Syntrophic pathways for microbial mercury methylation

Advances in mechanism for the microbial transformation of heavy metals: implications for bioremediation strategies

Heavy metals are extensively discharged through various anthropogenic activities, resulting in an environmental risk on a global scale. In this case, microorganisms can survive in an extreme heavy metal-contaminated environment via detoxification or resistance, playing a pivotal role in the speciation, bioavailability, and mobility of heavy metals. Therefore, studies on the mechanism for the microbial transformation of heavy metals are of great importance and can provide guidance for heavy metal bioremediation. Current research studies on the microbial transformation of heavy metals mainly focus on the single oxidation, reduction and methylation pathways. However, complex microbial transformation processes and corresponding bioremediation strategies have never been clarified, which may involve the inherent physicochemical properties of heavy metals. To uncover the underlying mechanism, we reclassified heavy metals into three categories based on their biological transformation pathways, namely, metals that can be chelated, reduced or oxidized, and methylated. Firstly, we comprehensively characterized the difference in transmembrane pathways between heavy metal cations and anions. Further, biotransformation based on chelation by low-molecular-weight organic complexes is thoroughly discussed. Moreover, the progress and knowledge gaps in the microbial redox and (de)methylation mechanisms are discussed to establish a connection linking theoretical advancements with solutions to the heavy metal contamination problem. Finally, several efficient bioremediation strategies for heavy metals and the limitations of bioremediation are proposed. This review presents a solid contribution to the design of efficient microbial remediation strategies applied in the real environment.

Soil Geobacteraceae are the key predictors of neurotoxic methylmercury bioaccumulation in rice

Contamination of rice by the potent neurotoxin methylmercury (MeHg) originates from microbe-mediated Hg methylation in soils. However, the high diversity of Hg methylating microorganisms in soils hinders the prediction of MeHg formation and challenges the mitigation of MeHg bioaccumulation via regulating soil microbiomes. Here we explored the roles of various cropland microbial communities in MeHg formation in the potentials leading to MeHg accumulation in rice and reveal that Geobacteraceae are the key predictors of MeHg bioaccumulation in paddy soil systems. We characterized Hg methylating microorganisms from 67 cropland ecosystems across 3,600 latitudinal kilometres. The simulations of a rice-paddy biogeochemical model show that MeHg accumulation in rice is 1.3-1.7-fold more sensitive to changes in the relative abundance of Geobacteraceae compared to Hg input, which is recognized as the primary parameter in controlling MeHg exposure. These findings open up a window to predict MeHg formation and accumulation in human food webs, enabling more efficient mitigation of risks to human health through regulations of key soil microbiomes.

Differential response of Hg-methylating and MeHg-demethylating microbiomes to dissolved organic matter components in eutrophic lake water

Methylmercury (MeHg) production in aquatic ecosystems is a global concern because of its neurotoxic effect. Dissolved organic matter (DOM) plays a crucial role in biogeochemical cycling of Hg. However, owing to its complex composition, the effects of DOM on net MeHg production have not been fully understood. Here, the Hg isotope tracer technique combined with different DOM treatments was employed to explore the influences of DOM with divergent compositions on Hg methylation/demethylation and its microbial mechanisms in eutrophic lake waters. Our results showed that algae-derived DOM treatments enhanced MeHg concentrations by 1.42-1.53 times compared with terrestrial-derived DOM. Algae-derived DOM had largely increased the methylation rate constants by approximately 1-2 orders of magnitude compared to terrestrial-derived DOM, but its effects on demethylation rate constants were less pronounced, resulting in the enhancement of net MeHg formation. The abundance of hgcA and merB genes suggested that Hg-methylating and MeHg-demethylating microbiomes responded differently to DOM treatments. Specific DOM components (e.g., aromatic proteins and soluble microbial byproducts) were positively correlated with both methylation rate constants and the abundance of Hg-methylating microbiomes. Our results highlight that the DOM composition influences the Hg methylation and MeHg demethylation differently and should be incorporated into future Hg risk assessments in aquatic ecosystems.

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.

Phylogenetic and ecophysiological novelty of subsurface mercury methylators in mangrove sediments

Mangrove sediment is a crucial component in the global mercury (Hg) cycling and acts as a hotspot for methylmercury (MeHg) production. Early evidence has documented the ubiquity of well-studied Hg methylators in mangrove superficial sediments; however, their diversity and metabolic adaptation in the more anoxic and highly reduced subsurface sediments are lacking. Through MeHg biogeochemical assay and metagenomic sequencing, we found that mangrove subsurface sediments (20-100 cm) showed a less hgcA gene abundance but higher diversity of Hg methylators than superficial sediments (0-20 cm). Regional-scale investigation of mangrove subsurface sediments spanning over 1500 km demonstrated a prevalence and family-level novelty of Hg-methylating microbial lineages (i.e., those affiliated to Anaerolineae, Phycisphaerae, and Desulfobacterales). We proposed the candidate phylum Zixibacteria lineage with sulfate-reducing capacity as a currently understudied Hg methylator across anoxic environments. Unlike other Hg methylators, the Zixibacteria lineage does not use the Wood-Ljungdahl pathway but has unique capabilities of performing methionine synthesis to donate methyl groups. The absence of cobalamin biosynthesis pathway suggests that this Hg-methylating lineage may depend on its syntrophic partners (i.e., Syntrophobacterales members) for energy in subsurface sediments. Our results expand the diversity of subsurface Hg methylators and uncover their unique ecophysiological adaptations in mangrove sediments.

Microbial community function and methylmercury production in oxygen-limited paddy soil

Methylmercury is a neurotoxic compound that can enter rice fields through rainfall or irrigation with contaminated wastewater, and then contaminate the human food chain through the consumption of rice. Flooded paddy soil has a porous structure that facilitates air exchange with the atmosphere, but the presence of trace amounts of oxygen in flooded rice field soil and its impact on microbial-mediated formation of methylmercury is still unclear. We compared the microbial communities and their functions in oxygen-depleted and oxygen-limited paddy soil. We discovered that oxygen-limited paddy soil had higher methylmercury concentration, which was strongly correlated with soil properties and methylation potential. Compared with oxygen-depleted soil, oxygen-limited soil altered the microbial composition based on 16 S rRNA sequences, but not based on hgcA sequences. Moreover, oxygen-limited soil enhanced microbial activity significantly, increasing the abundance of more than half of the KEGG pathways, especially the metabolic pathways that might be involved in methylation. Our study unveils how microbial communities influence methylmercury formation in oxygen-limited paddy soil. ENVIRONMENTAL IMPLICATIONS: This study examined how low oxygen input affects microbial-induced MeHg formation in anaerobic paddy soil. We found that oxygen-limited soil produced more MeHg than oxygen-depleted soil. Oxygen input altered the microbial community structure of 16 S rRNA sequencing in anaerobic paddy soil, but had little impact on the hgcA sequencing community structure. Microbial activity and metabolic functions related to MeHg formation were also higher in oxygen-limited paddy soil. We suggest that oxygen may not be a limiting factor for Hg methylators, and that insufficient oxygen input in flooded paddy soil increases the risk of human exposure to MeHg from rice consumption.

