobulbus propionicus, Desulfococcus multivorans, Desulfobacter sp, strain BG-8, and Desulfobacterium sp, strain BG-33) were grown in a strictly anoxic, minimal medium that received a dose of inorgan ic Bg 120 h after inoculation, The mercury methylation rates (MMR) normaliz ed per cell were up to 3 orders of magnitude higher in pure cultures of mem bers of SRB groups capable of acetate utilization (e.g., the family Desulfo bacteriaceae) than in pure cultures of members of groups that are not able to use acetate (e.g., the family Desulfovibrionaceae). Little or no Hg meth ylation was observed in cultures of Desulfobacterium or Desulfovibrio strai ns in the absence of sulfate, indicating that Hg methylation was coupled to respiration in these strains, Mercury methylation, sulfate reduction, and the identities of sulfate-reducing bacteria in marine sediment slurries wer e also studied. Sulfate-reducing consortia were identified by using group-s pecific oligonucleotide probes that targeted the 16S rRNA molecule. Acetate -amended slurries, which were dominated by members of the Desulfobacterium and Desulfobacter groups, exhibited a pronounced ability to methylate Ng wh en the MMR were normalized to the SRR, while lactate-amended and control sl urries had normalized MMR that were not statistically different. Collective ly, the results of pure-culture and amended-sediment experiments suggest th at members of the family Desulfobacteriaceae have a greater potential to me thylate Ng than members of the family Desulfovibrionaceae have when the MMR are normalized to the SRR Hg methylation potential may be related to genet ic composition and/or carbon metabolism in the SRB, Furthermore, we found t hat in marine sediments that are rich in organic matter and dissolved sulfi de rapid CH3Hg accumulation is coupled to rapid sulfate reduction. The obse rvations described above have broad implications for understanding the cont rol of CH3Hg formation and for developing remediation strategies For Hg-con taminated sediments.