Role of Ester Sulfate and Organic Disulfide in Mercury Methylation in Peatland Soils

Microbial communities and functions are structured by vertical geochemical zones in a northern peatland

Northern peatlands are important carbon pools; however, differences in the structure and function of microbiomes inhabiting contrasting geochemical zones within these peatlands have rarely been emphasized. Using 16S rRNA gene sequencing, metagenomic profiling, and detailed geochemical analyses, we investigated the taxonomic composition and genetic potential across various geochemical zones of a typical northern peatland profile in the Changbai Mountains region (Northeastern China). Specifically, we focused on elucidating the turnover of organic carbon, sulfur (S), nitrogen (N), and methane (CH4). Three geochemical zones were identified and characterized according to porewater and solid-phase analyses: the redox interface (<10 cm), shallow peat (10-100 cm), and deep peat (>100 cm). The redox interface and upper shallow peat demonstrated a high availability of labile carbon, which decreased toward deeper peat. In deep peat, anaerobic respiration and methanogenesis were likely constrained by thermodynamics, rather than solely driven by available carbon, as the acetate concentrations reached 90 μmol·L-1. Both the microbial community composition and metabolic potentials were significantly different (p < 0.05) among the redox interface, shallow peat, and deep peat. The redox interface demonstrated a close interaction between N, S, and CH4 cycling, mainly driven by Thermodesulfovibrionia, Bradyrhizobium, and Syntrophorhabdia metagenome-assembled genomes (MAGs). The archaeal Bathyarchaeia were indicated to play a significant role in the organic carbon, N, and S cycling in shallow peat. Although constrained by anaerobic respiration and methanogenesis, deep peat exhibited a higher metabolic potential for organic carbon degradation, primarily mediated by Acidobacteriota. In terms of CH4 turnover, subsurface peat (10-20 cm) was a CH4 production hotspot, with a net turnover rate of ∼2.9 nmol·cm-3·d-1, while the acetoclastic, hydrogenotrophic, and methylotrophic methanogenic pathways all potentially contributed to CH4 production. The results of this study improve our understanding of biogeochemical cycles and CH4 turnover along peatland profiles.

Suppression of mercury methylation in soil and methylmercury accumulation in rice by dissolved organic matter derived from sulfur-rich rape straw

Straw amendment significantly enhances mercury (Hg) methylation and subsequent methylmercury (MeHg) bioaccumulation in Hg-contaminated paddy fields by releasing dissolved organic matter (DOM). This study comprehensively investigates the regulatory mechanisms of DOM and its different molecular weights derived from sulfur-rich rape straw (RaDOM) and composted rape straw (CRaDOM) applied in the rice-filling stage on soil MeHg production and subsequent bioaccumulation in rice grains. The results indicated that the amendment of RaDOM and CRaDOM significantly reduced soil MeHg content by 42.40-62.42%. This reduction can be attributed to several factors, including the suppression of Hg-methylating bacteria in soil, the supply of sulfate from RaDOM and CRaDOM, and the increase in the humification, molecular weight, and humic-like fractions of soil DOM. Additionally, adding RaDOM increased the MeHg bioaccumulation factor in roots by 27.55% while inhibiting MeHg transportation by 12.24% and ultimately reducing MeHg content in grains by 21.24% compared to the control group. Similarly, CRaDOM enhanced MeHg accumulation by 25.19%, suppressed MeHg transportation by 39.65%, and reduced MeHg levels in the grains by 27.94%. The assimilation of sulfate derived from RaDOM and CRaDOM into glutathione may be responsible for the increased retention of MeHg in the roots. Over the three days, there was a significant decrease in soil MeHg content as the molecular weight of RaDOM increased; conversely, altering the molecular weight of CRaDOM demonstrated an inverse trend. However, this pattern was not observed after 12 days. Applying sulfur-rich rape DOM can help mitigate MeHg accumulation in paddy fields by regulating the quality of soil DOM, sulfur cycling, and Hg-methylating bacteria.

Sulfur speciation in Sphagnum peat moss modified by mutualistic interactions with cyanobacteria

Peat moss (Sphagnum spp.) develops mutualistic interactions with cyanobacteria by providing carbohydrates and S compounds in exchange for N-rich compounds, potentially facilitating N inputs into peatlands. Here, we evaluate how colonization of Sphagnum angustifolium hyaline cells by Nostoc muscorum modifies S abundance and speciation at the scales of individual cells and across whole leaves. For the first time, S K-edge X-ray Absorption Spectroscopy was used to identify bulk and micron-scale S speciation across isolated cyanobacteria colonies, and in colonized and uncolonized leaves. Uncolonized leaves contained primarily reduced organic S and oxidized sulfonate- and sulfate-containing compounds. Increasing Nostoc colonization resulted in an enrichment of S and changes in speciation, with increases in sulfate relative to reduced S and sulfonate. At the scale of individual hyaline cells, colonized cells exhibited localized enrichment of reduced S surrounded by diffuse sulfonate, similar to observations of cyanobacteria colonies cultured in the absence of leaves. We infer that colonization stimulates plant S uptake and the production of sulfate-containing metabolites that are concentrated in stem tissues. Sulfate compounds that are produced in response to colonization become depleted in colonized cells where they may be converted into reduced S metabolites by cyanobacteria.