Magnetite Alters the Metabolic Interaction between Methanogens and Sulfate-Reducing Bacteria

It is known that the presence of sulfate decreases the methane yield in the anaerobic digestion systems. Sulfate-reducing bacteria can convert sulfate to hydrogen sulfide competing with methanogens for substrates such as H2 and acetate. The present work aims to elucidate the microbial interactions in biogas production and assess the effectiveness of electron-conductive materials in restoring methane production after exposure to high sulfate concentrations. The addition of magnetite led to a higher methane content in the biogas and a sharp decrease in the level of hydrogen sulfide, indicating its beneficial effects. Furthermore, the rate of volatile fatty acid consumption increased, especially for butyrate, propionate, and acetate. Genome-centric metagenomics was performed to explore the main microbial interactions. The interaction between methanogens and sulfate-reducing bacteria was found to be both competitive and cooperative, depending on the methanogenic class. Microbial species assigned to the Methanosarcina genus increased in relative abundance after magnetite addition together with the butyrate oxidizing syntrophic partners, in particular belonging to the Syntrophomonas genus. Additionally, Ruminococcus sp. DTU98 and other species assigned to the Chloroflexi phylum were positively correlated to the presence of sulfate-reducing bacteria, suggesting DIET-based interactions. In conclusion, this study provides new insights into the application of magnetite to enhance the anaerobic digestion performance by removing hydrogen sulfide, fostering DIET-based syntrophic microbial interactions, and unraveling the intricate interplay of competitive and cooperative interactions between methanogens and sulfate-reducing bacteria, influenced by the specific methanogenic group.

Biogeochemical transformation of mercury driven by microbes involved in anaerobic digestion of municipal wastewater

Anaerobic digestion (AD) with municipal wastewater contained heavy metal mercury (Hg) highly affects the utilization of activated sludge, and poses severe threat to the health of human beings. However, the biogeochemical transformation of Hg during AD remains unclear. Here, we investigated the biogeochemical transformation and environmental characteristics of Hg and the variations of dominant microbes during AD. The results showed that Hg(II) methylation is dominant in the early stage of AD, while methylmercury (MeHg) demethylation dominates in the later stage. Dissolved total Hg (DTHg) in the effluent sludge decreased with time, while THg levels enhanced to varying degrees at the final stage. Sulfate significant inhibits MeHg formation, reduces bioavailability of Hg(II) by microbes and thus inhibits Hg(II) methylation. Microbial community analysis reveals that strains in Methanosarcina and Aminobacterium from the class of Methanomicrobia, rather than Deltaproteobacteria, may be directly related to Hg(II) methylation and MeHg demethylation. Overall, this research provide insights into the biogeochemical transformation of Hg in the anaerobic digestion of municipal wastewater treatment. This work is beneficial for scientific treatment of municipal wastewater and effluent sludge, thus reducing the risk of MeHg to human beings.

Responses of the structure and function of microbes in Yellow River Estuary sediments to different levels of mercury

The health and stability of the estuary of the Yellow River ecosystem have come under increasing pressure from land-based inputs of heavy metals. While it is known that heavy metals affect the function and health of the microbial community, there remains little knowledge on the responses of the microbial community to heavy metals, particularly highly toxic mercury. The research aimed to characterize the responses of the sediment microbial community of the estuary of the Yellow River to different levels of mercury stress. Estuary sediment samples were collected for microbial community analysis, measurement of mercury [including total mercury (THg) and methylmercury (MeHg)], and measurement of other physicochemical factors, including pH, total organic carbon (TOC), sulfide, iron ratio (Fe3+/Fe2+), ammonium salt (NH4+), and biochemical oxygen demand (BOD). The application of 16S rRNA sequencing identified 60 phyla of bacteria, dominated by Proteobacteria, Firmicutes, and Bacteroidetes. Stations with higher THg or MeHg and lower microbial abundance and diversity were generally distributed further outside of the estuary. Besides mercury, the measured physicochemical factors had impacts on microbial diversities and distribution. Metagenomics assessment of three stations, representative of low, moderate, and high mercury concentrations and measured physicochemical factors, revealed the abundances and functions of predicted genes. The most abundant genes regulating the metabolic pathways were categorized as metabolic, environmental information processing, and genetic information processing, genes. At stations with high levels of mercury, the dominant genes were related to energy metabolism, signal transport, and membrane transport. Functional genes with a mercury-resistance function were generally in the mer system (merA, merC, merT, merR), alkylmercury lyase, and metal-transporting ATPase. These results offer insight into the microbial community structure of the sediments in the Yellow River Estuary and the microbial function of mercury resistance under mercury stress.

Plastispheres as hotspots of microbially-driven methylmercury production in paddy soils

Microplastics (MPs) as emerging contaminants have accumulated extensively in agricultural ecosystems and are known to exert important effects on biogeochemical processes. However, how MPs in paddy soils influence the conversion of mercury (Hg) to neurotoxic methylmercury (MeHg) remains poorly understood. Here, we evaluated the effects of MPs on Hg methylation and associated microbial communities in microcosms using two typical paddy soils in China (i.e., yellow and red soils). Results showed that the addition of MPs significantly increased MeHg production in both soils, which could be related to higher Hg methylation potential in the plastisphere than in the bulk soil. We found significant divergences in the community composition of Hg methylators between the plastisphere and the bulk soil. In addition, the plastisphere had higher proportions of Geobacterales in the yellow soil and Methanomicrobia in the red soil compared with the bulk soil, respectively; and plastisphere also had more densely connected microbial groups between non-Hg methylators and Hg methylators. These microbiota in the plastisphere are different from those in the bulk soil, which could partially account for their distinct MeHg production ability. Our findings suggest plastisphere as a unique biotope for MeHg production and provide new insights into the environment risks of MP accumulation in agricultural soils.

MeHg production in eutrophic lakes: Focusing on the roles of algal organic matter and iron-sulfur-phosphorus dynamics

The mechanisms by which eutrophication affects methylmercury (MeHg) production have not been comprehensively summarized, which hinders accurately predicting the MeHg risk in eutrophic lakes. In this review, we first discussed the effects of eutrophication on biogeochemical cycle of mercury (Hg). Special attentions were paid to the roles of algal organic matter (AOM) and iron (Fe)-sulfur (S)-phosphorus (P) dynamics in MeHg production. Finally, the suggestions for risk control of MeHg in eutrophic lakes were proposed. AOM can affect in situ Hg methylation by stimulating the abundance and activities of Hg methylating microorganisms and regulating Hg bioavailability, which are dependent on bacteria-strain and algae species, the molecular weight and composition of AOM as well as environmental conditions (e.g., light). Fe-S-P dynamics under eutrophication including sulfate reduction, FeS formation and P release could also play crucial but complicated roles in MeHg production, in which AOM may participate through influencing the dissolution and aggregation processes, structural order and surface properties of HgS nanoparticles (HgS_NP). Future studies should pay more attention to the dynamics of AOM in responses to the changing environmental conditions (e.g., light penetration and redox fluctuations) and how such variations will subsequently affect MeHg production. The effects of Fe-S-P dynamics on MeHg production under eutrophication also deserve further investigations, especially the interactions between AOM and HgS_NP. Remediation strategies with lower disturbance, greater stability and less cost like the technology of interfacial O₂ nanobubbles are urgent to be explored. This review will deepen our understanding of the mechanisms of MeHg production in eutrophic lakes and provide theoretical guidance for its risk control.

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.

Amendments of nitrogen and sulfur mitigate carbon-promoting effect on microbial mercury methylation in paddy soils

The imbalance of nutrient elements in paddy soil could affect biogeochemical processes; however, how the key elements input influence microbially-driven conversion of mercury (Hg) to neurotoxic methylmercury (MeHg) remains virtually unknown. Herein, we conducted a series of microcosm experiments to explore the effects of certain species of carbon (C), nitrogen (N) and sulfur (S) on microbial MeHg production in two typical paddy soils (yellow and black soil). Results showed that the addition of C alone into the soils increased MeHg production approximately 2-13 times in the yellow and black soils; while the combined addition of N and C mitigated the C- promoting effect significantly. Added S also had a buffering effect on C-facilitated MeHg production in the yellow soil despite the extent being lower than that of N addition, whereas this effect was not obvious for the black soil. MeHg production was positively correlated with the abundance of Deltaproteobactera-hgcA in both soils, and the changes in MeHg production were related to the shifts of Hg methylating community resulting from C, N, and S imbalance. We further found that the changes in the proportions of dominant Hg methylators such as Geobacter and some unclassified groups could contribute to the variations in MeHg production under different treatments. Moreover, the enhanced microbial syntrophy with adding N and S might contribute to the reduced C-promoting effect on MeHg production. This study has important implications for better understanding of microbes-driven Hg conversion in paddies and wetlands with nutrient elements input.

