Competitive Binding Kinetics of Methylmercury-Cysteine with Dissolved Organic Matter: Critical Impacts of Thiols on Adsorption and Uptake of Methylmercury by Algae.

Complexes with low-molecular-weight thiols are crucial species of methylmercury (MeHg) excreted by anaerobic Hg-methylating microbes, notably, MeHg-cysteine (MeHg-Cys). As MeHg-Cys diffuses into surface water, it would undergo a ligand exchange process with dissolved organic matter (DOM) under nonsulfidic conditions, inevitably altering MeHg speciation and bioavailability to phytoplankton. In this study, we investigated the competitive binding kinetics between MeHg-Cys and Suwannee River natural organic matter, and their influence on the adsorption and uptake of MeHg by the cyanobacterium, Synechocystis sp. PCC6803. Liquid chromatography-inductively coupled plasma mass spectrometry was employed to monitor the kinetics processes involving competition of DOM with Cys for MeHg binding, which revealed that competitive binding kinetics were dictated by the abundance of thiol moieties in DOM. Thiol concentrations of 0.97 and 49.34 µmol of thiol (g C)-1 resulted in competitive binding rate constant (k values) of 0.30 and 3.47 h-1, respectively. Furthermore, the time-dependent competitive binding of DOM toward MeHg-Cys significantly inhibited MeHg adsorption and uptake by cyanobacteria, an effect that was amplified by an increased thiol abundance in DOM. These findings offer valuable insights into the kinetic characteristics of MeHg's fate and transport, as well as their impact on bioconcentration in aquatic organisms within natural aquatic ecosystems.

Uncovering the Role of Methylmercury on DNA Lesions at Cytotoxic Concentrations in Glutathione-Depleted Cells: Insights from Experimental and Computational Studies.

Organomercurials (RHg+), especially methylmercury (MeHg+) and ethylmercury (EtHg+), are considered to be more neurotoxic than the inorganic counterpart (Hg2+). They cause massive DNA damage in cells, especially in neurons, where cellular glutathione (GSH) levels are significantly low. However, the mechanism by which RHg+ exerts massive DNA damage at cytotoxic concentrations in brain cells remains obscure. In this study, we investigated the effect of RHg+ on the structural and electronic properties of nucleosides and its effects on DNA damage. The direct interaction of RHg+ with the nucleoside significantly weakens N-glycosidic bonds, decreases the C-H bond energy of sugar moieties, and increases the electrophilicity of the C8-center of purine bases. As a consequence, RHg+-conjugated DNA molecules are extremely labile and highly sensitive to any nucleophiles/radicals present in GSH-depleted cells and, thus, undergo enhanced oxidative and unusual alkylative DNA damage. We also report a functional model of organomercurial lyase, which showed excellent cytoprotective effect against RHg+-induced cytotoxicity; this reverses the activity of glutathione reductase inhibited by MeHgCl and ceases oxidative and alkylating DNA damage. This intriguing finding provides new mechanistic insight into the mode of action of organomercurials in GSH-depleted cells and their adverse effects on individuals with neurodegenerative disorders associated with oxidative stress.

Light-induced degradation of dimethylmercury in different natural waters.

Photo-induced degradation of dimethylmercury (DMHg) is considered to be an important source for the generation of methylmercury (MMHg). However, studies on DMHg photodegradation are scarce, and it is even debatable about whether DMHg can be degraded in natural waters. Herein, we found that both DMHg and MMHg could be photodegraded in three natural waters collected from the Yellow River Delta, while in pure water only DMHg photodegradation occurred under visible light irradiation. The effects of different environmental factors on DMHg photodegradation were investigated, and the underlying mechanisms were elucidated by density functional theory calculations and a series of control experiments. Our findings revealed that the DMHg degradation rate was higher in the tidal creek water compared to Yellow River, Yan Lake, and purified water. NO3-, NO2-, and DOM could promote the photodegradation with DOM and NO3- showing particularly strong positive effects. Different light sources were employed, and UV light was found to be more effective in DMHg photodegradation. Moreover, MMHg was detected during the photodegradation of DMHg, confirming that the photochemical demethylation of DMHg is a source of MMHg in sunlit water. This work may provide a novel mechanistic insight into the DMHg photodegradation in natural waters and enrich the study of the global biogeochemical cycle of Hg.

