Studies on the methylation of mercuric chloride by pure cultures of bacteria and fungi

Vonk, J. W.; Sijpesteijn, A. Kaars

doi:10.1007/bf02578894

Cited by 101 publications

(40 citation statements)

References 10 publications

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“…A spectrum of organisms have been shown to methylate mercury in pure culture, including Neurospora crassa, Clostridium cochlearium, Pseudomonas fluorescens, Enterobacter aerogenes, Desulfovibrio desulfuricans, and Desulfobacter sp. strain BG-8 (12,22,34,37,52,66,73). However, some of these organisms are not involved in dominant terminal-electron-accepting processes in anoxic sediments and consequently probably have a limited contribution to in situ production of methylmercury.…”

Section: Fig 2 Cell Density (Circles) and Methylmercury (Mehg) Concmentioning

confidence: 99%

Mercury Methylation from Unexpected Sources: Molybdate-Inhibited Freshwater Sediments and an Iron-Reducing Bacterium

Fleming

Mack

Green

et al. 2006

Appl Environ Microbiol

534

329

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Methylmercury has been thought to be produced predominantly by sulfate-reducing bacteria in anoxic sediments. Here we show that in circumneutral pH sediments (Clear Lake, CA) application of a specific inhibitor of sulfate-reducing bacteria at appropriate concentrations typically inhibited less than one-half of all anaerobic methylation of added divalent mercury. This suggests that one or more additional groups of microbes are active methylators in these sediments impacted by a nearby abandoned mercury mine. From Clear Lake sediments, we isolated the iron-reducing bacterium Geobacter sp. strain CLFeRB, which can methylate mercury at a rate comparable to Desulfobulbus propionicus strain 1pr3, a sulfate-reducing bacterium known to be an active methylator. This is the first time that an iron-reducing bacterium has been shown to methylate mercury at environmentally significant rates. We suggest that mercury methylation by iron-reducing bacteria represents a previously unidentified and potentially significant source of this environmental toxin in iron-rich freshwater sediments.Mercury has become a global concern due to its toxic properties and has contaminated water sources through atmospheric deposition, weathering of cinnabar, runoff from industrial sites and abandoned mines, and microbial production of acid-rock drainage. In California, acid-rock drainage from mercury deposits in the Coast Range and mercury used in the Sierra Nevada foothills for gold recovery are the dominant sources of water contamination. Clear Lake, a eutrophic lake in the mercury belt of the California Coast Range, receives acid-rock drainage from the Sulfur Bank Mercury mine located on the northeastern edge of the lake. There sediment concentrations of mercury can exceed 400 ppm and decline exponentially with distance from the mine (58, 59). In Clear Lake food chain bioaccumulation caused mercury to reach levels in fish tissues (Ͼ95% as methylmercury) that triggered a state health advisory limiting consumption of 10 fish species (16).Mercury is converted to methylmercury in anoxic sediments (72) via incompletely characterized mechanisms that are classically attributed to sulfate-reducing bacteria (12,17). This conclusion principally rests on the observation that estuarine sediments known to methylate exogenous mercury failed to do so when incubated under oxic conditions or in the presence of molybdate, an inhibitor that disrupts the central energy metabolism of sulfate-reducing bacteria. Sulfate-reducing bacteria have been regarded as the principal methylators in both marine and freshwater sediments, with no contribution consistently ascribed to other metabolically defined groups of Bacteria or Archaea. Recently, in certain riverine sediments from the southeastern United States a substantial portion of biological potential for mercury methylation could be attributed to activity of organisms other than sulfate-reducing bacteria (67).Because these sediments contained iron minerals and because reduction of iron was the dominant terminal electron...

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Section: Fig 2 Cell Density (Circles) and Methylmercury (Mehg) Concmentioning

confidence: 99%

Mercury Methylation from Unexpected Sources: Molybdate-Inhibited Freshwater Sediments and an Iron-Reducing Bacterium

Fleming

Mack

Green

et al. 2006

Appl Environ Microbiol

534

329

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“…Methylmercury production is an anaerobic process that occurs in saturated soils and wetlands (26,44,45,53), decaying periphyton mats (1, 14, 31), aquatic bottom sediments (16,27,33,36), and anaerobic bottom waters (56). Early investigations, prior to the advent of modern methylmercury (MeHg) analyses, reported a wide variety of aerobic and anaerobic Gram-positive and Gram-negative bacteria (30,49,55,58) and fungi (55) to be capable of MeHg production. However, subsequent studies with pure cultures have conclusively demonstrated a role only for sulfate-reducing bacteria (SRB) (4,8,11,13,20,23,38,50) and iron-reducing bacteria (FeRB; principally Geobacter spp.)…”

mentioning

confidence: 99%

Detailed Assessment of the Kinetics of Hg-Cell Association, Hg Methylation, and Methylmercury Degradation in Several Desulfovibrio Species

