Production of two morphologically different antimony trioxides by a novel antimonate-reducing bacterium, Geobacter sp. SVR

Draft genome sequence of Intrasporangium sp. DVR, an actinobacterium of the family Intrasporangiaceae isolated from soil in Japan

Intrasporangium sp. strain DVR is an actinobacterium of the family Intrasporangiaceae isolated from soil in Japan. Here we report the draft genome sequence of strain DVR.

Formation of Sb2O3 microcrystals by Rhodotorula mucilaginosa

Antimony (Sb) pollution seriously endangers ecological environment and human health. Microbial induced mineralization can effectively convert metal ions into more stable and less soluble crystalline minerals by extracellular polymeric substance (EPS). In this study, an efficient Sb-resistant Rhodotorula mucilaginosa (R. mucilaginosa) was screened, which can resist 41 mM Sb(III) and directly transform Sb(III) into Sb2O3 microcrystals by EPS. The removal efficiency of R. mucilaginosa for 22 mM Sb(III) reached 70% by converting Sb(III) to Sb2O3. The components of supernatants as well as the effects of supernatants and pH on Sb(III) mineralization verified that inducible and non-inducible extracellular protein/polysaccharide biomacromolecules play important roles in the morphologies and sizes control of Sb2O3 formed by R. mucilaginosa respectively. Sb2O3 microcrystals with different morphologies and sizes can be prepared by the regulation of inducible and non-inducible extracellular biomacromolecules secreted by R. mucilaginosa. This is the first time to identify that R. mucilaginosa can remove Sb(III) by transforming Sb(III) into Sb2O3 microcrystals under the control of EPS. This study contributes to our understanding for Sb(III) biomineralization mechanisms and provides strategies for the remediation of Sb-contaminated environment.

Identification of bacterial dissimilatory antimonate reductase AnrA: genes and proteins involved in antimonate respiration and resistance in Geobacter sp. strain SVR

Geobacter sp. strain SVR uses antimonate [Sb(V)] as a terminal electron acceptor for anaerobic respiration. Here, we visualized a possible key enzyme, periplasmic Sb(V) reductase (Anr), via active staining and non-denaturing gel electrophoresis. Liquid chromatography-tandem mass spectrometry analysis revealed that a novel dimethyl sulfoxide (DMSO) reductase family protein, WP_173201954.1, is involved in Anr. This protein was closely related with AnrA, a protein suggested to be the catalytic subunit of a respiratory Sb(V) reductase in Desulfuribacillus stibiiarsenatis. The anr genes of strain SVR (anrXSRBAD) formed an operon-like structure, and their transcription was upregulated under Sb(V)-respiring conditions. The expression of anrA gene was induced by more than 1 µM of antimonite [Sb(III)]; however, arsenite [As(III)] did not induce the expression of anrA gene. Tandem mass tag-based proteomic analysis revealed that, in addition to Anr proteins, proteins in the following categories were upregulated under Sb(V)-respiring conditions: (i) Sb(III) efflux systems such as Ant and Ars; (ii) antioxidizing proteins such as ferritin, rubredoxin, and thioredoxin; (iii) protein quality control systems such as HspA, HslO, and DnaK; and (iv) DNA repair proteins such as UspA and UvrB. These results suggest that strain SVR copes with antimony stress by modulating pleiotropic processes to resist and actively metabolize antimony. To the best of our knowledge, this is the first report to demonstrate the involvement of AnrA in Sb(V) respiration at the protein level. Furthermore, this is the first example to show high expression of the Ant system proteins in the Sb(V)-respiring bacterium.IMPORTANCEAntimony (Sb) exists mainly as antimonite [Sb(III)] or antimonate [Sb(V)] in the environment, and Sb(III) is more toxic than Sb(V). Recently, microbial involvement in Sb redox reactions has received attention. Although more than 90 Sb(III)-oxidizing bacteria have been reported, information on Sb(V)-reducing bacteria is limited. Especially, the enzyme involved in dissimilatory Sb(V) reduction, or Sb(V) respiration, is unclear, despite this pathway being very important for the circulation of Sb in nature. In this study, we demonstrated that the Sb(V) reductase (Anr) of an Sb(V)-respiring bacterium (Geobacter sp. SVR) is a novel member of the dimethyl sulfoxide (DMSO) reductase family. In addition, we found that strain SVR copes with Sb stress by modulating pleiotropic processes, including the Ant and Ars systems, and upregulating the antioxidant and quality control protein levels. Considering the abundance and diversity of putative anr genes in the environment, Anr may play a significant role in global Sb cycling in both marine and terrestrial environments.

