Microbial Mercury Methylation Research Articles

Mercury exposed: Advances in environmental analysis and ecotoxicology of a highly toxic metal

Mercury exposed: Advances in environmental analysis and ecotoxicology of a highly toxic metal

Mechanisms Regulating Mercury Bioavailability for Methylating Microorganisms in the Aquatic Environment: A Critical Review

Mercury is a potent neurotoxin for humans, particularly if the metal is in the form of methylmercury. Mercury is widely distributed in aquatic ecosystems as a result of anthropogenic activities and natural earth processes. A first step toward bioaccumulation of methylmercury in aquatic food webs is the methylation of inorganic forms of the metal, a process that is primarily mediated by anaerobic bacteria. In this Review, we evaluate the current state of knowledge regarding the mechanisms regulating microbial mercury methylation, including the speciation of mercury in environments where methylation occurs and the processes that control mercury bioavailability to these organisms. Methylmercury production rates are generally related to the presence and productivity of methylating bacteria and also the uptake of inorganic mercury to these microorganisms. Our understanding of the mechanisms behind methylation is limited due to fundamental questions related to the geochemical forms of mercury that persist in anoxic settings, the mode of uptake by methylating bacteria, and the biochemical pathway by which these microorganisms produce and degrade methylmercury. In anoxic sediments and water, the geochemical forms of mercury (and subsequent bioavailability) are largely governed by reactions between Hg(II), inorganic sulfides, and natural organic matter. These interactions result in a mixture of dissolved, nanoparticulate, and larger crystalline particles that cannot be adequately represented by conventional chemical equilibrium models for Hg bioavailability. We discuss recent advances in nanogeochemistry and environmental microbiology that can provide new tools and unique perspectives to help us solve the question of how microorganisms methylate mercury. An understanding of the factors that cause the production and degradation of methylmercury in the environment is ultimately needed to inform policy makers and develop long-term strategies for controlling mercury contamination.

Contribution of Coexisting Sulfate and Iron Reducing Bacteria to Methylmercury Production in Freshwater River Sediments

We investigated microbial methylmercury (CH(3)Hg) production in sediments from the South River (SR), VA, an ecosystem contaminated with industrial mercury (Hg). Potential Hg methylation rates in samples collected at nine sites were low in late spring and significantly higher in late summer. Demethylation of (14)CH(3)Hg was dominated by (14)CH(4) production in spring, but switched to producing mostly (14)CO(2) in the summer. Fine-grained sediments originating from the erosion of river banks had the highest CH(3)Hg concentrations and were potential hot spots for both methylation and demethylation activities. Sequencing of 16S rRNA genes of cDNA recovered from sediment RNA extracts indicated that at least three groups of sulfate-reducing bacteria (SRB) and one group of iron-reducing bacteria (IRB), potential Hg methylators, were active in SR sediments. SRB were confirmed as a methylating guild by amendment experiments showing significant sulfate stimulation and molybdate inhibition of methylation in SR sediments. The addition of low levels of amorphous iron(III) oxyhydroxide significantly stimulated methylation rates, suggesting a role for IRB in CH(3)Hg synthesis. Overall, our studies suggest that coexisting SRB and IRB populations in river sediments contribute to Hg methylation, possibly by temporally and spatially separated processes.

Genome Sequence of the Mercury-Methylating and Pleomorphic Desulfovibrio africanus Strain Walvis Bay

Desulfovibrio africanus strain Walvis Bay is an anaerobic sulfate-reducing bacterium capable of producing methylmercury (MeHg), a potent human neurotoxin. The mechanism of methylation by this and other organisms is unknown. We present the 4.2-Mb genome sequence to provide further insight into microbial mercury methylation and sulfate-reducing bacteria.

Genome Sequence of the Mercury-Methylating Strain Desulfovibrio desulfuricans ND132

Desulfovibrio desulfuricans strain ND132 is an anaerobic sulfate-reducing bacterium (SRB) capable of producing methylmercury (MeHg), a potent human neurotoxin. The mechanism of methylation by this and other organisms is unknown. We present the 3.8-Mb genome sequence to provide further insight into microbial mercury methylation.

