Effect of Exogenous and Endogenous Sulfide on The Production and The Export of Methylmercury by Sulfate Reducing Bacteria

Abstract

Mercury (Hg) is a global pollutant of environmental and health concern; its methylated form, methylmercury (MeHg) is a potent neurotoxin. Sulfur-containing molecules play a role in MeHg production by microorganisms. While sulfides are considered to limit Hg methylation, sulfate and cysteine were shown to favor this process. However, these two forms can be endogenously converted by microorganisms into sulfide. Here, we explore the effect of sulfide (produced by the cell or supplied exogenously) on Hg methylation. For this purpose, Pseudodesulfovibrio hydrargyri BerOc1 was cultivated in non-sulfidogenic conditions with addition of cysteine and sulfide as well as in sulfidogenic conditions. We report that Hg methylation depends on sulfide concentration in the culture rather than on the initial form of sulfur (cysteine, sulfide or sulfate) added, and was independent of hgcA expression. Interestingly, MeHg production was maximal at 0.1-0.5 mM of sulfides. Besides, a strong positive correlation between MeHg in the extracellular medium and the increase of sulfide concentrations was observed, suggesting a facilitated MeHg export with sulfide and/or higher desorption from the cell. We demonstrate that sulfides (exogenous or endogenous) play a key role in controlling mercury methylation, and should be considered when investigating the impact of Hg on natural environments.

Transcriptional control ofhgcABby an ArsR-like regulator inPseudodesulfovibrio mercuriiND132

“…Differences in Hg bioavailability may contribute to the differences in MeHg production among treatments. Sulfide has strong effects on Hg bioavailability for methylation(12, 30, 79, 80) and, although grown without sulfate, ND132 cleaves the S−C bond in cysteine provided in growth medium to release small amounts of sulfide, generally below 10 μM(51). We consistently measured significantly less sulfide in filtered samples from As(V)-amended washed-cell (Figure S7) and batch culture assays (Figure S11) compared to other treatments, including As(III).…”