Biological oxidation of As(III) and Sb(III) by a novel bacterium with Sb(III) oxidase rather than As(III) oxidase under anaerobic and aerobic conditions

Sb and As are chemically similar, but the sequences and structures of Sb(III) and As(III) oxidase are totally distinct. It is thus interesting to explore whether Sb(III) oxidase oxidizes As(III), and if so, how microbial oxidations of Sb(III) and As(III) influence one another. Previous investigations have yielded ambiguous or even erroneous conclusions. This study aimed to clarify this issue. Firstly, we prepared a consortium of Sb(III)-oxidizing prokaryotes (SOPs) by enrichment cultivation. Metagenomic analysis reveals that SOPs with the Sb(III) oxidase gene, but lacking the As(III) oxidase gene are predominant in the SOP community. Despite this, SOPs exhibit comparable Sb(III) and As(III)-oxidizing activities in both aerobic and anaerobic conditions, indicating that at the microbial community level, Sb(III) oxidase can oxidize As(III). Secondly, we isolated a representative cultivable SOP, Ralstonia sp. SbOX with Sb(III) oxidase gene but without As(III) oxidase gene. Genomic analysis of SbOX reveals that this SOP strain has a complete Sb(III) oxidase (AnoA) gene, but lacks As(III) oxidase (AioAB or ArxAB) gene. It is interesting to discover that, besides its Sb(III) oxidation activities, SbOX also exhibits significant capabilities in oxidizing As(III) under both aerobic and anaerobic conditions. Moreover, under aerobic conditions and in the presence of both Sb(III) and As(III), SbOX exhibited a preference for oxidizing Sb(III). Only after the near complete oxidation of Sb(III) did SbOX initiate rapid oxidation of As(III). In contrast, under anaerobic conditions and in the presence of both Sb(III) and As(III), Sb(III) oxidation notably inhibited the As(III) oxidation pathway in SbOX, while As(III) exhibited minimal effects on the Sb(III) oxidation. These findings suggest that SOPs can oxidize As(III) under both aerobic and anaerobic conditions, exhibiting a strong preference for Sb(III) over As(III) oxidation in the presence of both. This study unveils a novel mechanism of interaction within the Sb and As biogeochemical cycles.

Antimony precipitation and removal by antimony hyper resistant strain Achromobacter sp. 25-M

Microbes have been confirmed to play key role in biogeochemistry of antimony. However, the impact of indigenous bacteria (from active mines) on the behavior of dissolved antimony remained poorly understood. In current study, the hyper antimony-resistant strain, Achromobacter sp. 25-M, isolated from the world largest antimony deposit, Xikuangshan antimony deposit, was evaluated for its role in dissolved Sb(V) and Sb(III) precipitation and removal. Despite of the high resistance to Sb(III) (up to 50 mM), the facultative alkaliphile, 25-M was not capable of Sb(III) oxidation. Meanwhile 25-M can produce high amount of exopolymeric substance (EPS) with the presence of Sb, which prompted us to investigate the potential role of EPS in the precipitation and removal of Sb. To this end, 2 mM of Sb(III) and Sb(V) were added into the experimental systems with and without 25-M to discern the interaction mechanism between microbe and antimony. After 96 hrs' incubation, 88% [1.73 mM (210 mg/L)] of dissolved Sb(V) and 80% [1.57 mM (190 mg/L)] of dissolved Sb(III) were removed. X-ray diffraction and energy dispersive spectroscopy analysis confirmed the formation of valentinite (Sb2O3) in Sb(III) amended system and a solitary Sb(V) mineral mopungite [NaSb(OH)6] in Sb(V) amended group with microbes. Conversely, no precipitate was detected in abiotic systems. Morphologically valentinite was bowtie and mopungite was pseudo-cubic as indicated by scanning electronic microscopy. EPS was subjected to fourier transform infrared (FT-IR) analysis. FT-IR analysis suggested that -OH and -COO groups were responsible for the complexation and ligand exchange with Sb(III) and Sb(V), respectively. Additionally, the C-H group and N-H group could be involved in π-π interaction and chelation with Sb species. All these interactions between Sb and functional groups in EPS may subsequently favore the formation of valentinite and mopungite. Collectively, current results suggested that EPS play fundamental role in bioprecipitation of Sb, which offered a new strategy in Sb bioremediation.