Graham

Bullock

Maizel

et al. 2012

Appl Environ Microbiol

125

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. We tested the hypothesis that differences in Hg(II) i sorption and/or uptake rates drive observed differences in methylation rates among Desulfovibrio species. Hg(II) i associated rapidly and with high affinity to both methylating and nonmethylating species. MeHg production by Hg-methylating strains was rapid, plateauing after ϳ3 h. All MeHg produced was rapidly exported. We also tested the idea that all Desulfovibrio species are capable of Hg(II) i methylation but that rapid demethylation masks its production, but we found this was not the case. Therefore, the underlying reason why MeHg production capability is not universal in the Desulfovibrio is not differences in Hg affinity for cells nor differences in the ability of strains to degrade MeHg. However, Hg methylation rates varied substantially between Hg-methylating Desulfovibrio species even in these controlled experiments and after normalization to cell density. Thus, biological differences may drive crossspecies differences in Hg methylation rates. As part of this study, Microbial mercury methylation is the main driver of risk associated with Hg pollution. Methylmercury production is an anaerobic process that occurs in saturated soils and wetlands (26,44,45,53), decaying periphyton mats (1, 14, 31), aquatic bottom sediments (16,27,33,36), and anaerobic bottom waters (56). Early investigations, prior to the advent of modern methylmercury (MeHg) analyses, reported a wide variety of aerobic and anaerobic Gram-positive and Gram-negative bacteria (30,49,55,58) and fungi (55) to be capable of MeHg production. However, subsequent studies with pure cultures have conclusively demonstrated a role only for sulfate-reducing bacteria (SRB) (4,8,11,13,20,23,38,50) and iron-reducing bacteria (FeRB; principally Geobacter spp.) (21, 37), all belonging to the Deltaproteobacteria. Many field studies, using selective microbial stimulants (1, 10, 26, 44, 57), inhibitors (1, 16, 24, 26, 59), and biogeochemical correlates (6,39,40,45), have buttressed the paradigm of SRB and FeRB as the dominant Hg methylators in natural aquatic systems (16,24,59), though recent studies have hypothesized that methanogens may be significant in some systems (31).Only a subset of SRB and FeRB are capable of Hg methylation (11,23,37,50), but why this is the case remains unclear. Early work by Choi and Bartha (13) suggested that Hg methylation was a "metabolic mistake" of SRB utilizing the acetyl coenzyme A (acetyl-CoA) pathway for carbon metabolism. Subsequent studies, however, indicated that Hg methylation capability is not restricted to SRB possessing the acetyl-CoA pathway (20). At present, it is not possible to conclusively identify the methyltransferase or methyl donor in SRB (or other Deltaproteobacteria) responsible for in vivo Hg methylation. Hg methylation occurs intracellularly (23), and significant effort has therefore been devoted to elucidating the mechanism(s) of Hg uptake by Hg-methylating bacteria.Passive diffusion of neutral HgS species has been hypothesized to control Hg uptak...

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“…This proposition, like methylation in the oxygenated marine column (see above), implies methylation by aerobic microorganisms. Many aerobic microorganisms may methylate Hg, e.g., bacteria belonging to the Pseudomonas, Enterobacter, Bacillus, and Staphylococci genera and fungi such as Aspergillus niger, Scopulariopsis brevicattlis and Saccharomyces cerevisiae (Vonk & Sijpesteijn 1973), and the activity of these microbes may be environmentally relevant but remains to be demonstrated.…”

Section: Methylation In the Marine Water Columnmentioning

confidence: 99%

Some like it cold: microbial transformations of mercury in polar regions

2011

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The contamination of polar regions with mercury that is transported from lower latitudes as inorganic mercury has resulted in the accumulation of methylmercury (MeHg) in food chains, risking the health of humans and wildlife. While production of MeHg has been documented in polar marine and terrestrial environments, little is known about the responsible transformations and transport pathways and the processes that control them. We posit that as in temperate environments, microbial transformations play a key role in mercury geochemical cycling in polar regions by: (1) methylating mercury by one of four proposed pathways, some not previously described; (2) degrading MeHg by activities of mercury resistant and other bacteria; and (3) carrying out redox transformations that control the supply of the mercuric ion, the substrate of methylation reactions. Recent analyses have identified a high potential for mercury-resistant microbes that express the enzyme mercuric reductase to affect the production of gaseous elemental mercury when and where daylight is limited. The integration of microbially mediated processes in the paradigms that describe mercury geochemical cycling is therefore of high priority especially in light of concerns regarding the effect of global warming and permafrost thawing on input of MeHg to polar regions

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Studies on the methylation of mercuric chloride by pure cultures of bacteria and fungi

Cited by 101 publications

References 10 publications

Mercury Methylation from Unexpected Sources: Molybdate-Inhibited Freshwater Sediments and an Iron-Reducing Bacterium

Mercury Methylation from Unexpected Sources: Molybdate-Inhibited Freshwater Sediments and an Iron-Reducing Bacterium

Detailed Assessment of the Kinetics of Hg-Cell Association, Hg Methylation, and Methylmercury Degradation in Several Desulfovibrio Species

Some like it cold: microbial transformations of mercury in polar regions

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