The origin of methyl group in methanogen-mediated mercury methylation: From the Wolfe cycle.

Methylmercury (MeHg) is a bioaccumulating neurotoxin mainly produced by anaerobic microorganisms, with methanogen being one of the important methylators. A critical aspect for understanding the mechanism for microbial mercury (Hg) methylation is the origin of the methyl group. However, the origin of methyl group in methanogen-mediated Hg methylation remains unclear. This study aims to identify the source of methyl group for MeHg synthesis in methanogens. Our study revealed that Hg methylation in Methanospirillum hungatei JF-1 is closely related to methanogenesis process, according to the results of proteomic study and substrate limitation study. Next, we proved that nearly all methyl group in MeHg derives from the Wolfe cycle in this species, rather than the previously demonstrated acetyl-coenzyme A pathway, based on the results of 13C labeling study. We then proposed the Wolfe cycle-dependent Hg methylation mechanism in this species. Further genome analyses and 13C labeling experiments indicated that the involvement of the Wolfe cycle in Hg methylation is probably a universal feature among Hg-methylating methanogens. These findings reveal a unique Hg methylation mechanism in methanogens. Our study broadens the carbon substrates and controlling factors for MeHg synthesis in the environment, which can inform the prediction of MeHg production potential and remediation strategies for MeHg contamination.

Simultaneous Reduction and Methylation of Nanoparticulate Mercury: The Critical Role of Extracellular Electron Transfer.

Mercury nanoparticles are abundant in natural environments. Yet, understanding their contribution to global biogeochemical cycling of mercury remains elusive. Here, we show that microbial transformation of nanoparticulate divalent mercury can be an important source of elemental and methylmercury.Geobacter sulfurreducensPCA, a model bacterium predominant in anoxic environments (e.g., paddy soils), simultaneously reduces and methylates nanoparticulate Hg(II). Moreover, the relative prevalence of these two competing processes and the dominant transformation pathways differ markedly between nanoparticulate Hg(II) and its dissolved and bulk-sized counterparts. Notably, even when intracellular reduction of Hg(II) nanoparticles is constrained by cross-membrane transport (a rate-limiting step that also regulates methylation), the overall Hg(0) formation remains substantial due to extracellular electron transfer. With multiple lines of evidence based on microscopic and electrochemical analyses, gene knockout experiments, and theoretical calculations, we show that nanoparticulate Hg(II) is preferentially associated with c-type cytochromes on cell membranes and has a higher propensity for accepting electrons from the heme groups than adsorbed ionic Hg(II), which explains the surprisingly larger extent of reduction of nanoparticles than dissolved Hg(II) at relatively high mercury loadings. These findings have important implications for the assessment of global mercury budgets as well as the bioavailability of nanominerals and mineral nanoparticles.

Multiple effects of carbon, sulfur and iron on microbial mercury methylation in black-odorous sediments.

Black-odorous sediments provide ideal conditions for microbial mercury methylation. However, the multiple effects of carbon, sulfur, and iron on the microbial methylmercury of mercury in black-odorous sediments remains unclear. In this study, we conducted mercury methylation experiments using sediments collected from organically contaminated water bodies, as well as black-odorous sediments simulated in the laboratory. The results showed that black-odorous sediments exhibit a high capacity for mercury methylation. By simulating the blackening and odorization process in sediments, it was confirmed that dissolved oxygen, organic matter and sulfide were the primary factors triggering the black-odorous phenomenon in sediments. Regarding the influence of key factors in sediments on methylmercury formation, the batch tests demonstrated that high concentrations of organics additions (above 200 mg/L) may reduce bacterial activity and weaken mercury methylation in sediments. Under five different iron-sulfur ratios, the concentrations of methylmercury in the black-odorous sediments showed an increasing trend, the ratio of 5.0 Fe/S exhibited the highest MeHg accumulation. The iron-sulfur ratio in the sediment had a significant effect on the mercury methylation process, which was mainly due to the competition between Fe2+ and Hg2+ for sulfide sites and the adsorption/coprecipitation of Hg2+ by FeS. These findings offer a potential avenue for further understanding and controlling mercury methylation, contributing to the mitigation of the potential threat of mercury pollution to the environment and human health.