Reaction pathways and Sb(III) minerals formation during the reduction of Sb(V) by Rhodoferax ferrireducens strain YZ-1

Antimony (Sb), a non-essential metalloid, can be released into the environment through various industrial activities. Sb(III) is considered more toxic than Sb(V), but Sb(III) can be immobilized through the precipitation of insoluble Sb2S3 or Sb2O3. In the subsurface, Sb redox chemistry is largely controlled by microorganisms; however, the exact mechanisms of Sb(V) reduction to Sb(III) are still unclear. In this study, a new strain of Sb(V)-reducing bacterium, designated as strain YZ-1, that can respire Sb(V) as a terminal electron acceptor was isolated from Sb-contaminated soils. 16S-rRNA gene sequencing of YZ-1 revealed high similarity to a known Fe(III)-reducer, Rhodoferax ferrireducens. XRD and XAFS analyses revealed that bioreduction of Sb(V) to Sb(III) proceed through a transition from amorphous valentinite to crystalline senarmontite (allotropes of Sb2O3). Genomic DNA sequencing found that YZ-1 possesses arsenic (As) metabolism genes, including As(V) reductase arsC. The qPCR analysis showed that arsC was highly expressed during Sb(V)-reduction by YZ-1, and thus is proposed as the potential Sb(V) reductase in YZ-1. This study provides new insight into the pathways and products of microbial Sb(V) reduction and demonstrates the potential of a newly isolated bacterium for Sb bioremediation.

A novel antimony metallochaperone AntC in Comamonas testosteroni JL40 and its application in antimony immobilization

The microbial metabolism of toxic antimony (Sb) and the bioremediation of Sb-contaminated environments have attracted significant attention recently. This study identified an Sb(III) metallochaperone AntC in the Sb(III) efflux operon antRCA of Comamonas testosteroni JL40. The deletion of AntC significantly increased the intracellular Sb content in strain JL40 and concomitantly diminished resistance to Sb(III). By contrast, the complementary expression of AntC in the knockout strain resulted in a substantial recovery of Sb(III) resistance. The site-directed mutagenesis assay demonstrated the three conserved cysteine (Cys) residues (Cys30, Cys34, and Cys36) play an essential role in the binding of Sb(III) to AntC and its transfer. The function of the metallochaperone AntC was further investigated in an Sb(III) sensitive bacterium Escherichia coli AW3110 (Δars). The co-expression of AntC and AntA in AW3110 cells resulted in a four-fold increase in minimum inhibitory concentrations (MICs) toward Sb(III), while the intracellular Sb content decreased five-fold compared to cells expressing AntA alone. In addition, a genetically modified E. coli strain was engineered to co-express AntC and the Sb uptake protein GlpF, showing an eight-fold increase in Sb absorption and achieving a remarkable 90% removal of Sb from the solution. This engineered strain was also applied in a hydroponic experiment, displaying a significant 80% reduction in Sb uptake by rice seedlings. This finding provides new insights into the mechanisms of bacterial Sb detoxification and a potential bioremediation strategy for Sb pollution.

Mobility of antimony in contrasting surface environments of a mine site: influence of redox conditions and microbial communities

Microbial processes can influence the complex geochemical behaviour of the toxic metalloid antimony (Sb) in mining environments. The present study is aimed to evaluate the influence of microbial communities on the mobility of Sb from solid phases to water in different compartments and redox conditions of a mining site in southwest (SW) Spain. Samples of surface materials presenting high Sb concentrations, from two weathered mining waste dumps, and an aquatic sediment were incubated in slurries comparing oxic and anoxic conditions. The initial microbial communities of the three materials strongly differed. Incubations induced an increase of microbial biomass and an evolution of the microbial communities' structures and compositions, which diverged in different redox conditions. The presence of active bacteria always influenced the mobility of Sb, except in the neutral pH waste incubated in oxic conditions. The effect of active microbial activities in oxic conditions was dependent on the material: Sb oxic release was biologically amplified with the acidic waste, but attenuated with the sediment. Different bacterial genera involved in Sb, Fe and S oxidation or reduction were present and/or grew during incubation of each material. The results highlighted the wide diversity of microbial communities and metabolisms at the small geographic scale of a mining site and their strong implication in Sb mobility.

Draft genome sequence of Pelosinus sp. IPA-1, a bacterium isolated from arsenic-contaminated soil in Japan

Pelosinus sp. strain IPA-1 is a bacterium isolated from arsenic-contaminated soil in Japan. We here report the draft genome sequence of strain IPA-1.

Induced transformation of antimony trioxide by Mn(II) oxidation and their co-transformed mechanism

