85 Search Results

Dietary exposure to methylmercury (MeHg) causes irreversible damage to human cognition and is mitigated by photolysis and microbial demethylation of MeHg. Rice (Oryza sativa L.) has been identified as a major dietary source of MeHg. However, it remains unknown what drives the process within plants for MeHg to make its way from soils to rice and the subsequent human dietary exposure to Hg. Here we report a hidden pathway of MeHg demethylation independent of light and microorganisms in rice plants. This natural pathway is driven by reactive oxygen species generated in vivo, rapidly transforming MeHg to inorganic Hg and thenmore » eliminating Hg from plants as gaseous Hg°. MeHg concentrations in rice grains would increase by 2.4- to 4.7-fold without this pathway, which equates to intelligence quotient losses of 0.01–0.51 points per newborn in major rice-consuming countries, corresponding to annual economic losses of US$30.7–84.2 billion globally. Importantly, this discovered pathway effectively removes Hg from human food webs, playing an important role in exposure mitigation and global Hg cycling.« less

Dissolved elemental mercury [Hg(0)_aq] widely exists in natural waters, but its reactivity and purgeability in the presence of suspended particulate matter (SPM) remain controversial. Here, this study investigated reactions between Hg(0)_aq and various types of organic and inorganic SPM and found that Hg(0)_aq reacted weakly with the inorganic mineral SPM (i.e., kaolinite, montmorillonite, and hematite) but strongly with organic matter (OM) or OM-coated minerals in water. Nearly 100% of Hg(0)_aq could be recovered as purgeable gaseous Hg(0) after reactions with mineral SPM, irrespective of the mineral types, concentrations, and reaction time. However, incomplete Hg(0)_aq recoveries were observed in the presencemore » of OM or OM-coated minerals and in natural water containing OM and SPM, but the addition of borohydride, a reducing agent, immediately restored the Hg(0)_aq purgeability and recovery. The results suggest that Hg(0)_aq was oxidized and then retained by OM or OM-coated minerals. These findings clarify previous observations of so-called particulate Hg(0)_aq in water and have important implications for understanding the role of Hg(0)_aq in affecting Hg transformation and bioavailability in the aquatic environment.« less

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, in this study, 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 themore » 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.« less

Phytoplankton serves as a key entry point for the trophic transfer and bioaccumulation of the neurotoxin methylmercury (MeHg) in aquatic food webs. However, it is unclear whether and how phytoplankton itself may degrade and metabolize MeHg in the dark. Here, using several strains of the freshwater alga Chlorella vulgaris, the marine diatom Chaetoceros gracilis and two cyanobacteria (or blue-green algae), we report a light-independent pathway of MeHg degradation in water by phytoplankton, rather than its associated bacteria. About 36–85% of MeHg could be degraded intracellularly to inorganic Hg(II) and/or Hg(0) via dark reactions. Furthermore, endogenic reactive oxygen species, particularly singletmore » oxygen, were identified as the main driver of MeHg demethylation. Given the increasing incidence of algal blooms in lakes and marine systems globally, these findings underscore the potential roles of phytoplankton demethylation and detoxification of MeHg in aquatic ecosystems and call for improved modelling and assessment of MeHg bioaccumulation and environmental risks.« less

The cell surface adsorption and intracellular uptake of mercuric mercury Hg(II) and methylmercury (MeHg) are important in determining the fate and transformation of Hg in the environment. However, current information is limited about their interactions with two important groups of microorganisms, i.e., methanotrophs and Hg(II)-methylating bacteria, in aquatic systems. This study investigated the adsorption and uptake dynamics of Hg(II) and MeHg by three strains of methanotrophs, Methylomonas sp. strain EFPC3, Methylosinus trichosporium OB3b, and Methylococcus capsulatus Bath, and two Hg(II)-methylating bacteria, Pseudodesulfovibrio mercurii ND132 and Geobacter sulfurreducens PCA. Distinctive behaviors of these microorganisms towards Hg(II) and MeHg adsorption and intracellularmore » uptake were observed. The methanotrophs took up 55–80% of inorganic Hg(II) inside cells after 24 h incubation, lower than methylating bacteria (>90%). Approximately 80–95% of MeHg was rapidly taken up by all the tested methanotrophs within 24 h. In contrast, after the same time, G. sulfurreducens PCA adsorbed 70% but took up <20% of MeHg, while P. mercurii ND132 adsorbed <20% but took up negligible amounts of MeHg. These results suggest that microbial surface adsorption and intracellular uptake of Hg(II) and MeHg depend on the specific types of microbes and appear to be related to microbial physiology that requires further detailed investigation. Despite being incapable of methylating Hg(II), methanotrophs play important roles in immobilizing both Hg(II) and MeHg, potentially influencing their bioavailability and trophic transfer. Furthermore, methanotrophs are not only important sinks for methane but also for Hg(II) and MeHg and can influence the global cycling of C and Hg.« less

