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.