Role of low-proportion, hydrophobic dissolved organic matter components in inhibiting methylmercury uptake by phytoplankton.

Uptake of methylmercury (MeHg), a potent neurotoxin, by phytoplankton is a major concern due to its role as the primary pathway for MeHg entry into aquatic food webs, thereby posing a significant risk to human health. While it is widely believed that the MeHg uptake by plankton is negatively correlated with the concentrations of dissolved organic matter (DOM) in the water, ongoing debates continue regarding the specific components of DOM that exerts the dominant influence on this process. In this study, we employed a widely-used resin fractionation approach to separate and classify DOM derived from algae (AOM) and natural rivers (NOM) into distinct components: strongly hydrophobic, weakly hydrophobic, and hydrophilic fractions. We conduct a comparative analysis of different DOM components using a combination of spectroscopy and mass spectrometry techniques, aiming to identify their impact on MeHg uptake by Microcystis elabens, a prevalent alga in freshwater environments. We found that the hydrophobic components had exhibited more pronounced spectral characteristics associated with the protein structures while protein-like compounds between hydrophobic and hydrophilic components displayed significant variations in both distributions and the values of m/z (mass-to-charge ratio) of the molecules. Regardless of DOM sources, the low-proportion hydrophobic components usually dominated inhibition of MeHg uptake by Microcystis elabens. Results inferred from the correlation analysis suggest that the uptake of MeHg by the phytoplankton was most strongly and negatively correlated with the presence of protein-like components. Our findings underscore the importance of considering the diverse impacts of different DOM fractions on inhibition of phytoplankton MeHg uptake. This information should be considered in future assessments and modeling endeavors aimed at understanding and predicting risks associated with aquatic Hg contamination.

Degradation of methylmercury into Hg(0) by the oxidation of iron(II) minerals.

Mercury contamination is a global concern, and the degradation and detoxification of methylmercury have gained significant attention due to its neurotoxicity and biomagnification within the food chain. However, the currently known pathways of abiotic demethylation are limited to light-induced photodegradation process and little is known about light-independent abiotic demethylation of methylmercury. In this study, we reported a novel abiotic pathway for the degradation of methylmercury through the oxidation of both mineral structural iron(II) and surface-adsorbed iron(II) in the absence of light. Our findings reveal that methylmercury can be oxidatively degraded by reactive oxygen species, specifically hydroxyl and superoxide radicals, which are generated from the oxidation of iron(II) minerals under dark conditions. Surprisingly, Hg(0) trapping experiments demonstrated that inorganic Hg(II) resulting from the oxidative degradation of methylmercury was rapidly reduced to gaseous Hg(0) by iron(II) minerals. The demethylation of methylmercury, coupled with the generation of Hg(0), suggests a potential natural attenuation process for methylmercury. Our results highlight the underappreciated roles of iron(II) minerals in the abiotic degradation of methylmercury and the release of gaseous Hg(0) into the atmosphere.

Transformation and migration of Hg in a polluted alkaline paddy soil during flooding and drainage processes.

Mercury (Hg) contamination in paddy soils poses a health risk to rice consumers and the environmental behavior of Hg determines its toxicity. Thus, the variations of Hg speciation are worthy of exploring. In this study, microcosm and pot experiments were conducted to elucidate Hg transformation, methylation, bioaccumulation, and risk coupled with biogeochemical cycling of key elements in a Hg-polluted alkaline paddy soil. In microcosm and pot experiments, organic- and sulfide-bound and residual Hg accounted for more than 98% of total Hg, and total contents of dissolved, exchangeable, specifically adsorbed, and fulvic acid-bound Hg were less than 2% of total Hg, indicating a low mobility and environmental risk of Hg. The decrease of pH aroused from Fe(III), SO42-, and NO3- reduction promoted Hg mobility, whereas the increase of pH caused by Fe(II), S2-, and NH4+ oxidation reduced available Hg contents. Moreover, Fe-bearing minerals reduction and organic matter consumption promoted Hg mobility, whereas the produced HgS and Fe(II) oxidation increased Hg stability. During flooding, a fraction of inorganic Hg (IHg) could be transported into methylmercury (MeHg), and during drainage, MeHg would be converted back into IHg. After planting rice in an alkaline paddy soil, available Hg was below 0.3 mg kg-1. During rice growth, a portion of available Hg transport from paddy soil to rice, promoting Hg accumulation in rice grains. After rice ripening, IHg levels in rice tissues followed the trend: root > leaf > stem > grain, and IHg content in rice grain exceed 0.02 mg kg-1, but MeHg content in rice grain meets daily intake limit (37.45 µg kg-1). These results provide a basis for assessing the environmental risks and developing remediation strategies for Hg-contaminated redox-changing paddy fields as well as guaranteeing the safe production of rice grains.

