Mercury Uptake by Desulfovibrio desulfuricans ND132: Passive or Active?

Recent studies have identified HgcAB proteins as being responsible for mercury [Hg­(II)] methylation by certain anaerobic microorganisms. However, it remains controversial whether microbes take up Hg­(II) passively or actively. Here, we examine the dynamics of concurrent Hg­(II) adsorption, uptake, and methylation by both viable and inactivated cells (heat-killed or starved) or spheroplasts of the sulfate-reducing bacterium Desulfovibrio desulfuricans ND132 in laboratory incubations. We show that, without addition of thiols, >60% of the added Hg­(II) (25 nM) was taken up passively in 48 h by live and inactivated cells and also by cells treated with the proton gradient uncoupler, carbonylcyanide-3-chlorophenylhydrazone (CCCP). Inactivation abolished Hg­(II) methylation, but the cells continued taking up Hg­(II), likely through competitive binding or ligand exchange of Hg­(II) by intracellular proteins or thiol-containing cellular components. Similarly, treatment with CCCP impaired the ability of spheroplasts to methylate Hg­(II) but did not stop Hg­(II) uptake. Spheroplasts showed a greater capacity to adsorb Hg­(II) than whole cells, and the level of cytoplasmic membrane-bound Hg­(II) correlated well with MeHg production, as Hg­(II) methylation is associated with cytoplasmic HgcAB. Our results indicate that active metabolism is not required for cellular Hg­(II) uptake, thereby providing an improved understanding of Hg­(II) bioavailability for methylation