Methylmercury (MeHg) is a potent neurotoxin and has great adverse health impacts on humans. Organisms and sunlight-mediated demethylation are well-known detoxification pathways of MeHg, yet whether abiotic environmental components contribute to MeHg degradation remains poorly known. Here, in this paper, we report that MeHg can be degraded by trivalent manganese (Mn(III)), a naturally occurring and widespread oxidant. We found that 28 ± 4% MeHg could be degraded by Mn(III) located on synthesized Mn dioxide (MnO_2–x) surfaces during the reaction of 0.91 μg·L^–1 MeHg and 5 g·L^–1 mineral at an initial pH of 6.0 for 12 h in 10 mM NaNO₃more » at 25 °C. The presence of low-molecular-weight organic acids (e.g., oxalate and citrate) substantially enhances MeHg degradation by MnO_2–x via the formation of soluble Mn(III)-ligand complexes, leading to the cleavage of the carbon–Hg bond. MeHg can also be degraded by reactions with Mn(III)-pyrophosphate complexes, with apparent degradation rate constants comparable to those by biotic and photolytic degradation. Thiol ligands (cysteine and glutathione) show negligible effects on MeHg demethylation by Mn(III). This research demonstrates potential roles of Mn(III) in degrading MeHg in natural environments, which may be further explored for remediating heavily polluted soils and engineered systems containing MeHg.« less

At mercury (Hg)-contaminated sites, streambank erosion can act as a main mobilizer of Hg into nearby waterbodies.

Climate warming causes permafrost thaw predicted to increase toxic methylmercury (MeHg) and greenhouse gas [i.e., methane (CH₄), carbon dioxide (CO₂), and nitrous oxide (N₂O)] formation. A microcosm incubation study with Arctic tundra soil over 145 days demonstrates that N₂O at 0.1 and 1 mM markedly inhibited microbial MeHg formation, methanogenesis, and sulfate reduction, while it slightly promoted CO₂ production. Microbial community analyses indicate that N₂O decreased the relative abundances of methanogenic archaea and microbial clades implicated in sulfate reduction and MeHg formation. Following depletion of N₂O, both MeHg formation and sulfate reduction rapidly resumed, whereas CH₄ production remained low, suggestingmore » that N₂O affected susceptible microbial guilds differently. MeHg formation strongly coincided with sulfate reduction, supporting prior reports linking sulfate-reducing bacteria to MeHg formation in the Arctic soil. Here, this research highlights complex biogeochemical interactions in governing MeHg and CH₄ formation and lays the foundation for future mechanistic studies for improved predictive understanding of MeHg and greenhouse gas fluxes from thawing permafrost ecosystems.« less

Over 3000 mercury (Hg)-contaminated sites worldwide contain liquid metallic Hg [Hg(0)₁] representing a continuous source of elemental Hg(0) in the environment through volatilization and solubilization in water. Currently, there are few effective treatment technologies available to remove or sequester Hg(0)₁ in situ. We investigated sonochemical treatments coupled with complexing agents, polysulfide and sulfide, in oxidizing Hg(0)₁ and stabilizing Hg in water, soil and quartz sand. Results indicate that sonication is highly effective in breaking up and oxidizing liquid Hg(0)₁ beads via acoustic cavitation, particularly in the presence of polysulfide. Without complexing agents, sonication caused only minor oxidation of Hg(0)₁ butmore » increased headspace gaseous Hg(0)_g and dissolved Hg(0)_aq in water. However, the presence of polysulfide essentially stopped Hg(0) volatilization and solubilization. As a charged polymer, polysulfide was more effective than sulfide in oxidizing Hg(0)₁ and subsequently stabilizing the precipitated metacinnabar (β-HgS) nanocrystals. Sonochemical treatments with sulfide yielded incomplete oxidation of Hg(0)₁, likely resulting from the formation of HgS coatings on the dispersed µm-size Hg(0)₁ bead surfaces. Sonication with polysulfide also resulted in rapid oxidation of Hg(0)₁ and precipitation of HgS in quartz sand and in the Hg(0)₁-contaminated soil. This research indicates that sonochemical treatment with polysulfide could be an effective means in rapidly converting Hg(0)₁ to insoluble HgS precipitates in water and sediments, thereby preventing its further emission and release to the environment. We suggest that future studies are performed to confirm its technical feasibility and treatment efficacy for remediation applications.« less

We report some thiols, such as cysteine (CYS) at moderate concentrations (10–500 µM), can enhance methylmercury (MeHg) formation by Geobacter sulfurreducens PCA, whereas others such as dithiol 2,3-dimercaptopropanesulfonate (DMPS) and 2,3-dimercaptosuccinic acid (DMSA) abolish mercury [Hg(II)] methylation. Little is known, however, about whether Hg(II) methylation could be enhanced or inhibited by the presence of mixed thiol ligands at low concentrations observed in the environment. Surprisingly we found that mixing CYS (1 µM) with DMPS (0.025–0.5 µM) or DMSA (0.025–1 µM) substantially increased MeHg production by 1.5–3.5-fold, compared to the no-thiol control, whereas complexation with a single DMPS, or DMSA, or CYS (1 µM) strongly inhibitedmore » Hg(II) methylation. Pre-equilibration between Hg(II) and thiols before the addition of cells was necessary to observe enhanced methylation. Spectroscopic analyses indicated the formation of mixed or heteroleptic coordinated Hg(II)-S₃/S₄ complexes, which likely facilitated exchange of Hg(II) with cells and its uptake and internal transfer to the HgcAB proteins required for methylation. These results suggest that the effects of thiols on Hg(II) methylation were more complex than previously thought (using a single thiol) and thus underscore the importance of understanding how mixed thiols and their interactions with Hg(II) may ultimately influence MeHg production in the natural environment.« less