The influence of wetting-drying alternation on methylmercury degradation in Guangzhou soil

Photochemical demethylation of methylmercury (MeHg) in aquatic systems: A review of MeHg species, mechanisms, and influencing factors

Photodemethylation is the major pathway of methylmercury (MeHg) demethylation in surface water before uptake by the food chain, whose mechanisms and influence factors are still not completely understood. Here, we review the current knowledge on photodemethylation of MeHg and divide MeHg photolysis into three pathways: (1) direct photodemethylation, (2) free radical attack, and (3) intramolecular electron or energy transfer. In aquatic environments, dissolved organic matter is involved into all above pathways, and due to its complex compositions, properties and concentrations, DOM poses multiple functions during the PD of MeHg. DOM-MeHg complex (mainly by sulfur-containing molecules) might weaken the C-Hg bond and enhance PD through both direct and indirect pathways. In special, synergistic effects of both strong binding sites and chromophoric moieties in DOM might lead to intramolecular electron or energy transfer. Moreover, DOM might play a role of radical scavenger; while triplet state DOM, which is generated by chromophoric DOM under light, might become a source of free radicals. Apart from DOMs, transition metals, halides, NO3-, NO2-, and carbonates also act as radical initialaters or scavengers, and significantly pose effects on radical demethylation, which is generally mediated by hydroxyl radicals and singlet oxygen. Environmental factors such as pH, light wavelength, light intensity, dissolved oxygen, salinity, and suspended particles also affect the PD of MeHg. This study assessed previously published works on three major mechanisms, with the goal of providing general estimates for photodemethylation under various environment factors according to know effects, and highlighting the current uncertainties for future research directions.

Humic substances as electron acceptor for anaerobic oxidation of methane (AOM) and electron shuttle in Mn (IV)-dependent AOM

Anaerobic methanotrophic archaea (ANME) belonging to the family Methanoperedenaceae are crucial for the global carbon cycle and different biogeochemical processes, owing to their metabolic versatility to couple anaerobic oxidation of methane (AOM) with different electron acceptors. A universal feature of Methanoperedenaceae is the abundant genes encoded in their genomes associated with extracellular electron transfer (EET) pathways. Candidatus. 'Methanoperedens manganicus', an archaeon belonging to the family Methanoperedenaceae, was recently enriched in a bioreactor performing AOM coupled with Mn (IV) reduction. Using this EET-capable ANME, we tested the hypothesis in this study that ANME can catalyse the humic-dependent AOM for growth. A two-year incubation showed that AOM activity can be sustained by Ca. 'M. manganicus' consortium in a bioreactor fed only with humic acids and methane. An isotopic mass balance batch test confirmed that the observed AOM was coupled to the reduction of humic acids. The increase of relative abundance of Ca. 'M. manganicus', and the total archaea population in the microbial community suggested that Ca. 'M. manganicus' can grow on methane and humic acids. The observation of humic-dependent AOM led to a subsequent hypothesis that humic acids could be used as the electron shuttle to mediate the EET in dissimilatory Mn (IV) reduction by Ca. 'M. manganicus'. We tested this hypothesis by adding humic acids to a Ca. 'M. manganicus' dominated-culture, which showed that the AOM rate was doubled by the addition of humic acids. X-ray photoelectron spectroscopy (XPS) showed that quinone moieties were consumed when humic acids worked as electron acceptors while remaining stable when functioning as a shuttle for electron transfer. The results of our study suggest that humic acids may serve as electron shuttles to allow ANME to access more electron acceptors through long-range EET.

Role of light in methylmercury photodegradation: From irradiation to absorption in the presence of organic ligands

Photochemical degradation acts as the principal sink for methylmercury (MeHg) in surface water, which is regulated by light and solution matrix (especially the presence of dissolved organic matter, DOM). The spectral composition of light irradiation and the light absorption properties of reaction media (often exerted by DOM) are important in MeHg photodegradation, which has not yet been clearly resolved. Aiming to understand the role of light in MeHg photodegradation from the perspectives of both light irradiation and absorption, we investigated the photodegradation of MeHg under different simulated sunlight sources, with and without DOM model compounds of different molecular structures. The results show that the photodegradation of MeHg under different sunlight irradiation yields distinct first-order date constant, mainly due to the slight difference in UVB composition. The degradation of MeHg without DOM under a light source with high intensity in the UV region and no MeHg degradation under the UV-filtered light even in the presence of DOM showed the importance of UV lights in MeHg photodegradation. The use of ultrapure water as a reaction medium may be subject to MeHg loss through vessel adsorption, not just photolysis. Additionally, this work found that the type and position of coexisting substituents on aromatic thiols play a critical role in improving the photodegradation of MeHg, following amino > hydroxyl > carboxyl, para > meta > ortho. Based on the characterization of ultraviolet-visible absorption spectra and our previous work, it was concluded that the presence of DOM could induce red-shift in light absorption and reduce the electronic transition energy of the CHg bond, facilitating MeHg photodegradation. The structures of DOM affect the light absorption properties, which are related to MeHg photodegradation.