Growth and Methylmercury Accumulation in Juvenile Chinook Salmon in the Sacramento River and Its Floodplain, the Yolo Bypass

AbstractThe literature indicates a strong correlation between inundation of previously oxidized soils, as can occur on a floodplain, and increased microbial methylation of mercury. There is special concern over the potential for increased methylmercury levels in the Yolo Bypass, a 24,000‐ha floodplain for California's Sacramento River and its tributaries. The objective of our first study component was to compare methylmercury accumulation between juvenile Chinook salmon Oncorhynchus tshawytscha in the Yolo Bypass and those in the Sacramento River during a winter 2005 flood event. For each location, we tested accumulation in two groups of hatchery Chinook salmon juveniles: (1) free‐ranging, coded‐wire‐tagged fish that were released into the floodplain and river and recaptured by downstream sampling and (2) fish that were reared in enclosures at fixed locations in both the river and floodplain. We found that free‐ranging juvenile Chinook salmon in the floodplain accumulated 3.2% more methylmercury per day than did free‐ranging fish in the river. However, fish in the floodplain grew 0.7% more per day than fish in the river. Variance in growth and in methylmercury content was significantly higher in the free‐ranging fish than in the enclosure‐reared fish, suggesting suboptimal rearing conditions in the enclosures. In a second study component, we analyzed methylmercury levels of free‐ranging Chinook salmon released in the Yolo Bypass during hydrologically variable years (2001–2003 and 2005); the objective was to determine whether interannual differences in the primary source of floodwater to the Yolo Bypass were associated with different patterns of mercury accumulation in Yolo Bypass Chinook salmon. Fish in the Yolo Bypass showed different patterns of methylmercury accumulation in 2001, 2002, 2003, and 2005. Methylmercury accumulated linearly with time in years when Cache Creek provided the primary source of flood flow but followed a quadratic pattern in years when flood flow was dominated by the Sacramento River.

High methylation rates of mercury bound to cysteine by Geobacter sulfurreducens

Methylmercury bioaccumulates in aquatic food chains and can cross the blood–brain barrier, making this organometallic compound a much more worrisome pollutant than inorganic mercury. Experimental evidence now indicates that mercury methylation by the bacterium Geobacter sulfurreducens can be greatly enhanced in the presence of the amino-acid cysteine. Methylmercury bioaccumulates in aquatic food chains and is able to cross the blood–brain barrier, making this organometallic compound a much more worrisome pollutant than inorganic mercury. We know that methylation of inorganic mercury is carried out by microbes in the anoxic layers of sediments and water columns, but the factors that control the extent of this methylation are poorly known. Mercury methylation is generally thought to be catalysed accidentally by some methylating enzyme1,2, and it has been suggested that cellular mercury uptake results from passive diffusion of neutral mercury complexes3. Here, we show that mercury methylation by the bacterium Geobacter sulfurreducens is greatly enhanced in the presence of low concentrations of the amino acid cysteine. The formation of a mercury–cysteine complex promotes both the uptake of inorganic mercury by the bacteria and the enzymatic formation of methylmercury, which is subsequently released to the external medium. Our results suggest that mercury uptake and methylation by microbes are controlled more tightly by biological mechanisms than previously thought, and that the formation of specific mercury complexes in anoxic waters modulates the efficiency of the microbial methylation of mercury.

Mercury biomethylation assessment in the estuary of Bilbao (North of Spain)

The relationship between the microbial methylation of mercury and the microbial activities in sediments and water collected from the estuary of Bilbao (North of Spain) was studied in three different sampling points and in two different seasons. Three different cultures were prepared with a sediment slurry to distinguish between biotic and abiotic methylation pathways and the variations of the methylmercury concentration and the variations of the population of total number of bacteria (TDC), anaerobic heterotrophic bacteria (AHB), sulphate-reducing bacteria (SRB) and Desulfovibrio were measured. From this work, it can be concluded that the variation of MeHg concentrations is a result of the methylation/demethylation processes in the sediments, and that the abiotic processes have a negligible contribution to those processes. According to the statistical analysis of the results (partial least squares analysis) a significant statistical correlation was established between methylmercury and the SRB counts.

