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Distinguishing the respective contributions of various microbes to methylmercury (MeHg) production is critical for predicting MeHg bioaccumulation and exposure risk. Metabolic inhibitors have been commonly used to block the activity of specific microbial groups and identify primary Hg methylating microbes. By reviewing literatures and our empirical data, we demonstrate how multiple factors, including (1) the addition of inappropriate amounts of inhibitors, (2) a tendency to overlook microbial syntrophy, and (3) the absence of comprehensive proxy systems of Hg methylation, would impact result interpretation of this approach. We thus suggest that the design of inhibition assays should consider the environmental properties, e.g., background levels of electron acceptors, concentrations of metabolic substrates, and abundances of Hg methylating microbes. We also recommend that inhibitors should be added at multiple concentrations and that observed changes in Hg methylation should be assessed with comprehensive indicators. Revealing the key factors responsible for the improper usage of this method and inadequate interpretation of the results would help optimize inhibition assays for robust predictions of MeHg production in nature. [ABSTRACT FROM AUTHOR]

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. Unlabelled Image • A large range of mercury (Hg) concentrations is observed in mangrove systems. • Mangrove sediments exhibit several features that may favor MeHg production and accumulation. • Mangrove sediments may act as hotspots to export MeHg to adjacent coastal waters. [ABSTRACT FROM AUTHOR]

Abstract Eutrophication is a major environmental concern in lake systems, impacting the ecological risks of contaminants and drinking water safety. It has long been believed that eutrophication and thus algal blooms would reduce methylmercury (MeHg) levels in water, as well as MeHg bioaccumulation and trophic transfer (e.g., by growth dilution). In this study, however, we demonstrated that algae settlement and decomposition after algal blooms increased MeHg levels in sediments (54–514% higher), as evidenced by the results from sediments in 10 major lakes in China. These could in turn raise concerns about enhanced trophic transfer of MeHg and deterioration of water quality after algal blooms, especially considering that 9 out of the 10 examined lakes also serve as drinking water sources. The enhanced microbial MeHg production in sediments could be explained by the algal organic matter (AOM)-enhanced abundances of microbial methylators as well as the input of algae-inhabited microbes into sediments, but not Hg speciation in sediments: (1) Several AOM components (e.g., aromatic proteins and soluble microbial by product-like material with generally low molecular weights), rather than the bulk AOM, played key roles in enhancing the abundances of microbial methylators. The copies of Archaea- hgcA methylation genes were 51–397% higher in algae-added sediments; thus, MeHg production was also higher. (2) Input of algal biomass-inhabited microbial methylators contributed to 2–21% of total Archaea- hgcA in the 10 lake sediments with added algal biomass. (3) However, AOM-induced changes in Hg speciation, with implications on Hg availability to microbial methylators, played a minor role in enhancing microbial Hg methylation in sediments as seen in X-ray absorption near edge structure (XANES) data. Our results suggest the need to better understand the biogeochemistry and risks of contaminants in eutrophic lakes, especially during the period of algae settlement and decomposition following algal blooms. Graphical abstract Unlabelled Image Highlights • Algae settlement and decomposition enhanced net MeHg production in lake sediments. • Low molecular weight algal organic matter (AOM) increased the abundances of methylators. • Input of algae-inhabited microbes into sediments enhanced microbial MeHg production. • AOM-induced changes in Hg speciation played a minor role in Hg methylation. [ABSTRACT FROM AUTHOR]

Methylmercury (MeHg) production in paddy soils and its accumulation in rice raise global concerns since rice consumption has been identified as an important pathway of human exposure to MeHg. Sulfur (S) amendment via fertilization has been reported to facilitate Hg methylation in paddy soils under anaerobic conditions, while the dynamic of S-amendment induced MeHg production in soils with increasing redox potential remains unclear. This critical gap hinders a comprehensive understanding of Hg biogeochemistry in rice paddy system which is characterized by the fluctuation of redox potential. Here, we conducted soil incubation experiments to explore MeHg production in slow-oxidizing paddy soils amended with different species of S and doses of sulfate. Results show that the elevated redox potential (1) increased MeHg concentrations by 10.9%−35.2%, which were mainly attributed to the re-oxidation of other S species to sulfate and thus the elevated abundance of sulfate-reducing bacteria, and (2) increased MeHg phytoavailability by up to 75% due to the reductions in acid volatile sulfide (AVS) that strongly binds MeHg in soils. Results obtained from this study call for attention to the increased MeHg production and phytoavailability in paddy soils under elevated redox potentials due to water management, which might aggravate the MeHg production induced by S fertilization and thus enhance MeHg accumulation in rice. [ABSTRACT FROM AUTHOR]