Mercury isotope fractionation during methylmercury transport and transformation: A review focusing on analytical method, fractionation characteristics, and its application

Methylmercury (MeHg), a potent neurotoxin, can be formed, migrated and transformed in environmental compartments, accompanying with unique mass-dependent and mass-independent fractionation of mercury (Hg). These Hg isotope fractionation signals have great potential to probe the transformation and transport of MeHg in aquatic environments. However, the majority of studies to date have focused on total Hg isotopic composition, with less attention to the isotopic fractionation of MeHg due to technical difficulties in analysis, which severely hinders the understanding of MeHg isotopic fractionation and its applications. This review a) evaluates the reported analytical methods for Hg isotopic composition of MeHg, including online and offline measurement techniques; b) summarizes the extent and characteristics of Hg isotopic fractionation during MeHg transport and transformation, focusing on methylation, demethylation, trophic transfer and internal metabolism; and c) briefly discusses several applications of MeHg isotopic fractionation signatures in estimating the extent of photodemethylation, tracing the source of Hg species, and diagnosing reaction mechanisms. Additionally, the existing problems and future directions in MeHg isotope fractionation are highlighted to improve the analytical protocol for Hg isotope fractionation and deepen our understanding of Hg isotope fractionation in the biogeochemical cycling of MeHg.

Demethylation─The Other Side of the Mercury Methylation Coin: A Critical Review

The public and environmental health consequences of mercury (Hg) methylation have drawn much attention and considerable research to Hg methylation processes and their dynamics in diverse environments and under a multitude of conditions. However, the net methylmercury (MeHg) concentration that accumulates in the environment is equally determined by the rate of MeHg degradation, a complex process mediated by a variety of biotic and abiotic mechanisms, about which our knowledge is limited. Here we review the current knowledge on MeHg degradation and its potential pathways and mechanisms. We describe detoxification by resistant microorganisms that employ the Hg resistance (mer) system to reductively break the carbon-mercury (C-Hg) bond producing methane (CH₄) and inorganic mercuric Hg(II), which is then reduced by the mercuric reductase to elemental Hg(0). Very recent research has begun to elucidate a mechanism for the long-recognized mer-independent oxidative demethylation, likely involving some strains of anaerobic bacteria as well as aerobic methane-oxidizing bacteria, i.e., methanotrophs. In addition, photochemical and chemical demethylation processes are described, including the roles of dissolved organic matter (DOM) and free radicals as well as dark abiotic demethylation in the natural environment about which little is currently known. We focus on mechanisms and processes of demethylation and highlight the uncertainties and known effects of environmental factors leading to MeHg degradation. Finally, we suggest future research directions to further elucidate the chemical and biochemical mechanisms of biotic and abiotic demethylation and their significance in controlling net MeHg production in natural ecosystems.

Contrasting impacts of drying-rewetting cycles on the dissipation of di-(2-ethylhexyl) phthalate in two typical agricultural soils

Di-(2-ethylhexyl) phthalate (DEHP) pollution has become a growing problem in farmlands of China. Drying-rewetting (DW) cycle is one of frequent environmental changes that agricultural production is confronted with, and also a convenient and practical agronomic regulation measure. In this study, in order to explore the effects of DW cycles on the dissipation of DEHP and their driving mechanisms in different types of soils, we performed a 45-day microcosm culture experiment with two typical agricultural soils, Lou soil (LS) and Red soil (RS). High-throughput sequencing was applied to study the response of soil microbial communities in the process of DEHP dissipation under DW cycles. The results showed that the DW cycles considerably inhibited the dissipation of DEHP in LS while promoted that in RS. The DW cycles obviously decreased the diversity, the relative abundance of significantly differential bacteria, and the total abundance of potential degrading bacterial groups in LS, whereas have little effect on bacterial community in RS, except at the initial cultivation stage when the corresponding parameters were promoted. The inhibition of the DW cycles on DEHP dissipation in LS was mainly derived from microbial degradation, but the interplay between microbial functions and soil attributes contributed to the promotion of DEHP dissipation in RS under the DW cycles. This comprehensive understanding of the contrasting impacts and underlying driving mechanisms may provide crucial implications for the prevention and control of DEHP pollution in regional soils.