Treatment and remediation of metal-contaminated water and groundwater in mining areas by biological sulfidogenic processes: A review

Heavy metal pollution in the mining areas leads to serious environmental problems. The biological sulfidogenic process (BSP) mediated by sulfidogenic bacteria has been considered an attractive technology for the treatment and remediation of metal-contaminated water and groundwater. Notwithstanding, BSP driven by different sulfidogenic bacteria could affect the efficiency and cost-effectiveness of the treatment performance in practical applications, such as the microbial intolerance of pH and metal ions, the formation of toxic byproducts, and the consumption of organic electron donors. Sulfur-reducing bacteria (S⁰RB)-driven BSP has been demonstrated to be a promising alternative to the commonly used sulfate-reducing bacteria (SRB)-driven BSP for treating metal-contaminated wastewater and groundwater, due to the cost-saving in chemical addition, the high efficiency in sulfide production and metal removal efficiency. Although the S⁰RB-driven BSP has been developed and applied for decades, the present review works mainly focus on the developments in SRB-driven BSP for the treatment and remediation of metal-contaminated wastewater and groundwater. Accordingly, a comprehensive review for metal-contaminated wastewater treatment and groundwater remediation should be provided with the incorporation of the SRB- and S⁰RB-driven BSP. To identify the bottlenecks and to improve BSP performance, this paper reviews sulfidogenic bacteria presenting in metal-contaminated water and groundwater; highlight the critical factors for the metabolism of sulfidogenic bacteria during BSP; the ecological roles of sulfidogenic bacteria and the mechanisms of metal removal by sulfidogenic bacteria; and the application of the present sulfidogenic systems and their drawbacks. Accordingly, the research knowledge gaps, current process limitations, and future prospects were provided for improving the performance of BSP in the treatment and remediation of metal-contaminated wastewater and groundwater in mining areas.

Impacts of the invasive Spartina anglica on C-S-Hg cycles and Hg(II) methylating microbial communities revealed by hgcA gene analysis in intertidal sediment of the Han River estuary, Yellow Sea

We investigated the impact of invasive vegetation on mercury cycles, and identified microorganisms directly related to Hg(II) methylation using hgcA gene in vegetated mud flats (VMF) inhabited by native Suaeda japonica (SJ) and invasive Spartina anglica (SA), and unvegetated mud flats (UMF) in Ganghwa intertidal sediments. Sulfate reduction rate (SRR) and rate constants of Hg(II) methylation (K_m) and methyl-Hg demethylation (K_d) were consistently greater in VMF than in UMF, specifically 1.5, 2 and 11.7 times higher, respectively, for SA. Both K_m and K_d were significantly correlated with SRR and the abundance of sulfate-reducing bacteria. These results indicate that the rhizosphere of invasive SA provides a hotspot for Hg dynamics coupled with sulfate reduction. HgcA gene analysis revealed that Hg(II)-methylators were dominated by Deltaproteobacteria, Chloroflexi and Euryarchaeota, comprising 37.9%, 35.8%, and 6.5% of total hgcA gene sequences, respectively, which implies that coastal sediments harbor diverse Hg(II)-methylating microorganisms that previously underrepresented.

Microbial communities mediating net methylmercury formation along a trophic gradient in a peatland chronosequence

Peatlands are generally important sources of methylmercury (MeHg) to adjacent aquatic ecosystems, increasing the risk of human and wildlife exposure to this highly toxic compound. While microorganisms play important roles in mercury (Hg) geochemical cycles where they directly and indirectly affect MeHg formation in peatlands, potential linkages between net MeHg formation and microbial communities involving these microorganisms remain unclear. To address this gap, microbial community composition and specific marker gene transcripts were investigated along a trophic gradient in a geographically constrained peatland chronosequence. Our results showed a clear spatial pattern in microbial community composition along the gradient that was highly driven by peat soil properties and significantly associated with net MeHg formation as approximated by MeHg concentration and %MeHg of total Hg concentration. Known fermentative, syntrophic, methanogenic and iron-reducing metabolic guilds had the strong positive correlations to net MeHg formation, while methanotrophic and methylotrophic microorganisms were negatively correlated. Our results indicated that sulfate reducers did not have a key role in net MeHg formation. Microbial activity as interpreted from 16S rRNA sequences was significantly correlated with MeHg and %MeHg. Our findings shed new light on the role of microbial community in net MeHg formation of peatlands that undergo ontogenetic change.

Understanding of mercury and methylmercury transformation in sludge composting by metagenomic analysis

Municipal sewage especially the produced sewage sludge is a significant source releasing mercury (Hg) to the environment. However, the Hg speciation especially methylmercury (MeHg) transformation in sewage sludge treatment process remains poorly understood. This study investigated the transformation of Hg speciation especially MeHg in sludge composting. The distribution of Hg transformation related gene pairs hgcAB and merAB, and their putative microbial hosts were comprehensively analyzed. Both Hg (from 3.16±0.22 mg/kg to 3.20±0.19 mg/kg) and MeHg content (from 4.77±0.64 ng/g to 4.36±0.37 ng/g) were not obviously changed before and after composting, but about 19.69% of Hg and 27.36% of MeHg were lost according to mass balance calculation. The metagenomic analysis further revealed that anaerobes (Desulfobacterota and Euryarchaeota) were the mainly putative Hg methylators especially carrying high abundance of hgcA gene in the initial periods of composting. Among the 151 reconstructed metagenome-assembled genomes (MAGs), only 4 hgcA gene carriers (Myxococcota, Firmicutes, Cyclobacteriaceae, and Methanothermobacter) and 16 merB gene carriers were identified. But almost all of the MAGs carried hgcB gene and merA gene. The merA gene was widely distributed in genomes, which indicated the widespread functionality of microbes for reducing Hg(II) to Hg(0). The hgcA carrying microbes tends to present the similar metabolic pathways including methanogenesis and sulfur metabolism. Besides, both the irregular distribution of hgcA in various species (including Actinobacteria, Archaea, Bacteroidetes, Desulfobacterota, Euryarchaeota, and Nitrospirae, etc.) and opposite evolution trends between hgcA gene abundance and its host genome abundance can be an indication of horizontal gene transfer or gene deletions of hgcA during composting. Our findings thus revealed that sludge composting is not only a hotspot for Hg speciation transformation, but also a potential hotspot for MeHg transformation.

Increased water inputs fuel microbial mercury methylation in upland soils

Mercury (Hg) can be converted to neurotoxic methylmercury (MeHg) by certain microbes typically in anaerobic environments, threatening human health due to its bioaccumulation in food webs. However, it is unclear whether and how Hg can be methylated in legacy aerobic uplands with increasing water. Here, we conducted a series of incubation experiments to investigate the effects of increased water content on MeHg production in two typical upland soils (i.e., long-term and short-term use). Results showed that marked MeHg production occurred in water-saturated upland soils, which was strongly correlated with the proportions of significantly stimulated Hg methylating taxon (i.e., Geobacter). Elevated temperature further enhanced MeHg production by blooming proportions of typical Hg methylators (i.e., Clostridium, Acetonema, and Geobacter). Water saturation could also enhance microbial Hg methylation by facilitating microbial syntrophy between non-Hg methylators and Hg methylators. Taken together, the present work suggests that uplands could turn into a potential MeHg reservoir in response to water inputs resulting from rainfall or anthropogenic irrigation.

