Methanogens-Driven Arsenic Methylation Preceding Formation of Methylated Thioarsenates in Sulfide-Rich Hot Springs

Release and Redistribution of Arsenic Associated with Ferrihydrite Driven by Aerobic Humification of Exogenous Soil Organic Matter

Humification of exogenous soil organic matter (ESOM) remodels the organic compositions and microbial communities of soil, thus exerting potential impacts on the biogeochemical transformation of iron (hydr)oxides and associated trace metals. Here, we conducted a 70-day incubation experiment to investigate how aerobic straw humification influenced the repartitioning of arsenic (As) associated with ferrihydrite in paddy soil. Results showed that the humification was characterized by rapid OM degradation (1-14 days) and subsequent slow maturation (14-70 days). During the degradation stage, considerable As (13.1 mg·L-1) was released into the aqueous phase, which was reimmobilized to the solid phase in the maturation stage. Meanwhile, the low-crystalline structural As/Fe was converted to a more stable species, with a subtle crystalline phase transformation. The generated highly unsaturated and phenolic compounds and enriched Enterobacter and Sphingomonas induced ferrihydrite (∼3.1%) and As(V) reduction, leading to As release during the degradation stage. In the maturation stage, carboxylic-rich alicyclic molecules facilitated the aqueous As reimmobilization. Throughout the humification process, organo-mineral complexes formed between OM and ferrihydrite via C-O-Fe bond contributed to the solid-phase As/Fe stabilization. Collectively, this work highlighted the ESOM humification-driven iron (hydr)oxide transformation and associated As redistribution, advancing our understanding of the coupled biogeochemical behaviors of C, Fe, and As in soil.

Sulfur depletion through repetitive redox cycling unmasks the role of the cryptic sulfur cycle for (methyl)thioarsenate formation in paddy soils

Inorganic and oxymethylated thioarsenates form through the reaction of arsenite and oxymethylated arsenates with reduced sulfur, mainly as sulfide (SII-) but also as zerovalent sulfur (S0). In paddy soils, considered low-S systems, microbial reduction of the soil's "primary" sulfate pool is the principal SII- source for As thiolation. Under anoxic conditions, this primary pool is readily consumed, and the precipitation of iron (Fe) sulfides lowers SII- availability. Nonetheless, sulfate can be constantly replenished by the reoxidation of SII- coupled with the reduction of FeIII phases in the so-called cryptic S cycle (CSC). The CSC supplies a small secondary sulfate pool available for reduction and, according to previous studies, As thiolation. However, sulfate concentrations commonly found in paddy soils mask the biogeochemical processes associated with the CSC. Here, we depleted a paddy soil from excess S, Fe, and As from a paddy soil through repetitive flooding and draining (e.g., redox cycling). After 10, 20, and 30 such cycles, we followed thioarsenate formation during an anoxic incubation period of 10 days. Higher S/As ratios increased As thiolation contribution to total As up to 10-fold after 30 cycles. During the anoxic incubation, the depleted soils showed a transient first phase where the reduction of the primary sulfate pool led to inorganic thioarsenate formation. In the second phase, methylthioarsenate formation correlated with partially oxidized S species (S0, thiosulfate), suggesting CSC-driven sulfate replenishment, re-reduction, and thiolation. Methylthioarsenates formed even as inorganic thioarsenates de-thiolated, indicating thermodynamic preference under S-limited conditions. This study highlights the role of the CSC in sustaining thioarsenate formation in low-S systems.

Microbial changes, including methanogens, influenced by arsenic speciation in anaerobic wetland environments

Methylated and inorganic arsenic species are frequently detected in natural water systems due to anthropogenic activities and geological origins. However, the impact of arsenic species on microbial activity, particularly on methanogens responsible for methane production, remains underexplored. This study investigates the effects of arsenic on methanogenesis by adding iAs(V) and DMA(V) to wetland sediment batches and monitoring arsenic speciation in the supernatant and methane production over time. Changes in arsenic behavior in the sediment were analyzed using X-ray Absorption Fine Structure (XAFS). Microbial diversity analysis revealed that while iAs(V) acted as an electron acceptor and its reduction to iAs(III) inhibited methanogen activity due to its toxicity, DMA showed minimal effects on methane production. These findings demonstrate that arsenic dynamics in wetlands influence methane-producing microorganisms, with inorganic form of arsenic species posing significant inhibitory effects, whereas less toxic DMA(V) appears to have limited impact. This might shed light on the potential for regulating methane emissions in arsenic-contaminated wetlands.