Total mercury, methyl mercury, and carbon in fresh and burned plants and soil in Northwestern Ontario

Terrestrial plants and soil contain substantial amounts of organic carbon (C) and mercury. Flooding terrestrial areas stimulates microbial methyl mercury (MeHg) production and fish obtain elevated MeHg concentrations. Our purpose was to determine the loss of C, total mercury (THg), and MeHg from boreal plants and soil after burning to assess the potential of burning before flooding to lower MeHg. Fresh plants contained 4 to 52 ng g −1 dry weight (dw) of THg and 0.1 to 1.3 ng g −1 dw of MeHg. Upland soils contained 162±132 ng g −1 dw of THg and 0.6±0.6 ng g −1 dw of MeHg. Complete burning caused plants to lose 96, 98, 97, and 94% of the mass, C, THg, and MeHg, respectively. Upland soil lost 27, 95, 79, and 82% of the mass, C, THg, and MeHg, respectively. Our results demonstrated that a substantial loss of C, THg, and MeHg was caused by burning.

Microbial mercury transformation in anoxic freshwater sediments under iron-reducing and other electron-accepting conditions.

Potential rates of microbial methylation of inorganic mercury (added as HgCl2) and degradation of methyl mercury (MeHg) (added as CH3HgCl) were investigated in anoxic sediments from the Mobile Alabama River Basin (MARB) dominated by different terminal electron-accepting processes (TEAPs). Potential rates of methylation were comparable under methanogenic and sulfate-reducing conditions but suppressed under iron-reducing conditions, in slurries of freshwater wetland sediment In contrast, MeHg degradation rates were similar under all three TEAPs. Microbial Hg methylation and MeHg degradation were also investigated in surface sediment from three riverine sites, two of which had iron reduction and one sulfate reduction, as the dominant TEAP (as determined by 14C-acetate metabolism and other biogeochemical measurements). Methylation was active in sulfate-reducing sediments of a tributary creek and suppressed in iron-reducing, sandy sediments from the open river, whereas MeHg degradation was active at all three sites. Although iron-reducing conditions often suppressed methylation, some methylation activity was observed in two out of three replicates from iron-reducing sediments collected near a dam. Given that MeHg degradation was consistently observed under all TEAPs, our results suggest that the net flux of MeHg from iron-reducing surface sediments may be suppressed (due to inhibition of gross MeHg production) compared to sediments supporting other TEAPs.

Microbial mercury cycling in sediments of the San Francisco Bay-Delta

Microbial mercury (Hg) methylation and methylmercury (MeHg) degradation processes were examined using radiolabled model Hg compounds in San Francisco Bay-Delta surface sediments during three seasonal periods: late winter, spring, and fall. Strong seasonal and spatial differences were evident for both processes. MeHg production rates were positively correlated with microbial sulfate reduction rates during late winter only. MeHg production potential was also greatest during this period and decreased during spring and fall. This temporal trend was related both to an increase in gross MeHg degradation, driven by increasing temperature, and to a build-up in pore water sulfide and solid phase reduced sulfur driven by increased sulfate reduction during the warmer seasons. MeHg production decreased sharply with depth at two of three sites, both of which exhibited a corresponding increase in reduced sulfur compounds with depth. One site that was comparatively oxidized and alkaline exhibited little propensity for net MeHg production. These results support the hypothesis that net MeHg production is greatest when and where gross MeHg degradation rates are low and dissolved and solid phase reduced sulfur concentrations are low.