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.

Mechanism of methylmercury photodegradation in the yellow sea and East China Sea: Dominant pathways, and role of sunlight spectrum and dissolved organic matter.

Mercury (Hg) is among the most concerned contaminants in the world due to its high toxicity, prevalent existence in the environments, and bioaccumulation via food chain. Methylmercury (MeHg) is the major form of Hg that accumulates along the food chain and poses threat to humans and wild life. Photodegradation is the dominant process that MeHg is eliminated from freshwater system and upper ocean. The formation of MeHg-dissolved organic matter (DOM) complexes and a variety of free radicals (FR)/reactive oxygen species (ROS) have been previously proposed to be involved in MeHg photodegradation. However, most of these studies were conducted in freshwater, and the mechanism of MeHg photodegradation in seawater remains unclear. In this study, the main pathways of MeHg photodegradation in the seawater of Yellow Sea (YS) and East China Sea (ECS) were investigated using FR/ ROS scavenger addition and DOM competing-ligand addition techniques. The results showed that direct photodegradation of MeHg-DOM complexes is the major pathway of MeHg photodegradation in the YS and ECS, while indirect photolysis of MeHg by hydroxyl radical (·OH) also plays a certain role at some sites. MeHg photodegradation was found to be mainly induced by ultraviolet (UV) light rather than visible light in YS and ECS seawater, and the contribution of UV-B was higher than UV-A which was opposite to that previously reported in freshwater. The energy for breaking the bond of CHg in MeHg-Cl complexes formed in seawater is higher than that in MeHg-DOM complexes and this may cause the relatively greater contribution of UV-B with higher energy to MeHg photodegradation in seawater. In addition, MeHg photodegradation in various fractions of natural DOM with different molecular weights, hydrophilicity/hydrophobicity and acid-base was tested. MeHg photodegradation rates (kd) varied in these fractions and kd in high molecular weight DOM and hydrophobic Acid (HOA) fractions were faster than that in the other fractions. A significantly positive correlation was observed between kd and thiol concentrations while there was no significant correlation between kd and other measured parameters representing the composition of DOM (specific UV absorbance at 254 nm (SUVA254), spectral slope (SR), chromophoric dissolved organic matter (CDOM), humification index (HIX), biological index (BIX) and fluorescent components). These results indicate that thiol may be the key functional group in DOM affecting the photodegradation of MeHg in the YS and ECS.

N-Acetylcysteine Displaces Glutathionyl-Moieties from Hg2+ and MeHg+ to Form More Hydrophobic Complexes at Near-Physiological Conditions.

The anthropogenic release of Hg is associated with an increased human exposure risk. Since Hg2+ and MeHg+ have a high affinity for thiols, their interaction with L-glutathione (GSH) within mammalian cells is fundamentally involved in their toxicological chemistry and excretion. To gain insight into the interaction of these mercurials with multiple small molecular weight thiols, we have investigated their competitive interactions with GSH and N-acetylcysteine (NAC) at near-physiological conditions, using a liquid chromatographic approach. This approach involved the injection of each mercurial onto a reversed-phase (RP)-HPLC column (37 °C) using a PBS buffer mobile phase containing 5.0 mM GSH to simulate cytosolic conditions with Hg being detected in the column effluent by an inductively coupled plasma atomic emission spectrometer (ICP-AES). When the 5.0 mM GSH mobile phase was amended with up to 10 mM NAC, gradually increasing retention times of both mercurials were observed. To explain this behavior, the experiment with 5.0 mM NAC and 5.0 mM GSH was replicated using 50 mM Tris buffer (pH 7.4), and the Hg-containing fractions were analyzed by electrospray ionization mass spectrometry. The results revealed the presence of Hg(GS)(NAC) and Hg(NAC)2 for Hg2+ and MeHg(GS) and MeHg(NAC) for MeHg+, which suggests that the coordination/displacement of GS-moieties from each mercurial by the more hydrophobic NAC can explain their retention behavior. Since the biotransformations of both mercurials were observed at near-physiological conditions, they are of toxicological relevance as they provide a biomolecular explanation for some results that were obtained when animals were administered with each mercurial and NAC.