Sulfur-Intercalated Layered Double Hydroxides Minimize Microbial Mercury Methylation: Implications for In Situ Remediation of Mercury-Contaminated Sites.

Conventional approaches for in situ remediation of mercury (Hg)-contaminated soils and sediments rely mostly on precipitation or adsorption. However, this can generate Hg-rich surfaces that facilitate microbial production of methylmercury (MeHg), a potent, bioaccumulative neurotoxin. Herein, we prove the concept that the risk of mercury methylation can be effectively minimized by adding sulfur-intercalated layered double hydroxide (S-LDH) to Hg-contaminated soils. Hg bound to S-LDH has minimal methylation potential when incubated with model methylating bacteria Pseudodesulfovibrio mercurii ND132 and Geobacter sulfurreducens PCA. With a combination of spectroscopic and microscopic evidence, as well as theoretical calculations, we confirm that dissolved Hg(II) tends to enter the interlayers of S-LDH to bind to the sulfur groups intercalated within, leading to the formation of nanoscale metacinnabar (ß-HgS). This not only physically blocks the contact of methylating microorganisms but also inhibits secondary release of bound mercury in the presence of strong binding ligands in porewater. This study highlights the promising concept of in situ risk reduction of heavy metal contamination by inducing precipitation within (nano)confined domains, achieving a sustainable outcome of enhanced removal and reduced bioaccessibility for pollutants that may otherwise be bioavailable in the form of nanoprecipitates.

Bacterial mercury methylation modulated by vitamin B9: An overlooked pathway leads to increased environmental risks.

There has been a serious health and environmental concern in conversion of inorganic mercury (Hg) to the neurotoxin, methylmercury (MeHg) by anaerobic microbes, while very little is known about the potential role of vitamin B9 (VB9) regulator in the biochemical generation of MeHg. This study innovatively investigated bacterial Hg methylation by Geobacter sulfurreducens PCA in the presence of VB9 under two existing scenarios. In the low-complexing scenario, the bacterial MeHg yield reached 68 % higher than that without VB9 within 72 h, which was attributed to free VB9-protected PCA cells relieving oxidative stress, as manifested by the increased expression of Hg methylation gene (hgcAB cluster by 19-48 %). The high-complexing scenario emphasized the intracellular Hg accumulation (38-45 %) after 12 h, as indicated by the increased expression of outer membrane protein-related and mercuric reductase-encoding genes, indicating the inefficient bioavailability of Hg due to a gradual shift from Hg reduction toward Hg0 re-oxidation controlled by competitive ligand exchange. These results suggested that VB9 application significantly raised the potential for bacterial Hg methylation and cellular accumulation, thus proposing insights into the biochemical behaviors of hazardous Hg in farming environments where vulnerable organisms are more possibly co-exposed to higher levels of Hg and VB9.

The different effects of molybdate on Hg(II) bio-methylation in aerobic and anaerobic bacteria.

