The influence of sulfide on solid phase mercury bioavailability for methylation by pure cultures of Desulfobulbus propionicus (1pr3)

To help understand the mechanism and control of Hg uptake in Hg-methylating bacteria, we investigated the effect of sulfide on Hg methylation by pure cultures of the sulfate-reducing bacterium Desulfobulbus propionicus (1pr3) . Our previous research in natural sediments has Suggested that Hg methylat ion occurs most rapidly when sulfide concentrations favor formation of neut ral dissolved Hg-S species. In this study, the chemical speciation of Hg in culture media was manipulated by growing D, propionicus across a range of sulfide concentrations, with inorganic Hg (Hg-1) added in the form of groun d ores. A solid-phase, rather than a dissolved source of Hg, was used to si mulate the controls on Hg partitioning between solid and aqueous phases fou nd in natural sediments. Methylmercury (MeHg) production by cultures was no t related to the absolute solid-phase concentration of Hg in the ores, and it was only weakly related to the dissolved Hg-1 concentration in the mediu m. However, MeHg production was linearly related to the calculated concentr ation of the dominant neutral complex in solution, HgS degrees. Furthermore , the diffusive membrane permeability of HgS degrees, as estimated from its octanol-water partitioning coefficient, was found to be sufficient to supp ort MeHg production by cells. The present paper expands on our previous wor k by providing experimental support of our hypothesis that sulfide influenc es methylation by affecting the speciation of dissolved Hg-1 and its uptake via passive diffusion.