Expression Levels of hgcAB Genes and Mercury Availability Jointly Explain Methylmercury Formation in Stratified Brackish Waters

Neurotoxic methylmercury (MeHg) is formed by microbial methylation of inorganic divalent Hg (Hg^II) and constitutes severe environmental and human health risks. The methylation is enabled by hgcA and hgcB genes, but it is not known if the associated molecular-level processes are rate-limiting or enable accurate prediction of MeHg formation in nature. In this study, we investigated the relationships between hgc genes and MeHg across redox-stratified water columns in the brackish Baltic Sea. We showed, for the first time, that hgc transcript abundance and the concentration of dissolved Hg^II-sulfide species were strong predictors of both the Hg^II methylation rate and MeHg concentration, implying their roles as principal joint drivers of MeHg formation in these systems. Additionally, we characterized the metabolic capacities of hgc⁺ microorganisms by reconstructing their genomes from metagenomes (i.e., hgc⁺ MAGs), which highlighted the versatility of putative Hg^II methylators in the water column of the Baltic Sea. In establishing relationships between hgc transcripts and the Hg^II methylation rate, we advance the fundamental understanding of mechanistic principles governing MeHg formation in nature and enable refined predictions of MeHg levels in coastal seas in response to the accelerating spread of oxygen-deficient zones.

Microbial mercury transformations: Molecules, functions and organisms

Mercury (Hg) methylation, methylmercury (MeHg) demethylation, and inorganic redox transformations of Hg are microbe-mediating processes that determine the fate and cycling of Hg and MeHg in many environments, and by doing so influence the health of humans and wild life. The discovery of the Hg methylation genes, hgcAB, in the last decade together with advances in high throughput and genome sequencing methods, have resulted in an expanded appreciation of the diversity of Hg methylating microbes. This review aims to describe experimentally confirmed and recently discovered hgcAB gene-carrying Hg methylating microbes; phylogenetic and taxonomic analyses are presented. In addition, the current knowledge on transformation mechanisms, the organisms that carry them out, and the impact of environmental parameters on Hg methylation, MeHg demethylation, and inorganic Hg reduction and oxidation is summarized. This knowledge provides a foundation for future action toward mitigating the impact of environmental Hg pollution.

Unravelling biogeochemical drivers of methylmercury production in an Arctic fen soil and a bog soil

Arctic tundra soils store a globally significant amount of mercury (Hg), which could be transformed to the neurotoxic methylmercury (MeHg) upon warming and thus poses serious threats to the Arctic ecosystem. However, our knowledge of the biogeochemical drivers of MeHg production is limited in these soils. Using substrate addition (acetate and sulfate) and selective microbial inhibition approaches, we investigated the geochemical drivers and dominant microbial methylators in 60-day microcosm incubations with two tundra soils: a circumneutral fen soil and an acidic bog soil, collected near Nome, Alaska, United States. Results showed that increasing acetate concentration had negligible influences on MeHg production in both soils. However, inhibition of sulfate-reducing bacteria (SRB) completely stalled MeHg production in the fen soil in the first 15 days, whereas addition of sulfate in the low-sulfate bog soil increased MeHg production by 5-fold, suggesting prominent roles of SRB in Hg(II) methylation. Without the addition of sulfate in the bog soil or when sulfate was depleted in the fen soil (after 15 days), both SRB and methanogens contributed to MeHg production. Analysis of microbial community composition confirmed the presence of several phyla known to harbor microorganisms associated with Hg(II) methylation in the soils. The observations suggest that SRB and methanogens were mainly responsible for Hg(II) methylation in these tundra soils, although their relative contributions depended on the availability of sulfate and possibly syntrophic metabolisms between SRB and methanogens.

Role of sulfur biogeochemical cycle in mercury methylation in estuarine sediments: A review

Estuaries are sinks for mercury, in which the most toxic mercury form, neurotoxic methylmercury (MeHg), is produced by mercury methylators and accumulates in estuarine sediments. In the same area, the microbial sulfur cycle is triggered by sulfate-reducing bacteria (SRB), which is considered as the main mercury methylator. In this review, we analyzed the sulfur and mercury speciation in sediments from 70 estuaries globally. Abundant mercury and sulfur species were found in the global estuarine sediments. Up to 727 μg THg/g dw and 880 ng MeHg/g dw were found in estuarine sediments, showing the serious risk of mercury to aquatic ecological systems. Significant correlations between sulfur and MeHg concentrations were discovered. Especially, the porewater sulfate concentration positively correlated to MeHg production. The sulfur cycle affects MeHg formation via activating mercury methylator activities and limiting mercury bioavailability, leading to promote or inhibit MeHg formation at different sulfur speciation concentrations. These results suggest that sulfur biogeochemical cycle plays an important role in mercury methylation in estuarine sediments, and the effect of the sulfur cycle on mercury methylation deserves to be further explored in future research.

Transcriptomic evidence for versatile metabolic activities of mercury cycling microorganisms in brackish microbial mats

Methylmercury, biomagnifying through food chains, is highly toxic for aquatic life. Its production and degradation are largely driven by microbial transformations; however, diversity and metabolic activity of mercury transformers, resulting in methylmercury concentrations in environments, remain poorly understood. Microbial mats are thick biofilms where oxic and anoxic metabolisms cooccur, providing opportunities to investigate the complexity of the microbial mercury transformations over contrasted redox conditions. Here, we conducted a genome-resolved metagenomic and metatranscriptomic analysis to identify putative activity of mercury reducers, methylators and demethylators in microbial mats strongly contaminated by mercury. Our transcriptomic results revealed the major role of rare microorganisms in mercury cycling. Mercury methylators, mainly related to Desulfobacterota, expressed a large panel of metabolic activities in sulfur, iron, nitrogen, and halogen compound transformations, extending known activities of mercury methylators under suboxic to anoxic conditions. Methylmercury detoxification processes were dissociated in the microbial mats with methylmercury cleavage being carried out by sulfide-oxidizing Thiotrichaceae and Rhodobacteraceae populations, whereas mercury reducers included members of the Verrucomicrobia, Bacteroidetes, Gammaproteobacteria, and different populations of Rhodobacteraceae. However most of the mercury reduction was potentially carried out anaerobically by sulfur- and iron-reducing Desulfuromonadaceae, revising our understanding of mercury transformers ecophysiology.

Nutrient Inputs Stimulate Mercury Methylation by Syntrophs in a Subarctic Peatland

Climate change dramatically impacts Arctic and subarctic regions, inducing shifts in wetland nutrient regimes as a consequence of thawing permafrost. Altered hydrological regimes may drive changes in the dynamics of microbial mercury (Hg) methylation and bioavailability. Important knowledge gaps remain on the contribution of specific microbial groups to methylmercury (MeHg) production in wetlands of various trophic status. Here, we measured aqueous chemistry, potential methylation rates (k_meth), volatile fatty acid (VFA) dynamics in peat-soil incubations, and genetic potential for Hg methylation across a groundwater-driven nutrient gradient in an interior Alaskan fen. We tested the hypotheses that (1) nutrient inputs will result in increased methylation potentials, and (2) syntrophic interactions contribute to methylation in subarctic wetlands. We observed that concentrations of nutrients, total Hg, and MeHg, abundance of hgcA genes, and rates of methylation in peat incubations (k_meth) were highest near the groundwater input and declined downgradient. hgcA sequences near the input were closely related to those from sulfate-reducing bacteria (SRB), methanogens, and syntrophs. Hg methylation in peat incubations collected near the input source (FPF2) were impacted by the addition of sulfate and some metabolic inhibitors while those down-gradient (FPF5) were not. Sulfate amendment to FPF2 incubations had higher k_meth relative to unamended controls despite no effect on k_meth from addition of the sulfate reduction inhibitor molybdate. The addition of the methanogenic inhibitor BES (25 mM) led to the accumulation of VFAs, but unlike molybdate, it did not affect Hg methylation rates. Rather, the concurrent additions of BES and molybdate significantly decreased k_meth, suggesting a role for interactions between SRB and methanogens in Hg methylation. The reduction in k_meth with combined addition of BES and molybdate, and accumulation of VFA in peat incubations containing BES, and a high abundance of syntroph-related hgcA sequences in peat metagenomes provide evidence for MeHg production by microorganisms growing in syntrophy. Collectively the results suggest that wetland nutrient regimes influence the activity of Hg methylating microorganisms and, consequently, Hg methylation rates. Our results provide key information about microbial Hg methylation and methylating communities under nutrient conditions that are expected to become more common as permafrost soils thaw.