In nature, methylmercury (MeHg) is primarily generated through microbial metabolism, and the ability of bacteria to methylate Hg(II) depends on both bacterial properties and environmental factors. It is widely known that, as a metabolic analog, molybdate can inhibit the sulfate reduction process and affect the growth and methylation of sulfate-reducing bacteria (SRB). However, after it enters the cell, molybdate can be involved in various intracellular metabolic pathways as a molybdenum cofactor; whether fluctuations in its concentration affect the growth and methylation of aerobic mercury methylating strains remains unknown. To address this gap, aerobic γ-Proteobacteria strains Raoultella terrigena TGRB3 (B3) and Pseudomonas putida TGRB4 (B4), as well as an obligate anaerobic δ-Proteobacteria strain of the SRB Desulfomicrobium escambiense CGMCC 1.3481 (DE), were used as experimental strains. The growth and methylation ability of each strain were analyzed under conditions of 500 ng·L-1 Hg(II), 0 and 21% of oxygen, and 0, 0.25, 0.50, and 1 mM of MoO4 2-. In addition, in order to explore the metabolic specificity of aerobic strains, transcriptomic data of the facultative mercury-methylated strain B3 were further analyzed in an aerobic mercuric environment. The results indicated that: (a) molybdate significantly inhibited the growth of DE, while B3 and B4 exhibited normal growth. (b) Under anaerobic conditions, in DE, the MeHg content decreased significantly with increasing molybdate concentration, while in B3, MeHg production was unaffected. Furthermore, under aerobic conditions, the MeHg productions of B3 and B4 were not influenced by the molybdate concentration. (c) The transcriptomic analysis showed several genes that were annotated as members of the molybdenum oxidoreductase family of B3 and that exhibited significant differential expression. These findings suggest that the differential expression of molybdenum-binding proteins might be related to their involvement in energy metabolism pathways that utilize nitrate and dimethyl sulfoxide as electron acceptors. Aerobic bacteria, such as B3 and B4, might possess distinct Hg(II) biotransformation pathways from anaerobic SRB, rendering their growth and biomethylation abilities unaffected by molybdate.

Wildfires Influence Mercury Transport, Methylation, and Bioaccumulation in Headwater Streams of the Pacific Northwest.

The increasing frequency and severity of wildfires are among the most visible impacts of climate change. However, the effects of wildfires on mercury (Hg) transformations and bioaccumulation in stream ecosystems are poorly understood. We sampled soils, water, sediment, in-stream leaf litter, periphyton, and aquatic invertebrates in 36 burned (one-year post fire) and 21 reference headwater streams across the northwestern U.S. to evaluate the effects of wildfire occurrence and severity on total Hg (THg) and methylmercury (MeHg) transport and bioaccumulation. Suspended particulate THg and MeHg concentrations were 89 and 178% greater in burned watersheds compared to unburned watersheds and increased with burn severity, likely associated with increased soil erosion. Concentrations of filter-passing THg were similar in burned and unburned watersheds, but filter-passing MeHg was 51% greater in burned watersheds, and suspended particles in burned watersheds were enriched in MeHg but not THg, suggesting higher MeHg production in burned watersheds. Among invertebrates, MeHg in grazers, filter-feeders, and collectors was 33, 48, and 251% greater in burned watersheds, respectively, but did not differ in shredders or predators. Thus, increasing wildfire frequency and severity may yield increased MeHg production, mobilization, and bioaccumulation in headwaters and increased transport of particulate THg and MeHg to downstream environments.

Functional Genes and Transcripts Indicate the Existent and Active Microbial Mercury-Methylating Community in Mangrove Intertidal Sediments of an Urbanized Bay.

Mercury (Hg) methylation in mangrove sediments can result in the accumulation of neurotoxic methylmercury (MeHg). Identification of Hg methyltransferase gene hgcA provides the means to directly characterize the microbial Hg-methylating consortia in environments. Hitherto, the microbial Hg-methylating community in mangrove sediments was scarcely investigated. An effort to assess the diversity and abundance of hgcA genes and transcripts and link them to Hg and MeHg contents was made in the mangrove intertidal sediments along the urbanized Shenzhen Bay, China. The hgcA genes and transcripts associated with Thermodesulfobacteria [mainly Geobacteraceae, Syntrophorhabdaceae, Desulfobacterales, and Desulfarculales (these four lineages were previously classified into the Deltaproteobacteria taxon)], as well as Euryarchaeota (mainly Methanomicrobia and Theionarchaea) dominated the hgcA-harboring communities, while Chloroflexota, Nitrospirota, Planctomycetota, and Lentisphaerota-like hgcA sequences accounted for a small proportion. The hgcA genes appeared in greater abundance and diversity than their transcript counterparts in each sampling site. Correlation analysis demonstrated that the MeHg content rather than Hg content significantly correlated with the structure of the existent/active hgcA-harboring community and the abundance of hgcA genes/transcripts. These findings provide better insights into the microbial Hg methylation drivers in mangrove sediments, which could be helpful for understanding the MeHg biotransformation therein.