Methyltransferase: An enzyme assay for microbial methylmercury formation in acidic soils and sediments

Methylmercury (MeHg) is a highly toxic form of mercury that bioaccumulates in aquatic food chains. However, methods to reliably identify sites of MeHg formation or to quantify MeHg production require the use of isotopic tracers, which limits their use. In this paper, a method is presented to quantify the methylation of mercury by a methyltransferase pathway. This methyltransferase pathway is one of the biochemical pathways responsible for biological mercury methylation. Protein is extracted from environmental samples, and mercury methyltransferase (HgMT) activity of soil extracts is calculated by assessing increases in methyltransferase activity induced by Hg addition. In enzyme extracts from pure cultures or soil samples, HgMT activity correlated with net MeHg production and Hg consumption, suggesting that HgMT activity can be used to estimate MeHg production in field samples. Over the course of a three-month period in a freshwater wetland, HgMT activity correlated with net MeHg concentrations (r2 = 0.55; p < 0.057). Furthermore, HgMT activity predicted (r2 = 0.80; p < 0.01) gross MeHg formation in freshwater wetlands as well as in laboratory microcosms calculated using previously published rate constants. Our results show that a methyltransferase assay, in combination with demethylation estimates, accurately predicts MeHg formation under field and laboratory conditions. This assay does not require the use of mercury added to field samples to estimate activity but rather estimates the biological activity present in the soil by quantifying the amount of enzyme present in the soil. Such an assay is well suited for use in field surveillance programs assessing MeHg formation in a variety of environments.

Methyl-Mercury Degradation Pathways: A Comparison among Three Mercury-Impacted Ecosystems

We examined microbial methylmercury (MeHg) degradation in sediment of the Florida Everglades, Carson River (NV), and San Carlos Creek (CA), three freshwater environments that differ in the extent and type of mercury contamination and sediment biogeochemistry. Degradation rate constant (kdeg) values increased with total mercury (Hgt) contamination both among and within ecosystems. The highest kdeg's (2.8−5.8 d-1) were observed in San Carlos Creek, at acid mine drainage impacted sites immediately downstream of the former New Idria mercury mine, where Hgt ranged from 4.5 to 21.3 ppm (dry wt). A reductive degradation pathway (presumably mer-detoxification) dominated degradation at these sites, as indicated by the nearly exclusive production of 14CH4 from 14C-MeHg, under both aerobic and anaerobic conditions. At the upstream control site, and in the less contaminated ecosystems (e.g. the Everglades), kdeg's were low (≤0.2 d-1) and oxidative demethylation (OD) dominated degradation, as evident from 14CO2 production. kdeg increased with microbial CH4 production, organic content, and reduced sulfur in the Carson River system and increased with decreasing pH in San Carlos Creek. OD associated CO2 production increased with pore-water SO42- in Everglades samples but was not attributable to anaerobic methane oxidation, as has been previously proposed. This ecosystem comparison indicates that severely contaminated sediments tend to have microbial populations that actively degrade MeHg via mer-detoxification, whereas OD occurs in heavily contaminated sediments as well but dominates in those less contaminated.

Use of Path Analysis to Investigate Mercury Accumulation in Brown Trout (Salmo trutta) in Norway and the Influence of Environmental Factors

We studied relationships between environmental factors and mercury accumulation in resident brown trout (Salmo trutta) in 25 lakes in Norway. Our survey showed that mercury concentrations in fish are elevated in areas receiving high atmospheric depositions of mercury. A path analysis revealed that the concentrations of total organic carbon had a critical influence on the bioaccumulation of mercury. This reflects the role of humic matter in the transportation of mercury to lake water and sediments, and perhaps also a stimulating effect on the microbial methylation of mercury. Lake depth influenced the concentration of mercury in sediments, probably due to the complexation of mercury with sedimenting humic matter. The effect of such sediment on mercury concentrations in fish indicated that there exists a concentration-dependent production and release of methyl mercury to the biota. Selenium deposited from the atmosphere seemed to lower the bioavailability of mercury to fish. No effect of pH could be detected. The strong effect of total organic carbon implies that even moderate atmospheric deposition rates may result in relatively high mercury levels in fish, if other local environmental factors favour this.

