Nanomolar Copper Enhances Mercury Methylation by <i>Desulfovibrio desulfuricans</i> ND132

Methylmercury (MeHg) is produced by certain anaerobic microorganisms, such as the sulfate-reducing bacterium <i>Desulfovibrio desulfuricans</i> ND132, but environmental factors affecting inorganic mercury [Hg­(II)] uptake and methylation remain unclear. We report that the presence of a small amount of copper ions [Cu­(II), <100 nM] enhances Hg­(II) uptake and methylation by washed cells of ND132, while Hg­(II) methylation is inhibited at higher Cu­(II) concentrations because of the toxicity of copper to the microorganism. The enhancement or inhibitory effect of Cu­(II) is dependent on both time and concentration. The presence of nanomolar concentrations of Cu­(II) facilitates rapid uptake of Hg­(II) (within minutes) and doubles MeHg production within a 24 h period, but micromolar concentrations of Cu­(II) completely inhibit Hg­(II) methylation. Metal ions such as zinc [Zn­(II)] and nickel [Ni­(II)] also inhibit but do not enhance Hg­(II) methylation under the same experimental conditions. These observations suggest a synergistic effect of Cu­(II) on Hg­(II) uptake and methylation, possibly facilitated by copper transporters or metallochaperones in this organism, and highlight the fact that complex environmental factors affect MeHg production in the environment

Contrasting Effects of Dissolved Organic Matter on Mercury Methylation by <i>Geobacter sulfurreducens</i> PCA and <i>Desulfovibrio desulfuricans</i> ND132

Natural dissolved organic matter (DOM) affects mercury (Hg) redox reactions and anaerobic microbial methylation in the environment. Several studies have shown that DOM can enhance Hg methylation, especially under sulfidic conditions, whereas others show that DOM inhibits Hg methylation due to strong Hg–DOM complexation. In this study, we investigated and compared the effects of DOM on Hg methylation by an iron-reducing bacterium <i>Geobacter sulfurreducens</i> PCA and a sulfate-reducing bacterium <i>Desulfovibrio desulfuricans</i> ND132 under nonsulfidic conditions. The methylation experiment was performed with washed cells either in the absence or presence of DOM or glutathione, both of which form strong complexes with Hg via thiol-functional groups. DOM was found to greatly inhibit Hg methylation by <i>G. Sulfurreducens</i> PCA but enhance Hg methylation by <i>D. desulfuricans</i> ND132 cells with increasing DOM concentration. These strain-dependent opposing effects of DOM were also observed with glutathione, suggesting that thiols in DOM likely played an essential role in affecting microbial Hg uptake and methylation. Additionally, DOM and glutathione greatly decreased Hg sorption by <i>G. sulfurreducens</i> PCA but showed little effect on <i>D. desulfuricans</i> ND132 cells, demonstrating that ND132 has a higher affinity to sorb or take up Hg than the PCA strain. These observations indicate that DOM effects on Hg methylation are bacterial strain specific, depend on the DOM:Hg ratio or site-specific conditions, and may thus offer new insights into the role of DOM in methylmercury production in the environment

Biological Redox Cycling of Iron in Nontronite and Its Potential Application in Nitrate Removal

Biological redox cycling of structural Fe in phyllosilicates is an important but poorly understood process. The objective of this research was to study microbially mediated redox cycles of Fe in nontronite (NAu-2). During the reduction phase, structural Fe­(III) in NAu-2 served as electron acceptor, lactate as electron donor, AQDS as electron shuttle, and dissimilatory Fe­(III)-reducing bacterium <i>Shewanella putrefaciens</i> CN32 as mediator in bicarbonate- and PIPES-buffered media. During the oxidation phase, biogenic Fe­(II) served as electron donor and nitrate as electron acceptor. Nitrate-dependent Fe­(II)-oxidizing bacterium <i>Pseudogulbenkiania</i> sp. strain 2002 was added as mediator in the same media. For all three cycles, structural Fe in NAu-2 was able to reversibly undergo three redox cycles without significant dissolution. Fe­(II) in bioreduced samples occurred in two distinct environments, at edges and in the interior of the NAu-2 structure. Nitrate reduction to nitrogen gas was coupled with oxidation of edge-Fe­(II) and part of interior-Fe­(II) under both buffer conditions, and its extent and rate did not change with Fe redox cycles. These results suggest that biological redox cycling of structural Fe in phyllosilicates is a reversible process and has important implications for biogeochemical cycles of carbon, nitrogen, and other nutrients in natural environments

Anaerobic Mercury Methylation and Demethylation by <i>Geobacter bemidjiensis</i> Bem

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 <i>Geobacter bemidjiensis</i> Bem. Here we report, for the first time, that the strain <i>G. bemidjiensis</i> 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 <i>G. bemidjiensis</i> 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

Effect of Hg exposure on acidic compartmentalization of <i>A</i>. <i>parkinsoniana</i> labeled with AO.

<p>Epifluorescence micrographs of single optical sections showing overlay of AO green and red fluorescence for (A) T1-control, (B,C) T1-100 ppm, and (D) T2-100 ppm, Bars: 20μm. (E) Histogram of maximum dimension (diameter) of acidic vesicles. (F) Histogram of red Mean Fluorescence Intensity (MFI) expressed in arbitrary units (A.U.) for control and 100 ppm at both T1 and T2. Error bars indicate ± standard error of the mean.</p

Effect of Hg exposure on lipid distribution of <i>A</i>. <i>parkinsoniana</i> labeled with NR.

<p>Epifluoresecence micrographs of single optical sections showing overlay of NR yellow and red fluorescence for (A) T1- control, (B) T1-1 ppm, (C) T1-100 ppm and (D) T2-100 ppm, Bars: 20 μm. (E) Histogram of yellow Mean Fluorescence Intensity (MFI) expressed in arbitrary units (A.U.) for the three treatments over time (control, 1 ppm, and 100 ppm at both T1 and T2). Error bars indicate ± standard error of the mean.</p

Micrographs showing presence of Hg in <i>A</i>. <i>parkinsoniana</i> specimens (T2-100 ppm).

<p>(A) young chamber containing vacuoles; (B) high magnification of a young chamber; (C,D) basal part of pores; (E) foramen/septum. (F) Example EDS spectrum taken with a spot size of 4 on cross (E). Arrows mark the occurrence of Hg. Scale bar: (A) 10 μm; (B) 1.5 μm; (C) 2.5 μm; (D) 0.8 μm; (E) 5 μm.</p