Distribution of mercury and methylmercury in aquacultured fish in special waters formed by coal mining subsidence.

In China, fence net aquaculture practices have been established in some subsidence waters that have been formed in coal mining subsidence areas. Within this dynamic ecological context, diverse fish species grow continuously until being harvested at the culmination of their production cycle. The purpose of this study was to investigate diverse factors influencing the bioavailability and distribution of mercury (Hg) and methylmercury (MeHg), which have high physiological toxicity in fish, in the Guqiao coal mining subsidence area in Huainan, China. Mercury and MeHg were analyzed in 38 fish samples of eight species using direct mercury analysis (DMA-80) and gas chromatography-cold vapor atomic fluorescence spectrometry (GC-CVAFAS). The analysis results show that the ranges of Hg and MeHg content and methylation rate in the fish were 7.84-85.18 ng/g, 0.52-3.52 ng/g, and 0.81-42.68 %, respectively. Meanwhile, conclusions are also summarized as following: (1) Monophagous herbivorous fish that were fed continuously in fence net aquaculture areas had higher MeHg levels and mercury methylation rates than carnivorous fish. Hg and MeHg contents were affected by different feeding habits of fish. (2) Bottom-dwelling fish show higher MeHg levels, and habitat selection in terms of water depth also partially affected the MeHg content of fish. (3) The effect of fence net aquaculture on methylation of fish in subsidence water is mainly from feed and mercury-containing bottom sediments. However, a time-lag is observed in the physiological response of benthic fishes to the release of Hg from sediments. Our findings provides baseline reference data for the ecological impact of fence net aquaculture in waters affected by soil subsidence induced by coal mining in China. Prevalent environmental contaminants within coal mining locales, notably Hg, may infiltrate rain-induced subsidence waters through various pathways.

Molybdate inhibits mercury methylation capacity of Pseudodesulfovibrio hydrargyri BerOc1 regardless of the growth metabolism.

Molybdate inhibits sulfate respiration in sulfate-reducing bacteria (SRB). It is used as an inhibitor to indirectly evaluate the role of SRB in mercury methylation in the environment. Here, the SRB Pseudodesulfovibrio hydrargyri BerOc1 was used to assess the effect of molybdate on cell growth and mercury methylation under various metabolic conditions. Geobacter sulfurreducens PCA was used as the non-SRB counterpart strain with the ability to methylate mercury. While PCA growth and methylation are not affected by molybdate, 1 mM of molybdate inhibits BerOc1 growth under sulfate respiration (50% inhibition) but also under fumarate respiration (complete inhibition). Even more surprising, mercury methylation of BerOc1 is totally inhibited at 0.1 mM of molybdate when grown under sulfate or fumarate respiration with pyruvate as the electron donor. As molybdate is expected to reduce cellular ATP level, the lower Hg methylation observed with pyruvate could be the consequence of lower energy production. Although molybdate alters the expression of hgcA (mercury methylation marker) and sat (involved in sulfate reduction and molybdate sensitivity) in a metabolism-dependent manner, no relationship with mercury methylation rates could be found. Our results show, for the first time, a specific mercury methylation inhibition by molybdate in SRB.

Recent advance of microbial mercury methylation in the environment.