Microbial sulfur metabolism and environmental implications

Sulfur as a macroelement plays an important role in biochemistry in both natural environments and engineering biosystems, which can be further linked to other important element cycles, e.g. carbon, nitrogen and iron. Consequently, the sulfur cycling primarily mediated by sulfur compounds oxidizing microorganisms and sulfur compounds reducing microorganisms has enormous environmental implications, particularly in wastewater treatment and pollution bioremediation. In this review, to connect the knowledge in microbial sulfur metabolism to environmental applications, we first comprehensively review recent advances in understanding microbial sulfur metabolisms at molecular-, cellular- and ecosystem-levels, together with their energetics. We then discuss the environmental implications to fight against soil and water pollution, with four foci: (1) acid mine drainage, (2) water blackening and odorization in urban rivers, (3) SANI® and DS-EBPR processes for sewage treatment, and (4) bioremediation of persistent organic pollutants. In addition, major challenges and further developments toward elucidation of microbial sulfur metabolisms and their environmental applications are identified and discussed.

Nutrient Exposure Alters Microbial Composition, Structure, and Mercury Methylating Activity in Periphyton in a Contaminated Watershed

The conversion of mercury (Hg) to monomethylmercury (MMHg) is a critical area of concern in global Hg cycling. Periphyton biofilms may harbor significant amounts of MMHg but little is known about the Hg-methylating potential of the periphyton microbiome. Therefore, we used high-throughput amplicon sequencing of the 16S rRNA gene, ITS2 region, and Hg methylation gene pair (hgcAB) to characterize the archaea/bacteria, fungi, and Hg-methylating microorganisms in periphyton communities grown in a contaminated watershed in East Tennessee (United States). Furthermore, we examined how nutrient amendments (nitrate and/or phosphate) altered periphyton community structure and function. We found that bacterial/archaeal richness in experimental conditions decreased in summer and increased in autumn relative to control treatments, while fungal diversity generally increased in summer and decreased in autumn relative to control treatments. Interestingly, the Hg-methylating communities were dominated by Proteobacteria followed by Candidatus Atribacteria across both seasons. Surprisingly, Hg methylation potential correlated with numerous bacterial families that do not contain hgcAB, suggesting that the overall microbiome structure of periphyton communities influences rates of Hg transformation within these microbial mats. To further explore these complex community interactions, we performed a microbial network analysis and found that the nitrate-amended treatment resulted in the highest number of hub taxa that also corresponded with enhanced Hg methylation potential. This work provides insight into community interactions within the periphyton microbiome that may contribute to Hg cycling and will inform future research that will focus on establishing mixed microbial consortia to uncover mechanisms driving shifts in Hg cycling within periphyton habitats.

Fate of mercury and methylmercury in full-scale sludge anaerobic digestion combined with thermal hydrolysis

Methylmercury (MeHg) is one of the highly toxic and bio-accumulated forms of mercury. Its presence in wastewater treatment processes has been evidenced in recent studies. Considering its enrichment in sewage sludge and the ecological risk associated with its land application, this study investigated the fate of mercury and MeHg in full-scale anaerobic digestion combined with Cambi thermal hydrolysis based on one-year sampling. Results showed that the advanced anaerobic digestion could increase the total mercury (THg) content from 4.35 ± 0.43 mg/kg in raw sludge to 6.37 ± 1.05 mg/kg in digested sludge, and the MeHg content decreased from 1.61 to 8.94 ng/g in raw sludge to 0.21-2.03 ng/g after anaerobic digestion. The demethylation of MeHg was dominant in both thermal hydrolysis and anaerobic digestion; it was mostly derived from the physico-chemical impacts such as chemical decomposition in thermal hydrolysis and precipitation in anaerobic digestion. Although the reported microbial methylators, such as Methanosarcina and Clostridia, were dominant in anaerobic digestion, the relative abundances of hgcA and merA were relatively low and did not correlate with the MeHg profiles. Thus, microbial methylation or demethylation seems negligible in terms of MeHg transformation.

Effects of mercury, organic carbon, and microbial inhibition on methylmercury cycling at the profundal sediment-water interface of a sulfate-rich hypereutrophic reservoir

Methylmercury (MeHg) produced by anaerobic bacteria in lakes and reservoirs, poses a threat to ecosystem and human health due to its ability to bioaccumulate in aquatic food webs. This study used 48-hr microcosm incubations of profundal sediment and bottom water from a sulfate-rich, hypereutrophic reservoir to assess seasonal patterns of MeHg cycling under various treatments. Treatments included addition of air, Hg(II), organic carbon, and microbial inhibitors. Both aeration and sodium molybdate, a sulfate-reducing bacteria (SRB) inhibitor, generally decreased MeHg concentration in microcosm water, likely by inhibiting SRB activity. The methanogenic inhibitor bromoethanesulfonate increased MeHg concentration 2- to 4- fold, suggesting that methanogens were potent demethylators. Pyruvate increased MeHg concentration under moderately reduced conditions, likely by stimulating SRB, but decreased it under highly reduced conditions, likely by stimulating methanogens. Acetate increased MeHg concentration, likely due to the stimulation of acetotrophic SRB. Results suggest that iron-reducing bacteria (IRB) were not especially prominent methylators and MeHg production at the sediment-water interface is elevated under moderately reduced conditions corresponding with SRB activity. In contrast, it is suppressed under oxic conditions due to low SRB activity, and under highly reduced conditions (<-100 mV) due to enhanced demethylation by methanogens.

Production of methylmercury by methanogens in mercury contaminated estuarine sediments

Anaerobic bacteria are known to produce neurotoxic methylmercury [MeHg] when elemental mercury [Hg(0)] is provided as the sole mercury source. In this study, we examined the formation of MeHg in anaerobic incubations of sediment collected from the San Jacinto River estuary (Texas, USA) amended with aqueous Hg(0) to investigate the microbial communities involved in the conversion of Hg(0) to MeHg. The results show that the addition of the methanogen inhibitor 2-bromoethanesulfonate (BES) significantly decreased MeHg production. The mercury methylation gene, hgcA, was detected in these sediments using archaeal specific primers, and 16S rRNA sequencing showed that a member of the Methanosarcinaceae family of methanogens was active. These results suggest that methanogenic archaea play an underappreciated role in the production of MeHg in estuarine sediments contaminated with Hg(0).