Seasonal and Spatial Variations in Mercury Methylation and Demethylation in an Oligotrophic Lake

Microbial mercury methylation and methylmercury decomposition were examined in Lake Clara, an oligotrophic northern Wisconsin seepage lake, using radioisotopic tracers. Methylation activity was near background in the water column, was greatest in the profundal surficial sediments, and decreased with depth in sediment cores. Active demethylation occurred in the water column but was variable. Demethylation was greatest in the surficial sediments and decreased slightly with sediment depth. The methylation/demethylation ratio (M/D) was >1 in the water column, exhibited a sharp peak in surface sediments, and decreased in deeper sediments. Methylation and demethylation activity varied in surface sediments collected along a lake transect. The M/D ratio in surface sediments ranged from 1.4 to 5.8. Methylation in attached microbial communities was near background, while demethylation was high. The M/D ratios in the attached communities were all <0.20. Methylation activity in surface sediments incubated at in situ temperature increased from spring to late summer and decreased in the fall. Demethylation increased from early to midsummer and then declined. The M/D ratio in surface sediments increased from mid- to late summer, and decreased in the fall. These results indicate that the greatest potential for methylation in Lake Clara occurs in the surficial sediments and that methylation in surficial sediments is greatest from mid-July through September. In addition, the net rate of methylmercury production may be significantly affected by demethylation.

Microbial methylation of mercury in upper Wisconsin river sediments

The microbial methylation of Hg was studied in water and sediments from the Upper Wisconsin River by quantifying the methylation of radioactive Hg(II) (203Hg[NO3]2). Methylation activity was near detection limits in the water, highest in surface sediments (0 to 4 cm), and decreased with increasing sediment depth. Methylation had a temperature optimum of 35 °C. Organically enriched sediments exhibited higher methylation activity than less eutrophic sediments. Methylation activity in sediments was stimulated by the addition of peptone but not by glucose or starch. Oxygenation of sediments inhibited methylation activity. A summertime peak in methylation activity, observed in water, floc, and sediments, was related to factors other than temperature. More than 98% of the added 203Hg(II) was bound to sediments within 4 hr of inoculation, while more than 3% was methylated during a 10-day incubation. As much as 7% of the added 203Hg(II) was methylated in other experiments, suggesting that bound Hg is available for methylation. These data suggest that organic-rich surficial sediments in the Upper Wisconsin River have the potential to produce significant amounts of toxic methylmercury during late summer months.

Physiological role of mercury-methylation in Clostridium cochlearium T-2C.

Clostridium cochlearium T-2C, a mercury sensitive strain, was isolated by treating its original strain, T-2P, carrying a mercury-resistant plasmid with acridine orange. The T-2C strain obtained, having the ability to synthesize various types of cobalamin including methylcobalamin, can methylate inorganic mercury, probably by the participation of methylcobalamin. Results show that microbial methylation of inorganic mercury in the cells to produce less toxic methylmercury, in terms of the direct inhibitory effect on functional proteins, acts as a detoxification mechanism to build up bacterial tolerance to inorganic mercury.

Effect of salinity on methylation of mercury.

Monomethyl and dimethylmercury are potent neurotoxins subject to biomagnification in food webs. This fact was tragically demonstrated by the Minamata and Niigata poisoning incidents in Japan in which 168 persons who ate seafood from mercury polluted waters were poisoned, 52 fatally. Shortly after these two incidents, work conducted in freshwater environments demonstrated the microbial conversion of inorganic and phenylmercury compounds to mono- and di-methylmercury. Consideration of some fragmentary evidence from the literature, however, indicates that the rate and the significance of microbial methylation of mercury in freshwater and saltwater environments may not be the same. A demonstrated relationship between mercury methylation rates and water salinity would greatly influence our thinking about mercury pollution effects in marine versus freshwater environments. Since we were unable to locate published reports on this subject, we are investigating the influence of salinity on the rate of mercury methylation in an estuarine sediment.