Methylmercury formation is mainly driven by microbial-mediated process. The mechanism of microbial mercury methylation has become a crucial research topic for understanding methylation in the environment. Pioneering studies of microbial mercury methylation are focusing on functional strain isolation, microbial community composition characterization, and mechanism elucidation in various environments. Therefore, the functional genes of microbial mercury methylation, global isolations of Hg methylation strains, and their methylation potential were systematically analyzed, and methylators in typical environments were extensively reviewed. The main drivers (key physicochemical factors and microbiota) of microbial mercury methylation were summarized and discussed. Though significant progress on the mechanism of the Hg microbial methylation has been explored in recent decade, it is still limited in several aspects, including (1) molecular biology techniques for identifying methylators; (2) characterization methods for mercury methylation potential; and (3) complex environmental properties (environmental factors, complex communities, etc.). Accordingly, strategies for studying the Hg microbial methylation mechanism were proposed. These strategies include the following: (1) the development of new molecular biology methods to characterize methylation potential; (2) treating the environment as a micro-ecosystem and studying them from a holistic perspective to clearly understand mercury methylation; (3) a more reasonable and sensitive inhibition test needs to be considered. KEY POINTS: • Global Hg microbial methylation is phylogenetically and functionally discussed. • The main drivers of microbial methylation are compared in various condition. • Future study of Hg microbial methylation is proposed.

Cross-shelf processes of terrigenous organic matter drive mercury speciation on the east siberian shelf in the Arctic Ocean.

The cross-shelf distributions of total mercury (THg), methylmercury (MeHg) and organic and inorganic matter, as well as the presence of the hgcA gene were investigated on the East Siberian Shelf (ESS) to understand the processes underlying the speciation of sedimentary Hg. Samples were collected from 12 stations grouped into four zones based on water depth: inner shelf (5 stations), mid-shelf (3 stations), outer shelf (2 stations), and slope (2 stations). The THg concentration in the surface sediment increased from the inner shelf (0.25 ± 0.023 nmol g-1) toward the slope (0.52 nmol g-1), and, when normalized to total organic carbon content, the THg showed a positive correlation with the clay-to-sand ratio (r2 = 0.48, p = 0.012) and degree of chemical weathering (r2 = 0.79, p = 0.0001). The highest MeHg concentrations (3.0 ± 1.8 pmol g-1), as well as peaks in the S/C ratio (0.012 ± 0.002) of sediment-leached organic matter, were found on the mid-shelf, suggesting that the activities of sulfate reducers control the net Hg(II) methylation rates in the sediment. This was supported by results from a principal component analysis (PCA) performed with Hg species concentrations and sediment-leached organic matter compositions. The site-specific variation in MeHg showed the highest similarity with that of CHONS compounds in the PCA, where Deltaproteobacteria were projected to be putative Hg(II) methylators in the gene analysis. In summary, the hydrodynamic sorting of lithogenic particles appears to govern the cross-shelf distribution of THg, and in situ methylation is considered a major source of MeHg in the ESS sediment.

Impacts of forest harvesting on mercury concentrations and methylmercury production in boreal forest soils and stream sediment.

Methylmercury (MeHg) is the most neurotoxic and bioaccumulative form of mercury (Hg) present in the terrestrial and aquatic food sources of boreal ecosystems, posing potential risks to wildlife and human health. Harvesting impacts on Hg methylation and MeHg concentrations in forest soils and stream sediment are not fully understood. In this study, a field investigation was carried out in 4 harvested and 2 unharvested boreal forest watersheds, before and after harvest, to better understand impacts on Hg methylation and MeHg concentration in soils and stream sediment, including their responses to different forest management practices. Changes in total Hg (THg) and MeHg concentrations, first-order potential rate constants for Hg methylation and MeHg demethylation (Kmeth and Kdemeth) as well as total carbon content and carbon-to-nitrogen ratio post-harvest in upland, wetland and riparian soils and stream sediment were assessed and compared. Increases in MeHg production were minimal in upland, wetland or riparian soils after harvest. Sediment in streams with minor buffer protection (∼3 m), greater fractions (>75%) of harvested watershed area and more road construction had significantly increased THg and MeHg concentrations, %-MeHg, Kmeth and total carbon content post-harvest. From these patterns, we infer that inputs of carbon and inorganic Hg into harvest-impacted stream sediment are likely sourced from the harvested upland areas and stimulate in situ MeHg production in stream sediment. These findings indicate the importance of stream sediment as potential MeHg pools in harvested forest watersheds. The findings also demonstrate that forest management practices aiming to mitigate organic matter and Hg inputs to streams can effectively alleviate harvesting impacts on Hg methylation and MeHg concentrations in stream sediment.