Mercury Methylation Genes Identified across Diverse Anaerobic Microbial Guilds in a Eutrophic Sulfate-Enriched Lake

Mercury (Hg) methylation is a microbially mediated process that converts inorganic Hg into bioaccumulative, neurotoxic methylmercury (MeHg). The metabolic activity of methylating organisms is highly dependent on biogeochemical conditions, which subsequently influences MeHg production. However, our understanding of the ecophysiology of methylators in natural ecosystems is still limited. Here, we identified potential locations of MeHg production in the anoxic, sulfidic hypolimnion of a freshwater lake. At these sites, we used shotgun metagenomics to characterize microorganisms with the Hg-methylation gene hgcA. Putative methylators were dominated by hgcA sequences divergent from those in well-studied, confirmed methylators. Using genome-resolved metagenomics, we identified organisms with hgcA (hgcA+) within the Bacteroidetes and the recently described Kiritimatiellaeota phyla. We identified hgcA+ genomes derived from sulfate-reducing bacteria, but these accounted for only 22% of hgcA+ genome coverage. The most abundant hgcA+ genomes were from fermenters, accounting for over half of the hgcA gene coverage. Many of these organisms also mediate hydrolysis of polysaccharides, likely from cyanobacterial blooms. This work highlights the distribution of the Hg-methylation genes across microbial metabolic guilds and indicate that primary degradation of polysaccharides and fermentation may play an important but unrecognized role in MeHg production in the anoxic hypolimnion of freshwater lakes.

Deltaproteobacteria and Spirochaetes-Like Bacteria Are Abundant Putative Mercury Methylators in Oxygen-Deficient Water and Marine Particles in the Baltic Sea

Methylmercury (MeHg), a neurotoxic compound biomagnifying in aquatic food webs, can be a threat to human health via fish consumption. However, the composition and distribution of the microbial communities mediating the methylation of mercury (Hg) to MeHg in marine systems remain largely unknown. In order to fill this knowledge gap, we used the Baltic Sea Reference Metagenome (BARM) dataset to study the abundance and distribution of the genes involved in Hg methylation (the hgcAB gene cluster). We determined the relative abundance of the hgcAB genes and their taxonomic identity in 81 brackish metagenomes that cover spatial, seasonal and redox variability in the Baltic Sea water column. The hgcAB genes were predominantly detected in anoxic water, but some hgcAB genes were also detected in hypoxic and normoxic waters. Phylogenetic analysis identified putative Hg methylators within Deltaproteobacteria, in oxygen-deficient water layers, but also Spirochaetes-like and Kiritimatiellaeota-like bacteria. Higher relative quantities of hgcAB genes were found in metagenomes from marine particles compared to free-living communities in anoxic water, suggesting that such particles are hotspot habitats for Hg methylators in oxygen-depleted seawater. Altogether, our work unveils the diversity of the microorganisms with the potential to mediate MeHg production in the Baltic Sea and pinpoint the important ecological niches for these microorganisms within the marine water column.

Two Decades of Mercury Concentrations in Barents Sea Polar Bears (Ursus maritimus) in Relation to Dietary Carbon, Sulfur, and Nitrogen

Temporal trends of total mercury (THg) were examined in female polar bear (Ursus maritimus) hair (n = 199) from the Barents Sea in 1995-2016. In addition, hair values of stable isotopes (n = 190-197) of carbon (δ¹³C), sulfur (δ³⁴S), and nitrogen (δ¹⁵N) and information on breeding status, body condition, and age were obtained. Stable isotope values of carbon and sulfur reflect dietary source (e.g., marine vs terrestrial) and the nitrogen trophic level. Values for δ¹³C and δ³⁴S declined by -1.62 and -1.18‰ over the time of the study period, respectively, while values for δ¹⁵N showed no trend. Total Hg concentrations were positively related to both δ¹³C and δ³⁴S. Yearly median THg concentrations ranged from 1.61 to 2.75 μg/g and increased nonlinearly by 0.86 μg/g in total over the study. Correcting THg concentrations for stable isotope values of carbon and sulfur and additionally breeding status and age slightly accelerated the increase in THg concentrations; however, confidence intervals of the raw THg trend and the corrected THg trend had substantial overlap. The rise in THg concentrations in the polar bear food web was possibly related to climate-related re-emissions of previously stored Hg from thawing sea-ice, glaciers, and permafrost.

Organic fertilizer amendment increases methylmercury accumulation in rice plants

Rice grains are a methylmercury (MeHg) intake route for humans, especially in certain mercury (Hg)-contaminated areas. For rice plant growth, animal manure is commonly used as an organic fertilizer; however, its role in the formation of MeHg in paddy soils remains poorly understood. The aims of this study were thus to explore 1) the effect of chicken manure (CH) and cow manure (CO) addition on the production of soil MeHg and the accumulation of MeHg in rice plants and 2) the mechanism by which CH and CO addition affect the bioaccumulation of MeHg in grains. A pot experiment with different levels of CH and CO was carried out with newly deposited Hg-contaminated paddy soil. Two microcosm experiments were performed to explore the associated mechanisms. The results of the pot experiment showed that 0.1-1% CH and CO addition promoted the biomass of rice plants by 10-23% and increased the soil MeHg concentration by 34-143%, which exhibited a significant positive correlation with brown rice MeHg content. Organic fertilizer addition significantly increased MeHg bioaccumulation in rice plants and the ratio of MeHg to total Hg (THg) in brown rice. Organic fertilizer also increased the abundance of microbial methylators. The results of the microcosm experiments showed that organic fertilizer addition enhanced dissolved THg concentrations in soil and consequently increased the soil MeHg concentration. These results suggested that applying organic fertilizer to newly deposited Hg-contaminated soil may increase MeHg accumulation in grains due to enhanced Hg release and microbial methylator activity, leading to environmental health concerns.

Opposing spatial trends in methylmercury and total mercury along a peatland chronosequence trophic gradient

Peatlands are abundant elements of boreal landscapes where inorganic mercury (IHg) can be transformed into bioaccumulating and highly toxic methylmercury (MeHg). We studied fifteen peatlands divided into three age classes (young, intermediate and old) along a geographically constrained chronosequence to determine the role of biogeochemical factors and nutrient availability in controlling the formation of MeHg. In the 10 cm soil layer just below the average annual growing season water table, concentrations of MeHg and %MeHg (of total Hg) were higher in younger, more mesotrophic peatlands than in older, more oligotrophic peatlands. In contrast, total mercury (THg) concentrations were higher in the older peatlands. Partial least squares (PLS) analysis indicates that the net MeHg production was positively correlated to trophic demands of vegetation and an increased availability of potential electron acceptors and donors for Hg methylating microorganisms. An important question for further studies will be to elucidate why there is less THg in the younger peatlands compared to the older peatlands, even though the age of the superficial peat itself is similar for all sites. We hypothesize that ecosystem features which enhance microbial processes involved in Hg methylation also promote Hg reduction that makes previously deposited Hg more available for evasion back to the atmosphere.

Biotic formation of methylmercury: A bio-physico-chemical conundrum

Mercury (Hg) is a natural and widespread trace metal, but is considered a priority pollutant, particularly its organic form methylmercury (MMHg), because of human's exposure to MMHg through fish consumption. Pioneering studies showed the methylation of divalent Hg (Hg^II) to MMHg to occur under oxygen-limited conditions and to depend on the activity of anaerobic microorganisms. Recent studies identified the hgcAB gene cluster in microorganisms with the capacity to methylate Hg^II and unveiled a much wider range of species and environmental conditions producing MMHg than previously expected. Here, we review the recent knowledge and approaches used to understand Hg^II-methylation, microbial biodiversity and activity involved in these processes, and we highlight the current limits for predicting MMHg concentrations in the environment. The available data unveil the fact that Hg^II methylation is a bio-physico-chemical conundrum in which the efficiency of biological Hg^II methylation appears to depend chiefly on Hg^II and nutrients availability, the abundance of electron acceptors such as sulfate or iron, the abundance and composition of organic matter as well as the activity and structure of the microbial community. An increased knowledge of the relationship between microbial community composition, physico-chemical conditions, MMHg production, and demethylation is necessary to predict variability in MMHg concentrations across environments.