Molecular Basis of Thioredoxin-Dependent Arsenic Transformation in Methanogenic Archaea

Methanogenic archaea are known to play a crucial role in the biogeochemical cycling of arsenic (As); however, the molecular basis of As transformation mediated by methanogenic archaea remains poorly understood. Herein, the characterization of the redox transformation and methylation of As by Methanosarcina acetivorans, a model methanogenic archaeon, is reported. M. acetivorans was demonstrated to mediate As(V) reduction via a cytoplasmic As reductase (ArsC) in the exponential phase of methanogenic growth and to methylate As(III) via a cytoplasmic As(III) methyltransferase (ArsM) in the stationary phase. Characterization of the ArsC-catalyzed As(V) reduction and the ArsM-catalyzed As(III) methylation showed that a thioredoxin (Trx) encoded by MA4683 was preferentially utilized as a physiological electron donor for ArsC and ArsM, providing a redox link between methanogenesis and As transformation. The structures of ArsC and ArsM complexed with Trx were modeled using AlphaFold-Multimer. Site-directed mutagenesis of key cysteine residues at the interaction sites of the complexes indicated that the archaeal ArsC and ArsM employ evolutionarily distinct disulfide bonds for interacting with Trx compared to those used by bacterial ArsC or eukaryotic ArsM. The findings of this study present a major advance in our current understanding of the physiological roles and underlying mechanism of As transformation in methanogenic archaea.

Arsenic Methylation by a Sulfate-Reducing Bacterium from Paddy Soil Harboring a Novel ArsSM Fusion Protein

Microbial arsenic (As) methylation is an important process of As biogeochemistry. Only a few As-methylating microorganisms have been isolated from paddy soil, hindering the mechanistic understanding of the process involved. We isolated 54 anaerobic and 32 aerobic bacteria from paddy soil with a high As methylation potential. Among the 86 isolates, 14 anaerobes, including 7 sulfate-reducing bacteria (SRB), but none of the aerobes were able to methylate arsenite [As(III)] or monomethylarsenite [MMA(III)] or both, suggesting that the As-methylating ability is much more prevalent in anaerobes than in aerobes. We performed a detailed investigation on As methylation by a SRB isolate, Solidesulfovibrio sp. TC1, and identified a novel bifunctional enzyme consisting of a fusion of As(III) S-adenosylmethionine (SAM) methyltransferase (ArsM) and a radical SAM protein. The enzyme (ArsSM) can catalyze As(III) methylation to MMA and DMA and subsequent adenosylation of DMA to form 5'-deoxy-5'-dimethylarsinoyl-adenosine (DDMAA), which is a key intermediate in the biosynthesis of arsenosugars. High concentrations of sulfide produced by SRB did not affect As(III) methylation to MMA but inhibited MMA methylation to DMA. Genes encoding ArsSM fusion proteins are widespread in anaerobes, particularly SRB, suggesting that ArsSM-carrying anaerobes may play an important role in As methylation in an anoxic environment.

Occurrence and mobility of thiolated arsenic in legacy mine tailings

We studied the occurrence of dissolved thiolated Arsenic (As) in legacy tailings systems in Ontario and Nova Scotia, Canada, and used aqueous and mineralogical speciation analyses to assess its governing geochemical controls. Surface-accessible and inundated tailings in Cobalt, Ontario, contained ∼1 wt-% As mainly hosted in secondary arsenate minerals (erythrite, yukonite, and others) and traces of primary sulfide minerals (cobaltite, gersdorffite and others). Significant fractions of thiolated As (up to 5.9 % of total dissolved As) were detected in aqueous porewater and surface water samples from these sites, comprising mostly monothioarsenate, and smaller amounts of di- and tri-thioarsenates as well as methylated thioarsenates. Tailings at the Goldenville and Montague sites in Nova Scotia contained less (<0.5 wt-%) As, hosted mostly in arsenopyrite and As-bearing pyrite, than the Cobalt sites, but exhibited higher proportions of dissolved thiolated As (up to 17.3 % of total dissolved As, mostly mono- and di-thioarsenate and traces of tri-thioarsenate). Dissolved thiolated As was most abundant in sub-oxic porewaters and inundated tailings samples across the studied sites, and its concentrations were strongly related to the prevailing redox conditions and porewater hydrochemistry, and to a lesser extent, the As-bearing mineralogy. Our novel results demonstrate that thiolated As species play an important role in the cycling of As in mine waste systems and surrounding environments, and should be considered in mine waste management strategies for high-As sites.

Screening and characterization of a denitrifying Chelatococcus with strong desulfurization capacity

A denitrifying bacteria, which we named Chelatococcus DNB-15, was screened and isolated from the Shengli Oilfield polymer-containing wastewater. The strain was characterized by 16S rDNA analysis and the effects of nutrient substrate type, pH, mineralization and temperature on the activity of the strain were also investigated. The strain was identified as Chelatococcus, of which the most efficient carbon source is sodium citrate, the most suitable nitrogen source is potassium nitrate, the suitable temperature for growth is 35～45 °C, the suitable pH range for growth is 7.0～9.0, and the maximal tolerable mineralization is 4 × 104 mg/L. The desulfurization experiment showed that Chelatococcus DNB-15 has desulfurization ability to some extent. When the initial sulfide concentration is lower than 50 mg/L, Chelatococcus DNB-15 grows rapidly, and the sulfides are removed quickly within 24 h, displaying a sulfur removal rate higher than 99 %; When the initial sulfide concentration is higher than 150 mg/L, the growth of Chelatococcus DNB-15 is severely restricted, the desulfurization effect is not obvious, and the desulfurization rate is almost stagnant.