Aging rice straw reduces the bioavailability of mercury and methylmercury in paddy soil.

Straw amendment is a prevalent agricultural practice worldwide, which can reduce air pollution and improve soil fertility. However, the impact of aging straw amendment on the bioavailability of mercury (Hg) and methylmercury (MeHg) in paddy soil remains unclear. To investigate this, incubation experiments were conducted using the diffusive gradient in thin-film technique. Results showed that amendments of dry-wet aging (DRS), photochemical aging (LRS), and freeze-thaw aging rice straw (FRS) reduced the bioavailable MeHg in paddy soil by 2.2-27.6%, 13.5-69.8%, and 23.5-86.1%, respectively, compared to fresh rice straw (RS) amendment. This result could be due to changes in soil properties such as soil pH and overlying water Fe and Mn as well as microbial abundance (including Clostridiaceae, Firmicutes, and Actinobacteriota). Simultaneously, The LRS and FRS amendments reduced bioavailable Hg in paddy soil by 20.0-40.8% and 17.1-48.6%, respectively, while DRS increased the bioavailable Hg by 15.8-120.0%. This could be attributed to changes in soil oxidation-reduction potential and overlying water SO42- content. Additionally, the results of sand culture experiments showed that the concentrations of Hg uptake by rice seedlings were 97.1-118.2%, 28.1-35.6%, and 198.0-217.1% higher in dissolved organic matter (DOM) derived from DRS, LRS, and FRS than RS, indicating that aging straw leached DOM may promote the Hg bioavailable when straw amendment. This result could be due to lower molecular weight and higher CO functional group content. These results provide new insight into how aging straw amendment affects the bioavailability of Hg and MeHg in paddy soil under different climates.

Redox gradient shapes the abundance and diversity of mercury-methylating microorganisms along the water column of the Black Sea.

In the global context of seawater deoxygenation triggered by climate change and anthropogenic activities, changes in redox gradients impacting biogeochemical transformations of pollutants, such as mercury, become more likely. Being the largest anoxic basin worldwide, with high concentrations of the potent neurotoxic methylmercury (MeHg), the Black Sea is an ideal natural laboratory to provide new insights about the link between dissolved oxygen concentration and hgcAB gene-carrying (hgc+) microorganisms involved in the formation of MeHg. We combined geochemical and microbial approaches to assess the effect of vertical redox gradients on abundance, diversity, and metabolic potential of hgc+ microorganisms in the Black Sea water column. The abundance of hgcA genes [congruently estimated by quantitative PCR (qPCR) and metagenomics] correlated with MeHg concentration, both maximal in the upper part of the anoxic water. Besides the predominant Desulfobacterales, hgc+ microorganisms belonged to a unique assemblage of diverse-previously underappreciated-anaerobic fermenters from Anaerolineales, Phycisphaerae (characteristic of the anoxic and sulfidic zone), Kiritimatiellales, and Bacteroidales (characteristic of the suboxic zone). The metabolic versatility of Desulfobacterota differed from strict sulfate reduction in the anoxic water to reduction of various electron acceptors in the suboxic water. Linking microbial activity and contaminant concentration in environmental studies is rare due to the complexity of biological pathways. In this study, we disentangle the role of oxygen in shaping the distribution of Hg-methylating microorganisms consistently with MeHg concentration, and we highlight their taxonomic and metabolic niche partitioning across redox gradients, improving the prediction of the response of marine communities to the expansion of oxygen-deficient zones. IMPORTANCE Methylmercury (MeHg) is a neurotoxin detected at high concentrations in certain marine ecosystems, posing a threat to human health. MeHg production is mainly mediated by hgcAB gene-carrying (hgc+) microorganisms. Oxygen is one of the main factors controlling Hg methylation; however, its effect on the diversity and ecology of hgc+ microorganisms remains unknown. Under the current context of seawater deoxygenation, mercury cycling is expected to be disturbed. Here, we show the strong effect of oxygen gradients on the distribution of potential Hg methylators. In addition, we show for the first time the significant contribution of a unique assemblage of potential fermenters from Anaerolineales, Phycisphaerae, and Kiritimatiellales to Hg methylation, stratified in different redox niches along the Black Sea gradient. Our results considerably expand the known taxonomic diversity and ecological niches prone to the formation of MeHg and contribute to better apprehend the consequences of oxygen depletion in seawater.