Dissolved organic matter thiol concentrations determine methylmercury bioavailability across the terrestrial-marine aquatic continuum.

The most critical step for methylmercury (MeHg) bioaccumulation in aquatic food webs is phytoplankton uptake of dissolved MeHg. Dissolved organic matter (DOM) has been known to influence MeHg uptake, but the mechanisms have remained unclear. Here we show that the concentration of DOM-associated thiol functional groups (DOM-RSH) varies substantially across contrasting aquatic systems and dictates MeHg speciation and bioavailability to phytoplankton. Across our 20 study sites, DOM-RSH concentrations decrease 40-fold from terrestrial to marine environments whereas dissolved organic carbon (DOC), the typical proxy for MeHg binding sites in DOM, only has a 5-fold decrease. MeHg accumulation into phytoplankton is shown to be directly linked to the concentration of specific MeHg binding sites (DOM-RSH), rather than DOC. Therefore, MeHg bioavailability increases systematically across the terrestrial-marine aquatic continuum as the DOM-RSH concentration decreases. Our results strongly suggest that measuring DOM-RSH concentrations will improve empirical models in phytoplankton uptake studies and will form a refined basis for modeling MeHg incorporation in aquatic food webs under various environmental conditions.

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

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

Algal organic matter inhibits methylmercury photodegradation in eutrophic lake water: A dynamic study.

Algal organic matter (AOM) is a major component of dissolved organic matter (DOM) in eutrophic lakes and could impact the photodegradation of neurotoxic methylmercury (MeHg) in water. Predicting these effects, however, is challenging, largely due to the dynamic changes of AOM during algal decomposition. Here, we investigated the effects of AOM on MeHg photodegradation throughout the algal decomposition process and elucidated these effects by characterizing dynamic changes of AOM and exploring the respective roles of various reactive oxygen species (ROS). Our results reveal that AOM derived from algal decomposition significantly inhibits MeHg photodegradation, and the extent of this inhibition varies depending on the specific lakes (8-21 %, p < 0.05) and their eutrophication states (16-28 %, p < 0.05). The inhibitory effect gradually weakened as the decomposition progressed, which may be attributed to the dynamic changes in the quantity and quality of AOM. Moreover, hydroxyl radical (·OH) was found to be the main contributor in driving MeHg photodegradation (15-23 %) during the early stages of decomposition (day 0-3), while in the later stage (day 12-24), the role of singlet oxygen (1O2, 15-20 %) and (3DOM*, 21-30 %) gradually strengthened and these three ROS jointly drove MeHg photodegradation. Based on our findings and recent studies, we propose that AOM derived from algal decomposition plays a vital role in increasing the risk of MeHg in eutrophic lakes. It promotes MeHg formation while simultaneously inhibiting its photodegradation. Integrating AOM-MeHg interactions into Hg biogeochemical cycling models would reduce uncertainties when predicting MeHg risks.

Geochemical mercury pools regulate diverse communities of hgcA microbes and MeHg levels in paddy soils.