Recent advance of microbial mercury methylation in the environment

Methylmercury formation is mainly driven by microbial-mediated process. The mechanism of microbial mercury methylation has become a crucial research topic for understanding methylation in the environment. Pioneering studies of microbial mercury methylation are focusing on functional strain isolation, microbial community composition characterization, and mechanism elucidation in various environments. Therefore, the functional genes of microbial mercury methylation, global isolations of Hg methylation strains, and their methylation potential were systematically analyzed, and methylators in typical environments were extensively reviewed. The main drivers (key physicochemical factors and microbiota) of microbial mercury methylation were summarized and discussed. Though significant progress on the mechanism of the Hg microbial methylation has been explored in recent decade, it is still limited in several aspects, including (1) molecular biology techniques for identifying methylators; (2) characterization methods for mercury methylation potential; and (3) complex environmental properties (environmental factors, complex communities, etc.). Accordingly, strategies for studying the Hg microbial methylation mechanism were proposed. These strategies include the following: (1) the development of new molecular biology methods to characterize methylation potential; (2) treating the environment as a micro-ecosystem and studying them from a holistic perspective to clearly understand mercury methylation; (3) a more reasonable and sensitive inhibition test needs to be considered. KEY POINTS: • Global Hg microbial methylation is phylogenetically and functionally discussed. • The main drivers of microbial methylation are compared in various condition. • Future study of Hg microbial methylation is proposed.

Habitat and dissolved organic carbon modulate variation in the biogeochemical drivers of mercury bioaccumulation in dragonfly larvae at the national scale

We paired mercury (Hg) concentrations in dragonfly larvae with water chemistry in 29 U.S. national parks to highlight how ecological and biogeochemical context (habitat, dissolved organic carbon [DOC]) influence drivers of Hg bioaccumulation. Although prior studies have defined influences of biogeochemical variables on Hg production and bioaccumulation, it has been challenging to determine their influence across diverse habitats, regions, or biogeochemical conditions within a single study. We compared global (i.e., all sites), habitat-specific, and DOC-class models to illuminate how these controls on biotic Hg vary. Although the suite of important biogeochemical factors across all sites (e.g., aqueous Hg, DOC, sulfate [SO42-], and pH) was consistent with general findings in the literature, contrasting the restricted models revealed more nuanced controls on biosentinel Hg. Comparing habitats, aqueous (filtered) total mercury (THg) and SO42- were important in lentic systems whereas aqueous (filtered) methylmercury (MeHg), DOC, pH, and SO42- were important in lotic and wetland systems. The ability to identify important variables varied among habitats, with less certainty in lentic (model weight (W) = 0.05) than lotic (W = 0.11) or wetland habitats (W = 0.23), suggesting that biogeochemical drivers of bioaccumulation are more variable, or obscured by other aspects of Hg cycling, in these habitats. Results revealed a contrast in the importance of aqueous MeHg versus aqueous THg between DOC-classes: in low-DOC sites (<8.5 mg/L), availability of upstream inputs of MeHg appeared more important for bioaccumulation; in high-DOC sites (>8.5 mg/L) THg was more important, suggesting a link to in-situ controls on bioavailability of Hg for MeHg production. Mercury bioaccumulation (indicated by bioaccumulation factor) was more efficient in low DOC-class sites, likely due to reduced partitioning of aqueous MeHg to DOC. Together, findings highlight substantial variation in the drivers of Hg bioaccumulation and suggest consideration of these factors in natural resource management and decision-making.

Microbial community function and methylmercury production in oxygen-limited paddy soil

Methylmercury is a neurotoxic compound that can enter rice fields through rainfall or irrigation with contaminated wastewater, and then contaminate the human food chain through the consumption of rice. Flooded paddy soil has a porous structure that facilitates air exchange with the atmosphere, but the presence of trace amounts of oxygen in flooded rice field soil and its impact on microbial-mediated formation of methylmercury is still unclear. We compared the microbial communities and their functions in oxygen-depleted and oxygen-limited paddy soil. We discovered that oxygen-limited paddy soil had higher methylmercury concentration, which was strongly correlated with soil properties and methylation potential. Compared with oxygen-depleted soil, oxygen-limited soil altered the microbial composition based on 16 S rRNA sequences, but not based on hgcA sequences. Moreover, oxygen-limited soil enhanced microbial activity significantly, increasing the abundance of more than half of the KEGG pathways, especially the metabolic pathways that might be involved in methylation. Our study unveils how microbial communities influence methylmercury formation in oxygen-limited paddy soil. ENVIRONMENTAL IMPLICATIONS: This study examined how low oxygen input affects microbial-induced MeHg formation in anaerobic paddy soil. We found that oxygen-limited soil produced more MeHg than oxygen-depleted soil. Oxygen input altered the microbial community structure of 16 S rRNA sequencing in anaerobic paddy soil, but had little impact on the hgcA sequencing community structure. Microbial activity and metabolic functions related to MeHg formation were also higher in oxygen-limited paddy soil. We suggest that oxygen may not be a limiting factor for Hg methylators, and that insufficient oxygen input in flooded paddy soil increases the risk of human exposure to MeHg from rice consumption.

