Time-Resolved Dynamics of Mercury Uptake, Methylmercury Production, and Export by Pseudodesulfovibrio hydrargyri BerOc1 at Different Mercury Concentrations

Methylmercury (MeHg) is highly toxic and is mainly produced in anoxic environments by certain microorganisms. Net MeHg production involves a series of separate cellular processes: the uptake of inorganic divalent Hg (Hg(II)) by the cell, intracellular enzymatic Hg(II) methylation, and the release of MeHg into the extracellular medium, as well as MeHg demethylation. As a biological process, saturation at the cellular level can be anticipated at all stages of the Hg transformation. The aim of this study was to investigate the kinetics of Hg(II) methylation and MeHg demethylation over a 24-h period in the model sulfate-reducing strain Pseudodesulfovibrio hydrargyri BerOc1, across a range of Hg(II) concentrations from 0.03 to 3.15 μM. The distribution of Hg(II) and MeHg over 24 h within three cellular fractions (extracellular, adsorbed to the cells, and intracellular) was determined to estimate Hg uptake and export. With increasing Hg(II) concentrations, we observed (i) an increase in the accumulated intracellular Hg(II), (ii) a reduction in the methylation rate, and (iii) an increase in MeHg associated with the cells after a short Hg(II) exposure time (<1 h). Our study suggests that the saturation of MeHg production is likely not driven by Hg(II) uptake but rather by Hg(II) intracellular speciation, Hg(II) methylation by HgcAB proteins, and/or MeHg export. These results are essential to better predict and understand the parameters influencing MeHg production within more complex environments, such as anoxic sediments and soils.