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

Abstract

Differences in methylmercury (CH(3)Hg) production normalized to the sulfate reduction rate (SRR) in various species of sulfate-reducing bacteria (SRB) were 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 (Desulfovibrio 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 inorganic Hg 120 h after inoculation. The mercury methylation rates (MMR) normalized per cell were up to 3 orders of magnitude higher in pure cultures of members of SRB groups capable of acetate utilization (e.g., the family Desulfobacteriaceae) than in pure cultures of members of groups that are not able to use acetate (e.g., the family Desulfovibrionaceae). Little or no Hg methylation was observed in cultures of Desulfobacterium or Desulfovibrio strains 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 were also studied. Sulfate-reducing consortia were identified by using group-specific 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 Hg when the MMR were normalized to the SRR, while lactate-amended and control slurries had normalized MMR that were not statistically different. Collectively, the results of pure-culture and amended-sediment experiments suggest that members of the family Desulfobacteriaceae have a greater potential to methylate Hg than members of the family Desulfovibrionaceae have when the MMR are normalized to the SRR. Hg methylation potential may be related to genetic composition and/or carbon metabolism in the SRB. Furthermore, we found that in marine sediments that are rich in organic matter and dissolved sulfide rapid CH(3)Hg accumulation is coupled to rapid sulfate reduction. The observations described above have broad implications for understanding the control of CH(3)Hg formation and for developing remediation strategies for Hg-contaminated sediments.

FIG. 1

Growth curves for five pure cultures of SRB during Hg methylation experiments. Inorganic Hg (100 ng ml⁻¹) was added to cultures 120 h after inoculation.

FIG. 2

(A) Sulfate reduction in Desulfovibrio desulfuricans cultures over a 120-h period. The SRR was calculated from the results of a linear regression analysis of the amount of sulfate reduced over time. Inorganic Hg (100 ng ml⁻¹) was added at 12 h. (B) MMR in pure cultures of Desulfovibrio desulfuricans. Inorganic Hg (100 ng ml⁻¹) was added at zero time. A linear regression analysis was performed with CH₃Hg concentrations obtained between 24 and 96 h.

FIG. 3

(A) CH₃Hg concentrations in pure cultures of SRB. (B) CH₃Hg production in the presence of Desulfovibrio and Desulfobacterium cells and in control cultures. Inorganic Hg (100 ng ml⁻¹) was added at zero time in both experiments.

FIG. 4

(A) Sulfate reduction in sediment slurries amended with lactate and acetate. Lactate or acetate was added to slurries 20 days before inorganic Hg was added. Inorganic Hg (950 ng g⁻¹) was added at zero time. (B) Sulfate reduction in unamended or control sediment slurries. Inorganic Hg (950 ng g⁻¹) was added to slurries at zero time. The data points at each time represent samples acquired from three slurries that received the same treatment, and the lines represent the results of linear regression analysis which was used to generate an average SRR for the duration of the experiment.

FIG. 5

(A) CH₃Hg production in sediment slurries amended with lactate and acetate. (B) CH₃Hg concentrations in control sediment slurries. Inorganic Hg (950 ng g⁻¹) was added to slurries at zero time. The data points each represent triplicate samples which received the same sediment treatment. The slurries represented above were the same slurries as those used in the experiments whose results are shown in Fig. 4.

FIG. 6

Plot of MMR normalized to SRR in sediment slurries as a function of total soluble Hg concentrations. The MMR for each sediment slurry was normalized to the observed SRR for the same slurry and was plotted with respect to the average observed total soluble Hg concentration.