Selective Photochemical Oxidation of Reduced Dissolved Organic Sulfur to Inorganic Sulfate

The chemical nature and stability of reduced dissolved organic sulfur (DOS_Red) have implications on the biogeochemical cycling of trace and major elements across fresh and marine aquatic environments, but the underlying processes governing DOS_Red stability remain obscure. Here, dissolved organic matter (DOM) was isolated from a sulfidic wetland, and laboratory experiments quantified dark and photochemical oxidation of DOS_Red using atomic-level measurement of sulfur X-ray absorption near-edge structure (XANES) spectroscopy. DOS_Red was completely resistant to oxidation by molecular oxygen in the dark and underwent rapid and quantitative oxidation to inorganic sulfate (SO₄^2-) in the presence of sunlight. The rate of DOS_Red oxidation to SO₄^2- greatly exceeded that of DOM photomineralization, resulting in a 50% loss of total DOS and 78% loss of DOS_Red over 192 h of irradiance. Sulfonates (DOS_SO3) and other minor oxidized DOS functionalities were not susceptible to photochemical oxidation. The observed susceptibility of DOS_Red to photodesulfurization, which has implications on carbon, sulfur, and mercury cycling, should be comprehensively evaluated across diverse aquatic environments of differing DOM composition.

Long-Term Experimental Manipulation of Atmospheric Sulfate Deposition to a Peatland: Response of Methylmercury and Related Solute Export in Streamwater

Changes in sulfate (SO₄^2-) deposition have been linked to changes in mercury (Hg) methylation in peatlands and water quality in freshwater catchments. There is little empirical evidence, however, of how quickly methyl-Hg (MeHg, a bioaccumulative neurotoxin) export from catchments might change with declining SO₄^2- deposition. Here, we present responses in total Hg (THg), MeHg, total organic carbon, pH, and SO₄^2- export from a peatland-dominated catchment as a function of changing SO₄^2- deposition in a long-term (1998-2011), whole-ecosystem, control-impact experiment. Annual SO₄^2- deposition to half of a 2-ha peatland was experimentally increased 6-fold over natural levels and then returned to ambient levels in two phases. Sulfate additions led to a 5-fold increase in monthly flow-weighted MeHg concentrations and yields relative to a reference catchment. Once SO₄^2- additions ceased, MeHg concentrations in the outflow streamwater returned to pre-SO₄^2- addition levels within 2 years. The decline in streamwater MeHg was proportional to the change in the peatland area no longer receiving experimental SO₄^2- inputs. Importantly, net demethylation and increased sorption to peat hastened the return of MeHg to baseline levels beyond purely hydrological flushing. Overall, we present clear empirical evidence of rapid and proportionate declines in MeHg export from a peatland-dominated catchment when SO₄^2- deposition declines.

Mercury methylation and methylmercury demethylation in boreal lake sediment with legacy sulphate pollution

Sulphate and dissolved organic matter (DOM) in freshwater systems may regulate the formation of methylmercury (MeHg), a potent neurotoxin that biomagnifies in aquatic ecosystems. While many boreal lakes continue to recover from decades of elevated atmospheric sulphate deposition, little research has examined whether historically high sulphate concentrations can result in persistently elevated MeHg production and accumulation in aquatic systems. This study used sediment from a historically sulphate-impacted lake and an adjacent reference lake in northwestern Ontario, Canada to investigate the legacy effects of sulphate pollution, as well as the effects of newly added sulphate, natural organic matter (NOM) of varying sulphur content and a sulphate reducing bacteria (SRB) inhibitor on enhancing or inhibiting the Hg methylation and demethylation activity (K_meth and K_demeth) in the sediment. We found that K_meth and MeHg concentrations in sulphate-impacted lake sediment were significantly greater than in reference lake sediment. Further adding sulphate or NOM with different sulphur content to sediment of both lakes did not significantly change K_meth. The addition of a SRB inhibitor resulted in lower K_meth only in sulphate-impacted sediment, but methylation was not entirely depressed. Methylmercury demethylation potentials in sediment were consistent across lakes and experimental treatments, except for some impacts related to SRB inhibitor additions in the reference lake sediment. Overall, a broader community of microbes beyond SRB may be methylating Hg and demethylating MeHg in this system. This study reveals that legacies of sulphate pollution in boreal lakes may persist for decades in stimulating elevated Hg methylation in sediment.