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

TLDR

Sulfate reducers methylate Hg²⁺ only under sulfate‑limiting, fermentable substrate conditions, a capability previously unrecognized. The study aimed to directly identify the microorganisms responsible for monomethylmercury production in anoxic saltmarsh sediment spiked with Hg²⁺. The authors applied substrate‑electron acceptor combinations and specific metabolic inhibitors to the Hg²⁺‑spiked sediment to pinpoint the methylating microorganisms. Inhibiting methanogens increased monomethylmercury synthesis, whereas inhibiting sulfate reducers reduced it by >95%; enrichment isolated a Desulfovibrio desulfuricans strain that vigorously methylated Hg²⁺, establishing sulfate reducers as the principal methylators in anoxic sediments and questioning prior pure‑culture methylation work.

Abstract

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.