The Combined Effect of Hg(II) Speciation, Thiol Metabolism, and Cell Physiology on Methylmercury Formation by Geobacter sulfurreducens.

The chemical and biological factors controlling microbial formation of methylmercury (MeHg) are widely studied separately, but the combined effects of these factors are largely unknown. We examined how the chemical speciation of divalent, inorganic mercury (Hg(II)), as controlled by low-molecular-mass thiols, and cell physiology govern MeHg formation by Geobacter sulfurreducens. We compared MeHg formation with and without addition of exogenous cysteine (Cys) to experimental assays with varying nutrient and bacterial metabolite concentrations. Cysteine additions initially (0-2 h) enhanced MeHg formation by two mechanisms: (i) altering the Hg(II) partitioning from the cellular to the dissolved phase and/or (ii) shifting the chemical speciation of dissolved Hg(II) in favor of the Hg(Cys)2 complex. Nutrient additions increased MeHg formation by enhancing cell metabolism. These two effects were, however, not additive since cysteine was largely metabolized to penicillamine (PEN) over time at a rate that increased with nutrient addition. These processes shifted the speciation of dissolved Hg(II) from complexes with relatively high availability, Hg(Cys)2, to complexes with lower availability, Hg(PEN)2, for methylation. This thiol conversion by the cells thereby contributed to stalled MeHg formation after 2-6 h Hg(II) exposure. Overall, our results showed a complex influence of thiol metabolism on microbial MeHg formation and suggest that the conversion of cysteine to penicillamine may partly suppress MeHg formation in cysteine-rich environments like natural biofilms.

Mercury methylation potential and bioavailability in the sediments of two distinct aquatic systems.

This study explored mercury (Hg) methylation potential in two distinct aquatic systems. Fourmile Creek (FMC) was historically polluted with Hg effluents from groundwater as it is a typical gaining stream, where organic matter and microorganisms in streambed are continuously winnowed. The H02 constructed wetland only receives atmospheric Hg and is rich in organic matter and microorganisms. Both systems receive Hg from atmospheric deposition now. Surface sediments were collected from FMC and H02, spiked with inorganic Hg, and cultivated in an anaerobic chamber to stimulate microbial Hg methylation reactions. Total mercury (THg) and methylmercury (MeHg) concentrations were measured at each spiking stage. Mercury methylation potential (MMP, %MeHg in THg) and Hg bioavailability were assessed with the deployment of diffusive gradients in thin films (DGTs). During the methylation process and at the same incubation stage, FMC sediment showed faster increasing rates of %MeHg and higher MeHg concentrations than H02, demonstrating a stronger MMP in the FMC sediment. Similarly, higher Hg bioavailability was observed in FMC sediment compared to the H02 as indicated by DGT-Hg concentrations. In conclusion, the H02 wetland with high levels of organic matter and microorganisms presented low MMP. But the Fourmile Creek as a gaining stream and a historical site of Hg pollution showed strong MMP and high Hg bioavailability. A related study on microbial community activities characterized the microorganisms between FMC and H02, which is attributed to be the main reason for their different methylation capabilities. Our study further brought up the considerations on remediated sites from Hg contamination: Hg bioaccumulation and biomagnification can still be elevated and higher than the surrounding environment due to lagged changes in microbial community structures. This study supported the sustainable ecological modifications of legacy Hg contamination and raised the necessity of long-term monitoring actions even after executing a remediation plan.

