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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]

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]

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]