Understanding mercury methylation in the changing environment: Recent advances in assessing microbial methylators and mercury bioavailability

Methylmercury (MeHg) is a neurotoxin, mainly derived from microbial mercury methylation in natural aquatic environments, and poses threats to human health. Polar regions and paddy soils are potential hotspots of mercury methylation and represent environmental settings that are susceptible to natural and anthropogenic perturbations. The effects of changing environmental conditions on the methylating microorganisms and mercury speciation due to global climate change and farming practices aimed for sustainable agriculture were discussed for polar regions and paddy soils, respectively. To better understand and predict microbial mercury methylation in the changing environment, we synthesized current understanding of how to effectively identify active mercury methylators and assess the bioavailability of different mercury species for methylation. The application of biomarkers based on the hgcAB genes have demonstrated the occurrence of potential mercury methylators, such as sulfate-reducing bacteria, iron-reducing bacteria, methanogen and syntrophs, in a diverse variety of microbial habitats. Advanced techniques, such as enriched stable isotope tracers, whole-cell biosensor and diffusive gradient thin film (DGT) have shown great promises in quantitatively assessing mercury availability to microbial methylators. Improved understanding of the complex structure of microbial communities consisting mercury methylators and non-methylators, chemical speciation of inorganic mercury under geochemically relevant conditions, and the pathway of cellular mercury uptake will undoubtedly facilitate accurate assessment and prediction of in situ microbial mercury methylation.

Shifts in mercury methylation across a peatland chronosequence: From sulfate reduction to methanogenesis and syntrophy

Peatlands are globally important ecosystems where inorganic mercury is converted to bioaccumulating and highly toxic methylmercury, resulting in high risks of methylmercury exposure in adjacent aquatic ecosystems. Although biological mercury methylation has been known for decades, there is still a lack of knowledge about the organisms involved in mercury methylation and the drivers controlling their methylating capacity. In order to investigate the metabolisms responsible for mercury methylation and methylmercury degradation as well as the controls of both processes, we studied a chronosequence of boreal peatlands covering fundamentally different biogeochemical conditions. Potential mercury methylation rates decreased with peatland age, being up to 53 times higher in the youngest peatland compared to the oldest. Methylation in young mires was driven by sulfate reduction, while methanogenic and syntrophic metabolisms became more important in older systems. Demethylation rates were also highest in young wetlands, with a gradual shift from biotic to abiotic methylmercury degradation along the chronosequence. Our findings reveal how metabolic shifts drive mercury methylation and its ratio to demethylation as peatlands age.

Reduced sulphur sources favour HgII reduction during anoxygenic photosynthesis by Heliobacteria

The consumption of rice has become a global food safety issue because rice paddies support the production of high levels of the potent neurotoxin, methylmercury. The production of methylmercury is carried out by chemotrophic anaerobes that rely on a diversity of terminal electron acceptors, namely sulphate. Sulphur can be a limiting nutrient in rice paddies, and sulphate amendments are often used to stimulate crop production, which can increase methylmercury production. Mercury (Hg) redox cycling can affect Hg methylation by controlling the delivery of inorganic Hg substrates to methylators in anoxic habitats. Whereas sulphur is recognized as a key substrate controlling methylmercury production, the controls sulphur exerts on other microbe-mediated Hg transformations remain poorly understood. To explore the potential coupling between sulphur assimilation and anaerobic Hg^II reduction to Hg⁰ , we studied Heliobacillus mobilis, a mesophilic anoxygenic phototroph representative from the Heliobacteriacea family originally isolated from a rice paddy. Here, we tested whether the redox state of the sulphur sources available to H. mobilis would affect its ability to reduce Hg^II . By comparing Hg⁰ production over a redox gradient of sulphur sources, we demonstrate that phototrophic Hg^II reduction is favoured in the presence of reduced sulphur sources such as thiosulphate and cysteine. We also show that cysteine exerts dynamic control on Hg cycling by affecting not only Hg's bioavailability but also its abiotic photoreduction under low light conditions. Specifically, in the absence of cells we show that organic matter (as yeast extract) and cysteine are both required for photoreduction to occur. This study offers insights into how one of the most primitive forms of photosynthesis affects Hg redox transformations and frames Heliobacteria as key players in Hg cycling within paddy soils, forming a basis for management strategies to mitigate Hg accumulation in rice.

Overlooked Role of Putative Non-Hg Methylators in Predicting Methylmercury Production in Paddy Soils

Rice ingestion has been recognized as an important route of dietary exposure to neurotoxic methylmercury (MeHg) that is commonly synthesized in rice paddy soils. Although Hg methylators are known to regulate soil MeHg formation, the effect of non-Hg methylating communities on MeHg production remains unclear. Here, we collected 141 paddy soil samples from main rice-producing areas across China to identify associations between bacterial community composition (including both Hg and putative non-Hg methylators) and MeHg production. Results showed that the MeHg content in the paddy soils varied from 0.11 to 8.36 ng g^-1 at a national spatial scale, which could be due to the shifts of soil microbial community composition across different areas. Our structure equation modeling suggested a strong link between bacterial community composition and MeHg content and %MeHg. More importantly, random forest analyses suggested a more significant role of putative non-Hg methylators than Hg methylators in predicting variations of soil MeHg content. The relative abundance of putative non-Hg methylators such as unclassified Xanthomonadales and Chitinophagaceae were strongly correlated with soil MeHg contents. Further, microbial network analysis revealed strong co-occurrence patterns between the putative non-Hg and Hg methylators. These findings highlight an overlooked role of non-Hg methylating communities in predicting MeHg production in paddy soils.

Mercury species in the nests and bodies of soil-feeding termites, Silvestritermes spp. (Termitidae, Syntermitinae), in French Guiana

Mercury pollution is currently a major public health concern, given the adverse effects of mercury on wildlife and humans. Soil plays an essential role in speciation of mercury and its global cycling, while being a habitat for a wide range of terrestrial fauna. Soil fauna, primarily soil-feeding taxa that are in intimate contact with soil pollutants are key contributors in the cycling of soil mercury and might provide relevant indications about soil pollution. We studied the enrichment of various mercury species in the nests and bodies of soil-feeding termites Silvestritermes spp. in French Guiana. Soil-feeding termites are the only social insects using soil as both shelter and food and are major decomposers of organic matter in neotropical forests. Nests of S. minutus were depleted in total and mobile mercury compared to nearby soil. In contrast, they were enriched 17 times in methylmercury. The highest concentrations of methylmercury were found in body of both studied termite species, with mean bioconcentration factors of 58 for S. minutus and 179 for S. holmgreni relative to the soil. The assessment of the body distribution of methylmercury in S. minutus showed concentrations of 221 ng g^-1 for the guts and even higher for the gut-free carcasses (683 ng g^-1), suggesting that methylmercury is not confined to the gut where it was likely produced, but rather stored in various tissues. This enrichment in the most toxic form of Hg in termites may be of concern on termite predators and the higher levels in the food chain that may be endangered through prey-to-predator transfers and bioaccumulation. Soil-feeding termites appear to be promising candidates as bio-indicators of mercury pollution in soils of neotropical rainforest ecosystems.

