Anaerobic Mercury Methylation and Demethylation by Geobacter bemidjiensis Bem

Abstract

Microbial methylation and demethylation are two competing processes controlling the net production and bioaccumulation of neurotoxic methylmercury (MeHg) in natural ecosystems. Although mercury (Hg) methylation by anaerobic microorganisms and demethylation by aerobic Hg-resistant bacteria have both been extensively studied, little attention has been given to MeHg degradation by anaerobic bacteria, particularly the iron-reducing bacterium Geobacter bemidjiensis Bem. Here we report, for the first time, that the strain G. bemidjiensis Bem can mediate a suite of Hg transformations, including Hg(II) reduction, Hg(0) oxidation, MeHg production and degradation under anoxic conditions. Results suggest that G. bemidjiensis utilizes a reductive demethylation pathway to degrade MeHg, with elemental Hg(0) as the major reaction product, possibly due to the presence of genes encoding homologues of an organomercurial lyase (MerB) and a mercuric reductase (MerA). In addition, the cells can strongly sorb Hg(II) and MeHg, reduce or oxidize Hg, resulting in both time and concentration-dependent Hg species transformations. Moderate concentrations (10-500 μM) of Hg-binding ligands such as cysteine enhance Hg(II) methylation but inhibit MeHg degradation. These findings indicate a cycle of Hg methylation and demethylation among anaerobic bacteria, thereby influencing net MeHg production in anoxic water and sediments.

Methylmercury uptake and degradation by methanotrophs

“…However, with a longer incubation time (120 hours), the cells were able to recover and degrade a substantial amount of CH 3 Hg + (71%). This initially inhibited CH 3 Hg + degradation may be interpreted as a result of potential toxic effects of CH 3 Hg + on M. trichosporium OB3b, similar to that observed with Geobacter bemidjiensis Bem ( 17 ). …”

“…CH 3 Hg + working solution (10 nM) was prepared by diluting 5 μM stock solution (CH 3 HgOH in 0.5% acetic acid and 0.2% HCl from Brooks Rand Labs) in MOPS. The reaction was initiated by mixing 0.5 ml of CH 3 Hg + working solution with 0.5 ml of washed cells to give a final concentration of CH 3 Hg + at 5 nM and of cells at 1 × 10 8 cells ml −1 ( 17 ), or otherwise specified. Samples were then placed on a rotary shaker, kept at 30°C for M. trichosporium OB3b and its mutants, and at 45°C for M. capsulatus Bath.…”

“…Replicate sample vials were taken at selected time points and analyzed as follows. For CH 3 Hg + sorption (or uptake) analysis, triplicate samples were filtered through 0.2-μm syringe filters (to remove cells) and analyzed for CH 3 Hg + sol ( 17 , 32 ). The unfiltered samples were used to determine the total Hg and total CH 3 Hg + (CH 3 Hg + Total ) so that the cell-associated or total sorbed Hg can be calculated by their difference.…”

“…The unfiltered samples were used to determine the total Hg and total CH 3 Hg + (CH 3 Hg + Total ) so that the cell-associated or total sorbed Hg can be calculated by their difference. For Hg species distribution analyses, six replicate samples (in separate vials) were taken, and three of them were filtered as above and analyzed for total soluble Hg (Hg sol ) and CH 3 Hg + sol ( 17 , 33 ). The remaining three samples were used to determine cellular uptake of CH 3 Hg + (CH 3 Hg + up ) and cell surface–adsorbed CH 3 Hg + (CH 3 Hg + ad ).…”

“…However, only certain aerobic prokaryotes have this CH 3 Hg + degradation pathway. mer -mediated pathway is operative only at extremely high Hg concentrations (that is, micromolar) ( 7 , 17 , 21 ), conditions that are largely irrelevant to most natural waters and sediments, where Hg or CH 3 Hg + concentrations are usually at picomolar to low nanomolar ranges ( 7 , 17 ). In addition, in vitro experiments with the isolated MerB enzyme showed that demethylation by MerB occurs generally at above neutral pH conditions, with an optimal pH of ~10 ( 21 ).…”