Approximately 60% or more of the added Hg(II) is taken up passively by both live and inactivated cells (left). A schematic representation of passive uptake of Hg and its handoff to HgcAB leading to methylmercury (MeHg) production (right). OM = outer membrane; IM = inner membrane.
Study reveals that, contrary to current views of active Hg uptake, microbial cells can take up Hg passively without a specific transporter
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) was taken up passively in 48 hours 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 methylmercury (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 improved understanding of Hg(II) bioavailability for methylation.
Contrary to current views of active Hg(II) uptake, we found that active metabolism is not required for cellular Hg(II) uptake, and Hg can get into cells without a specific transporter.
Although recent studies have identified HgcAB proteins as being responsible for Hg(II) methylation by certain anaerobic microorganisms, it remains controversial whether microbes take up Hg(II) passively or actively, and what factors control Hg(II) uptake by methylating organisms. Study suggests that intracellular Hg binding and handoff to HgcA, rather than uptake, is likely the driving factor for Hg methylation.
An, J., L. Zhang, X. Lu, E. M. Pierce, A. Johs, J. M. Parks, and B. Gu. 2019. “Mercury uptake by Desulfovibrio desulfuricans ND132: Passive or active?” Environmental Science & Technology. 53(11): 6264–272. DOI: 10.1021/acs.est.9b00047.
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