Rice paddies are unique artificial wetlands generating methylmercury (MeHg), a highly potent neurotoxin. However, the impact of diverse mercury (Hg) pools on the Hg-methylating communities during rice growth is unclear. This study investigates soil treated with five mercury forms (HgCl2, α-HgS, ß-HgS, nano-HgS, and Hg-DOM) at two levels (5 mg/kg and 50 mg/kg). The results showed a varying abundance of sulphate-reducing bacteria, Geobacteraceae, methanogens, and hgcA microbes in the soils across rice grown under different mercury treatments and concentrations. Soils treated with HgCl2, nano-HgS and ß-HgS had higher than average levels of hgcA-methanogen abundance, and the abundance significantly and positively correlated with MeHg concentration in all samples (p < 0.05). The shifting trends in Hg-methylating microbial structure following treatment with α-HgS, ß-HgS, nano-HgS and Hg-DOM at both 5 and 50 mg/kg Hg levels were diverse compared with the control group. HgCl2 treatment showed contrasting trends in community distribution of Hg methylators at 5 and 50 mg/kg Hg levels during rice growth. Dissolved organic carbon, redox potential and sulphate levels significantly correlated with variation in the Hg-methylating microbial community structure and MeHg production in soils.

Controlling Factors and Predictive Models of Total Mercury and Methylmercury Accumulation in Rice (Oryza sativa L.) from Mercury-Contaminated Paddy Soils.

It is urgent to detect the major controlling factors and establish predictive models of mercury (Hg) accumulation in rice. A pot trial was conducted, exogenous Hg was added to 19 paddy soils at 4 concentration levels in this study. The major controlling factors of total Hg (THg) in brown rice were soil THg, pH and organic matter (OM) content, while those of methylmercury (MeHg) in brown rice were soil MeHg and OM. THg and MeHg in brown rice could be well predicted by soil THg, pH and clay content. The data from previous studies were collected to validate the predictive models of Hg in brown rice. The predicted values of Hg in brown rice were within the twofold prediction intervals of the observations, which demonstrated the predictive models in this study were reliable. The results could provide theoretical foundation for the risk assessment of Hg in paddy soils.

Biodegradability of algal-derived dissolved organic matter and its influence on methylmercury uptake by phytoplankton.

Methylmercury (MeHg) uptake by phytoplankton represents a key step in determining the exposure risks of aquatic organisms and human beings to this potent neurotoxin. Phytoplankton uptake is believed to be negatively related to dissolved organic matter (DOM) concentration in water. However, microorganisms can rapidly change DOM concentration and composition and subsequent impact on MeHg uptake by phytoplankton has rarely been tested. Here, we explored the influences of microbial degradation on the concentrations and molecular compositions of DOM derived from three common algal sources and tested their subsequent impacts on MeHg uptake by the widespread phytoplankton species Microcystis elabens. Our results indicated that dissolved organic carbon was degraded by 64.3‒74.1% within 28 days of incubating water with microbial consortia from a natural meso­eutrophic river. Protein-like components in DOM were more readily degraded, while the numbers of molecular formula for peptides-like compounds had increased after 28 days' incubation, probably due to the production and release of bacterial metabolites. Microbial degradation made DOM more humic-like which was consistent with the positive correlations between changes in proportions of Peaks A and C and bacterial abundance in bacterial community structures as illustrated by 16S rRNA gene sequencing. Despite rapid losses of the bulk DOM during the incubation, we found that DOM degraded after 28 days still reduced the MeHg uptake by Microcystis elabens by 32.7‒52.7% relative to a control without microbial decomposers. Our findings emphasize that microbial degradation of DOM would not necessarily enhance the MeHg uptakes by phytoplankton and may become more powerful in inhibiting MeHg uptakes by phytoplankton. The potential roles of microbes in degrading DOM and changing the uptakes of MeHg at the base of food webs should now be incorporated into future risk assessments of aquatic Hg cycling.

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 (HgSNP). 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 HgSNP. 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 and methylmercury in Hg-contaminated paddy soil and their uptake in rice as regulated by DOM from different agricultural sources.