There is growing evidence that incorporating crop straw into soils, which is being widely encouraged in many parts of the world, could increase net methylmercury (MeHg) production in soils and MeHg accumulation in crops. We explored the possibility of mitigating the risk of increased MeHg levels under straw amendment by transforming straw into biochar (BC). Greenhouse and batch experiments were conducted, in which soil MeHg concentrations, MeHg phytoavailability and accumulation in rice, dynamics of sulfate, nitrate and abundances of sulfate reducing bacteria (SRB) were compared in ‘Control’ (Hg contaminated soil), ‘Straw’ (soil with 1% rice straw), ‘Straw + BC’ (soil with 1% straw and 1% biochar), and ‘Straw + BC + N’ (soil with 1% straw, 1% biochar and 0.12% nitrate). Our results indicate that straw amendment increased MeHg concentrations in soils (28–136% higher) and rice plants (26% higher in grains, ‘Straw’ versus ‘Control’), while co-application of biochar with straw reduced grain MeHg levels (60% lower, ‘Straw + BC’ versus ‘Straw’). This could be mainly attributed to the reduced MeHg availability to rice plants (phytoavailability, extraction rates of MeHg by ammonium thiosulfate) under biochar amendment (64–99% lower, ‘Straw + BC’ versus ‘Straw’). However, biochar amendment enhanced soil MeHg levels (5–75% higher, ‘Straw + BC’ versus ‘Control’). Interestingly, nitrate addition helped reduce soil MeHg concentrations (11–41% lower, ‘Straw + BC + N’ versus ‘Straw + BC’) by facilitating nitrate reduction while inhibiting SRB activities. Subsequently, addition of nitrate with biochar, compared with biochar alone, further reduced grain MeHg levels by 34%. Therefore, straw biochar together with nitrate could possibly be effective in mitigating the risk of MeHg under straw amendment. Furthermore, the results evidence the impacts of straw management on the risk posed by MeHg in soils and emphasize the necessity to carefully consider the straw management policy in Hg-contaminated areas. [ABSTRACT FROM AUTHOR]

Mangrove ecosystems serve as an important carbon sink but also could be a hotspot that produces neurotoxic methylmercury (MeHg). Although many studies have focused on mercury (Hg) contamination in this carbon-rich ecosystem, our understanding of the effects and mechanisms of the organic matter (OM) regulation of MeHg production in mangrove sediments is still limited. Here, we examined the effects of Hg contamination and OM enrichment on MeHg production in anoxic mangrove sediments and identified the major microbial guilds attending this process. The mangrove sediments possessed a high potential for producing MeHg, but this was counterbalanced by its rapid degradation. Sulfate-reducing bacteria (SRB) such as Desulfobacterales , Desulfovibrionales , and Syntrophobacterales were the major methylators. OM diagenesis significantly changed the biogeochemical conditions, accelerating MeHg degradation in the sediments. The enhanced MeHg degradation could be attributed to the abundant sulfide produced during OM decomposition, which could potentially inhibit the Hg methylation by immobilization of inorganic Hg, abiotically degrade MeHg, and favor the non- mer -mediated degradation of MeHg by SRB. Our study provides both geochemical and microbial clues that can partly explain the low MeHg levels widely observed in mangrove sediments. [Display omitted] • Geochemistry and microbial analysis explain the low MeHg in mangrove sediments. • Hg methylation counterbalanced with MeHg degradation in mangrove sediments. • OM enrichment facilitated MeHg degradation in the sediments. • SRB played critical roles in both Hg methylation and MeHg demethylation. [ABSTRACT FROM AUTHOR]

[Display omitted] • Both methylation and demethylation were promoted in AD with thermal hydrolysis. • Thermal hydrolysis directly reduced MeHg content from 4.24 ng/g to 0.95 ng/g. • Further improved demethylation with MeHg content reducing to 0.39 ng/g after AD. • Potential methylator, gene marker, relation with microbiota were identified. Reducing health risk of mercury (Hg)/methylmercury (MeHg) in sewage sludge is vital to its land application. This study revealed that thermal hydrolysis reduced MeHg content both during pretreatment process and subsequent anaerobic digestion (AD), which resulted in decrease of MeHg content from 4.24 ng/g to 0.95 ng/g after thermal hydrolysis (150 ℃) and further decreased to 0.39 ng/g after AD. Notably, thermal hydrolysis at high temperature (120 ℃ and 150 ℃) promoted both Hg methylation and MeHg demethylation rather than the control or at low temperature (100 ℃). Hg methylation dominated in hydrolysis and acidogenesis stage, whereas MeHg demethylation dominated in methanogenesis stage. Though abundance of related genes (HgcA and merA) was dramatically reduced, Ruminococcaceae , Peptococcaceae , and Lachnospiraceae were potentially Hg methylators in hydrolysis and acidogenesis stage. Whereas, MeHg demethylation dominated in the late period of AD due to the improved syntrophic methanogenesis and possibly reduced Hg2+ biodegradability by precipitation. [ABSTRACT FROM AUTHOR]

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. ga1 • The fate of mercury and methylmercury in anaerobic digestion was investigated. • Total mercury was concentrated while methylmercury was dramatically reduced. • Health risk of mercury derived from methylmercury was lowered. • Demethylation of MeHg was dominant as the result of physico-chemical impacts. [ABSTRACT FROM AUTHOR]