Sulfate-reducing bacteria and methanogens are involved in arsenic methylation and demethylation in paddy soils

Microbial arsenic (As) methylation and demethylation are important components of the As biogeochemical cycle. Arsenic methylation is enhanced under flooded conditions in paddy soils, producing mainly phytotoxic dimethylarsenate (DMAs) that can cause rice straighthead disease, a physiological disorder occurring widely in some rice growing regions. The key microbial groups responsible for As methylation and demethylation in paddy soils are unknown. Three paddy soils were incubated under flooded conditions. DMAs initially accumulated in the soil porewater, followed by a rapid disappearance coinciding with the production of methane. The soil from a rice straighthead disease paddy field produced a much larger amount of DMAs than the other two soils. Using metabolic inhibition, quantification of functional gene transcripts, microbial enrichment cultures and 13C-labeled DMAs, we show that sulfate-reducing bacteria (SRB) and methanogenic archaea are involved in As methylation and demethylation, respectively, controlling the dynamics of DMAs in paddy soils. We present a model of As biogeochemical cycle in paddy soils, linking the dynamics of changing soil redox potential with arsenite mobilization, arsenite methylation and subsequent demethylation driven by different microbial groups. The model provides a basis for controlling DMAs accumulation and incidence of straighthead disease in rice.

A novel biological sulfur reduction process for mercury-contaminated wastewater treatment

The sulfidogenic process driven by sulfate-reducing bacteria (SRB) is not suitable for mercury-contaminated wastewater treatment due to the highly toxic methyl-mercury (MeHg) produced by SRB. In our previous study, we observed in short-term batch tests that sulfur-reducing bacteria (S⁰RB) could remove mercury ions without MeHg production. Thus, the aim of this study is to develop a biological sulfur reduction process driven by S⁰RB for mercury-contaminated wastewater, and investigate its long-term performance on mercury removal and MeHg accumulation. Receiving mercury-contaminated wastewater containing 0-50 mg Hg(II)/L for 326 days, S⁰RB in the sulfur-reducing bioreactor showed high tolerance with mercury toxicity, and removed 99.4% ± 1.4% of the influent Hg(II) by biogenic sulfide. MeHg was always found to be undetectable in the bioreactor, even though the sulfidogenic bacteria were exposed to high levels of Hg(II) in long-term trials. The result of qPCR analysis further revealed that the mercury-methylation functional gene (hgcA) concentration in the bioreactor sludge was found to be extremely lower than in the SRB-enriched sludge, Geobacter sulfurreducens PCA and Desulfomicrobium baculatum DSM 4028, implying that there was no or few mercury methylators in the bioreactor. In short, the biological sulfur reduction process using S⁰RB can efficiently treat mercury-contaminated wastewater, with high Hg(II) removal efficiency and no MeHg accumulation.

A review on mercury biogeochemistry in mangrove sediments: Hotspots of methylmercury production?

Wetlands are highly productive and biologically diverse environments that provide numerous ecosystem services, but can also be sources of methylmercury (MeHg) production and export. Mangrove wetlands contribute up to 15% of the coastal sediment carbon storage and ~10% of the particulate terrestrial carbon exported to the ocean. Thus, mercury (Hg) methylation in mangrove sediments and subsequent MeHg output to adjacent waters could have a great impact on global Hg cycling. In this review, we provide a comprehensive analysis of the literature on worldwide Hg concentrations in mangrove ecosystems, and the results reveal that a large range of total Hg (THg) and MeHg concentrations is detected in mangrove systems. Then, we discuss the potential roles of organic matter (OM) in controlling the Hg biogeochemistry in mangrove sediments. The intense OM decomposition by anoxic reduction (e.g., sulfate reduction) drastically affects sediment chemistries, such as redox potential, pH, and sulfur speciation, all of which may have a great impact on MeHg production. While the outwelling of carbon from mangroves has been extensively examined, little is known about their roles in exporting MeHg to adjacent waters. Our understanding of Hg biogeochemical processes in mangrove systems is constrained by the limited MeHg data and a lack of in-depth studies on the Hg methylation potential in this ecologically important environment. More efforts are needed to gain better insights into the contributions mangrove wetlands to the global Hg cycle.

Geochemical Factors Controlling Dissolved Elemental Mercury and Methylmercury Formation in Alaskan Wetlands of Varying Trophic Status

The transformations of aqueous inorganic divalent mercury (Hg(II)_i) to volatile dissolved gaseous mercury (Hg(0)_(aq)) and toxic methylmercury (MeHg) govern mercury bioavailability and fate in northern ecosystems. This study quantified concentrations of aqueous mercury species (Hg(II)_i, Hg(0)_(aq), MeHg) and relevant geochemical constituents in pore waters of eight Alaskan wetlands that differ in trophic status (i.e., bog-to-fen gradient) to gain insight on processes controlling dark Hg(II)_i reduction and Hg(II)_i methylation. Regardless of wetland trophic status, positive correlations were observed between pore water Hg(II)_i and dissolved organic carbon (DOC) concentrations. The concentration ratio of Hg(0)_(aq) to Hg(II)_i exhibited an inverse relationship to Hg(II)_i concentration. A ubiquitous pathway for Hg(0)_(aq) formation was not identified based on geochemical data, but we surmise that dissolved organic matter (DOM) influences mercury retention in wetland pore waters by complexing Hg(II)_i and decreasing the concentration of volatile Hg(0)_(aq) relative to Hg(II)_i. There was no evidence of Hg(0)_(aq) abundance directly limiting mercury methylation. The concentration of MeHg relative to Hg(II)_i was greatest in wetlands of intermediate trophic status, and geochemical data suggest mercury methylation pathways vary between wetlands. Our insights on geochemical factors influencing aqueous mercury speciation should be considered in context of the long-term fate of mercury in northern wetlands.

Unraveling Microbial Communities Associated with Methylmercury Production in Paddy Soils

Rice consumption is now recognized as an important pathway of human exposure to the neurotoxin methylmercury (MeHg), particularly in countries where rice is a staple food. Although the discovery of a two-gene cluster hgcAB has linked Hg methylation to several phylogenetically diverse groups of anaerobic microorganisms converting inorganic mercury (Hg) to MeHg, the prevalence and diversity of Hg methylators in microbial communities of rice paddy soils remain unclear. We characterized the abundance and distribution of hgcAB genes using third-generation PacBio long-read sequencing and Illumina short-read metagenomic sequencing, in combination with quantitative PCR analyses in several mine-impacted paddy soils from southwest China. Both Illumina and PacBio sequencing analyses revealed that Hg methylating communities were dominated by iron-reducing bacteria (i.e., Geobacter) and methanogens, with a relatively low abundance of hgcA ⁺ sulfate-reducing bacteria in the soil. A positive correlation was observed between the MeHg content in soil and the relative abundance of Geobacter carrying the hgcA gene. Phylogenetic analysis also uncovered some hgcAB sequences closely related to three novel Hg methylators, Geobacter anodireducens, Desulfuromonas sp. DDH964, and Desulfovibrio sp. J2, among which G. anodireducens was validated for its ability to methylate Hg. These findings shed new light on microbial community composition and major clades likely driving Hg methylation in rice paddy soils.

Shining light on recent advances in microbial mercury cycling

Mercury (Hg) is a global pollutant emitted primarily as gaseous Hg0 that is deposited in aquatic and terrestrial ecosystems following its oxidation to HgII. From that point, microbes play a key role in determining Hg’s fate in the environment by participating in sequestration, oxidation, reduction, and methylation reactions. A wide diversity of chemotrophic and phototrophic microbes occupying oxic and anoxic habitats are known to participate directly in Hg cycling. Over the last few years, new findings have come to light that have greatly improved our mechanistic understanding of microbe-mediated Hg cycling pathways in the environment. In this review, we summarize recent advances in microbially mediated Hg cycling and take the opportunity to compare the relatively well-studied chemotrophic pathways to poorly understood phototrophic pathways. We present how the use of genomic and analytical tools can be used to understand Hg transformations and the physiological context of recently discovered cometabolic Hg transformations supported in anaerobes and phototrophs. Finally, we propose a conceptual framework that emphasizes the role that phototrophs play in environmental Hg redox cycling and the importance of better characterizing such pathways in the face of the environmental changes currently underway.