In this study, from the perspectives of structural and compositional variations of soil-dissolved organic matter (DOM), we explored the effects of agricultural DOM inputs on methylmercury (MeHg) accumulation in the soil and mercury (Hg) bioaccumulation in rice grains. Pot experiments with the addition of DOMs from maize straw (MaS), rape straw (RaS), rice straw (RiS), composted rice straw (CRiS), cow dung (CD), and composted cow dung (CCD) were then conducted. Results showed that, relative to the control, the DOM amendment from each agricultural source elevated MeHg concentrations in the soil, with an increase of 18-227%, but only parts of DOMs elevated total dissolved Hg (DHg) and MeHg (DMeHg) concentrations in pore water. Among all DOM species, RiS, CRiS, and CCD significantly increased total Hg (THg) and MeHg contents in rice grains by 34-64% and 32-118%, respectively. Compared with RiS, THg and MeHg contents in rice grains in the CRiS treatment decreased slightly, which was consistent with the distributions of DHg and DMeHg concentrations in pore water and the aromaticity variation of soil DOM. In contrast, the CCD input significantly enhanced the enrichment of THg and MeHg in rice grains relative to CD because it significantly reduced the humification of soil DOM at all rice-growing stages while increasing the low-molecular-weight fractions in soil DOM. The THg and MeHg contents in the rice grains were significantly lower treated by RaS than those by MaS and RiS, which may be related to the higher sulfur-containing compounds such as sulfate and cysteine in rape straw or its DOM solution. Overall, DOM amendment from different agricultural sources resulted in significantly discriminative effects on the MeHg accumulation in soil and Hg enrichment in rice in the Hg-contaminated paddy field by shaping soil DOM properties.

Spatial and seasonal patterns of mercury concentrations, methylation and demethylation in central Canadian boreal soils and stream sediment.

Terrestrial ecosystems store large amounts of mercury (Hg), which may be subject to methylation, mobilization and uptake into downstream aquatic ecosystems. Mercury concentrations, methylation and demethylation potentials are not well characterized simultaneously across different habitats in boreal forest ecosystems, particularly not so in stream sediment, leading to uncertainties about the importance of various habitats as primary production areas of the bioaccumulative neurotoxin methylmercury (MeHg). In this study, we collected soil and sediment samples from 17 undisturbed, central Canadian boreal forested watersheds during spring, summer and fall to robustly characterize the spatial (upland and riparian/wetland soils, and stream sediment) and seasonal patterns of total Hg (THg) and MeHg concentrations. Mercury methylation and MeHg demethylation potentials (Kmeth and Kdemeth) in the soils and sediment were also assessed using enriched stable Hg isotope assays. We found the highest Kmeth and %-MeHg in stream sediment. In both riparian and wetland soils, Hg methylation was lower and less seasonally variable compared to stream sediment, but had comparable MeHg concentrations, suggesting longer-term storage of MeHg produced in these soils. Soil and sediment carbon content, and THg and MeHg concentrations were strong covariates across habitats. Additionally, sediment carbon content was important for delineating between stream sediment with relatively high vs. relatively low Hg methylation potential, which generally separated between different landscape physiographies. Broadly, this large and spatiotemporally diverse dataset is an important baseline for understanding Hg biogeochemistry in boreal forests both in Canada and possibly in many other boreal systems globally. This work is particularly important with respect to future possible impacts from natural and anthropogenic perturbations, which are increasingly straining boreal ecosystems in various parts of the world.

Mercury and Sulfur Redox Cycling Affect Methylmercury Levels in Rice Paddy Soils across a Contamination Gradient.

Methylmercury (MeHg) contamination in rice via paddy soils is an emerging global environmental issue. An understanding of mercury (Hg) transformation processes in paddy soils is urgently needed in order to control Hg contamination of human food and related health impacts. Sulfur (S)-regulated Hg transformation is one important process that controls Hg cycling in agricultural fields. In this study, Hg transformation processes, such as methylation, demethylation, oxidation, and reduction, and their responses to S input (sulfate and thiosulfate) in paddy soils with a Hg contamination gradient were elucidated simultaneously using a multi-compound-specific isotope labeling technique (200HgII, Me198Hg, and 202Hg0). In addition to HgII methylation and MeHg demethylation, this study revealed that microbially mediated reduction of HgII, methylation of Hg0, and oxidative demethylation-reduction of MeHg occurred under dark conditions; these processes served to transform Hg between different species (Hg0, HgII, and MeHg) in flooded paddy soils. Rapid redox recycling of Hg species contributed to Hg speciation resetting, which promoted the transformation between Hg0 and MeHg by generating bioavailable HgII for fuel methylation. Sulfur input also likely affected the microbial community structure and functional profile of HgII methylators and, therefore, influenced HgII methylation. The findings of this study contribute to our understanding of Hg transformation processes in paddy soils and provide much-needed knowledge for assessing Hg risks in hydrological fluctuation-regulated ecosystems.