Inhibition of Methylmercury and Methane Formation by Nitrous Oxide in Arctic Tundra Soil Microcosms.

Climate warming causes permafrost thaw predicted to increase toxic methylmercury (MeHg) and greenhouse gas [i.e., methane (CH4), carbon dioxide (CO2), and nitrous oxide (N2O)] formation. A microcosm incubation study with Arctic tundra soil over 145 days demonstrates that N2O at 0.1 and 1 mM markedly inhibited microbial MeHg formation, methanogenesis, and sulfate reduction, while it slightly promoted CO2 production. Microbial community analyses indicate that N2O decreased the relative abundances of methanogenic archaea and microbial clades implicated in sulfate reduction and MeHg formation. Following depletion of N2O, both MeHg formation and sulfate reduction rapidly resumed, whereas CH4 production remained low, suggesting that N2O affected susceptible microbial guilds differently. MeHg formation strongly coincided with sulfate reduction, supporting prior reports linking sulfate-reducing bacteria to MeHg formation in the Arctic soil. This research highlights complex biogeochemical interactions in governing MeHg and CH4 formation and lays the foundation for future mechanistic studies for improved predictive understanding of MeHg and greenhouse gas fluxes from thawing permafrost ecosystems.

Methylmercury formation in biofilms of Geobacter sulfurreducens.

Introduction: Mercury (Hg) is a major environmental pollutant that accumulates in biota predominantly in the form of methylmercury (MeHg). Surface-associated microbial communities (biofilms) represent an important source of MeHg in natural aquatic systems. In this work, we report MeHg formation in biofilms of the iron-reducing bacterium Geobacter sulfurreducens. Methods: Biofilms were prepared in media with varied nutrient load for 3, 5, or 7 days, and their structural properties were characterized using confocal laser scanning microscopy, cryo-scanning electron microscopy and Fourier-transform infrared spectroscopy. Results: Biofilms cultivated for 3 days with vitamins in the medium had the highest surface coverage, and they also contained abundant extracellular matrix. Using 3 and 7-days-old biofilms, we demonstrate that G. sulfurreducens biofilms prepared in media with various nutrient load produce MeHg, of which a significant portion is released to the surrounding medium. The Hg methylation rate constant determined in 6-h assays in a low-nutrient assay medium with 3-days-old biofilms was 3.9 ± 2.0 ∙ 10-14 L ∙ cell-1 ∙ h-1, which is three to five times lower than the rates found in assays with planktonic cultures of G. sulfurreducens in this and previous studies. The fraction of MeHg of total Hg within the biofilms was, however, remarkably high (close to 50%), and medium/biofilm partitioning of inorganic Hg (Hg(II)) indicated low accumulation of Hg(II) in biofilms. Discussion: These findings suggest a high Hg(II) methylation capacity of G. sulfurreducens biofilms and that Hg(II) transfer to the biofilm is the rate-limiting step for MeHg formation in this systems.

Anaerobic mercury methylators inhabit sinking particles of oxic water columns.

Increased concentration of mercury, particularly methylmercury, in the environment is a worldwide concern because of its toxicity in severely exposed humans. Although the formation of methylmercury in oxic water columns has been previously suggested, there is no evidence of the presence of microorganisms able to perform this process, using the hgcAB gene pair (hgc+ microorganisms), in such environments. Here we show the prevalence of hgc+ microorganisms in sinking particles of the oxic water column of Lake Geneva (Switzerland and France) and its anoxic bottom sediments. Compared to anoxic sediments, sinking particles found in oxic waters exhibited relatively high proportion of hgc+genes taxonomically assigned to Firmicutes. In contrast hgc+members from Nitrospirae, Chloroflexota and PVC superphylum were prevalent in anoxic sediment while hgc+ Desulfobacterota were found in both environments. Altogether, the description of the diversity of putative mercury methylators in the oxic water column expand our understanding on MeHg formation in aquatic environments and at a global scale.