Sulfate-Reducing Bacteria: Principal Methylators of Mercury in Anoxic Estuarine Sediment

Substrate-electron acceptor combinations and specific metabolic inhibitors were applied to anoxic saltmarsh sediment spiked with mercuric ions (Hg ²⁺ ) in an effort to identify, by a direct approach, the microorganisms responsible for the synthesis of hazardous monomethylmercury. 2-Bromoethane sulfonate (30 mM), a specific inhibitor of methanogens, increased monomethylmercury synthesis, whereas sodium molybdate (20 mM), a specific inhibitor of sulfate reducers, decreased Hg ²⁺ methylation by more than 95%. Anaerobic enrichment and isolation procedures yielded a Desulfovibrio desulfuricans culture that vigorously methylated Hg ²⁺ in culture solution and also in samples of presterilized sediment. The Hg ²⁺ methylation activity of sulfate reducers is fully expressed only when sulfate is limiting and fermentable organic substrates are available. To date, sulfate reducers have not been suspected of Hg ²⁺ methylation. Identification of these bacteria as the principal methylators of Hg ²⁺ in anoxic sediments raises questions about the environmental relevance of previous pure culture-based methylation work.