Sulfate-reducing bacteria (SRB) in anoxic waters and sediments are the major producers of methylmercury in aquatic systems. Although a considerable amount of work has addressed the environmental factors that control methylmercury formation and the conditions that control bioavailability of inorganic mercury to SRB, little work has been undertaken analyzing the biochemical mechanism of methylmercury production. The acetyl-coenzyme A (CoA) pathway has been implicated as being key to mercury methylation in one SRB strain, Desulfovibrio desulfuricans LS, but this result has not been extended to other SRB species. To probe whether the acetyl-CoA pathway is the controlling biochemical process for methylmercury production in SRB, five incomplete-oxidizing SRB strains and two Desulfobacter strains that do not use the acetyl-CoA pathway for major carbon metabolism were assayed for methylmercury formation and acetyl-CoA pathway enzyme activities. Three of the SRB strains were also incubated with chloroform to inhibit the acetyl-CoA pathway. So far, all species that have been found to have acetyl-CoA activity are complete oxidizers that require the acetyl-CoA pathway for basic metabolism, as well as methylate mercury. Chloroform inhibits Hg methylation in these species either by blocking the methylating enzyme or by indirect effects on metabolism and growth. However, we have identified four incomplete-oxidizing strains that clearly do not utilize the acetyl-CoA pathway either for metabolism or mercury methylation (as confirmed by the absence of chloroform inhibition). Hg methylation is thus independent of the acetyl-CoA pathway and may not require vitamin B 12 in some and perhaps many incomplete-oxidizing SRB strains.

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Aluminum, one of the most abundant elements in soils and sediments, is commonly found co-precipitated with Fe in natural Fe(III) (hydr)oxides; yet, little is known about how Al substitution impacts bacterial Fe(III) reduction. Accordingly, we investigated the reduction of Al substituted (0-13 mol% Al) goethite, lepidocrocite, and ferrihydrite by the model dissimilatory Fe(III)-reducing bacterium (DIRB), Shewanella putrefaciens CN32. Here we reveal that the impact of Al on microbial reduction varies with Fe(III) (hydr)oxide type. No significant difference in Fe(III) reduction was observed for either goethite or lepidocrocite as a function of Al substitution. In contrast, Fe(III) reduction rates significantly decreased with increasing Al substitution of ferrihydrite, with reduction rates of 13% Al-ferrihydrite more than 50% lower than pure ferrihydrite. Although Al substitution changed the minerals' surface area, particle size, structural disorder, and abiotic dissolution rates, we did not observe a direct correlation between any of these physiochemical properties and the trends in bacterial Fe(III) reduction. Based on projected Al-dependent Fe(III) reduction rates, reduction rates of ferrihydrite fall below those of lepidocrocite and goethite at substitution levels equal to or greater than 18 mol% Al. Given the prevalence of Al substitution in natural Fe(III) (hydr)oxides, our results bring into question the conventional assumptions about Fe (hydr)oxide bioavailability and suggest a more prominent role of natural lepidocrocite and goethite phases in impacting DIRB activity in soils and sediments.

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 physiologyand 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-oxidizer that 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 3 times 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.

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