Antimony (Sb) is a toxic and carcinogenic element that often enters soil in the form of antimony trioxide (Sb₂O₃) and coexists with manganese (Mn) in weakly alkaline conditions. Mn oxides such as birnessite have been found to promote the oxidative dissolution of Sb₂O₃, but few researches concerned the co-transformations of Sb₂O₃ and Mn(II) in environment. This study investigated the mutual effect of abiotic oxidation of Mn(II) and the coupled oxidative dissolution of Sb₂O₃. The influencing factors, such as Mn(II) concentrations, pH and oxygen were also discussed. Furthermore, their co-transformed mechanism was also explored based on the analysis of Mn(II) oxidation products with or without Sb₂O₃ using XRD, SEM and XPS. The results showed that the oxidative dissolution of Sb₂O₃ was enhanced under higher pH and higher Mn(II) loadings. With a lower Mn(II) concentration such as 0.01 mmol/L Mn(II) at pH 9.0, the improved dissolution of Sb₂O₃ was attributed to the generation of dissolved intermediate Mn(III) species with strong oxidation capacity. However, under higher Mn(II) concentrations, both amorphous Mn(III) oxides and intermediate Mn(III) species were responsible for promoting the oxidative dissolution of Sb₂O₃. Most released Sb (∼72%) was immobilized by Mn oxides and Sb(V) was dominant in the adsorbed and dissolved total Sb. Meanwhile, the presence of Sb₂O₃ not only inhibited the removal of Mn(II) by reducing Mn(III) to Mn(II) but also affected the final products of Mn oxides. For example, amorphous Mn oxides were formed instead of crystalline Mn(III) oxides, such as MnOOH. Furthermore, rhodochrosite (MnCO₃) was formed with the high Mn(II)/Sb₂O₃ ratio_, but without being observed in the low Mn(II)/Sb₂O₃ ratio. The results of study could help provide more understanding about the fate of Sb in the environment and the redox transformation of Mn.

Anaerobic selenite-reducing bacteria and their metabolic potentials in Se-rich sediment revealed by the combination of DNA-stable isotope probing, metagenomic binning, and metatranscriptomics

Microorganisms play a critical role in the biogeochemical cycling of selenium (Se) in aquatic environments, particularly in reducing the toxicity and bioavailability of selenite (Se(IV)). This study aimed to identify putative Se(IV)-reducing bacteria (Se^IVRB) and investigate the genetic mechanisms underlying Se(IV) reduction in anoxic Se-rich sediment. Initial microcosm incubation confirmed that Se(IV) reduction was driven by heterotrophic microorganisms. DNA stable-isotope probing (DNA-SIP) analysis identified Pseudomonas, Geobacter, Comamonas, and Anaeromyxobacter as putative Se^IVRB. High-quality metagenome-assembled genomes (MAGs) affiliated with these four putative Se^IVRB were retrieved. Annotation of functional gene indicated that these MAGs contained putative Se(IV)-reducing genes such as DMSO reductase family, fumarate and sulfite reductases. Metatranscriptomic analysis of active Se(IV)-reducing cultures revealed significantly higher transcriptional levels of genes associated with DMSO reductase (serA/PHGDH), fumarate reductase (sdhCD/frdCD), and sulfite reductase (cysDIH) compared to those in cultures not amended with Se(IV), suggesting that these genes played important roles in Se(IV) reduction. The current study expands our knowledge of the genetic mechanisms involved in less-understood anaerobic Se(IV) bio-reduction. Additinally, the complementary abilities of DNA-SIP, metagenomics, and metatranscriptomics analyses are demonstrated in elucidating the microbial mechanisms of biogeochemical processes in anoxic sediment.

Response of soil protists to antimony and arsenic contamination

Microorganisms can mediate antimony (Sb) and arsenic (As) transformation and thus change their mobility and toxicity. Having similar geochemical behavior, Sb and As are generally considered to exert similar environmental pressure on microbiome. However, it needs further validation, especially for protists. In this study, the responses of protistan communities to Sb and As were investigated by collecting soils from Xikuangshan Sb mine and Shimen As mine in China. Antimony and As contamination taxonomically and functionally (consumer and phototroph) changed the alpha and beta diversities of protistan communities, but exerted different impacts on the parasitic community. Based on multiple statistical tools, As contamination had a greater impact on protistan communities than Sb. The ecological networks of highly contaminated sites were less complex but highly positively connected compared to less contaminated sites. High As contamination raised the ratio of consumers and decreased the ratio of phototrophs in ecological networks, while the opposite tendency was observed in Sb contaminated soils. High Sb and As contamination enriched different keystone taxa resistant to Sb and As. These results demonstrate that protistan community respond differently to Sb and As.

Identification of chlorpyrifos-degrading microorganisms in farmland soils via cultivation-independent and -dependent approaches

Microorganisms have important roles in the bioremediation of organophosphorus pesticides (OPs) in farmland soils. However, the majority of functional microorganisms (>99%) are yet to be cultivated. In this study, magnetic nanoparticle-mediated isolation (MMI) coupled with high-throughput sequencing was applied to identify the microorganisms responsible for chlorpyrifos (CPF) degradation in farmland soils. Various microorganisms have been identified as CPF degraders via MMI, in which the roles of genera Citrobacter, Exiguobacterium, Azoarcus, Azohydromonas and Massilia have not previously been related to CPF degradation. Two organophosphorus hydrolase genes (ophB and ophC) were involved in CPF metabolism in MMI enrichments, compared to only ophB found in the cultivable CPF degrader Alcaligenes L1. Also, a more thorough degradation of CPF was found in MMI enrichments, where 3,5,6-trichloro-2-pyridinol (TCP), O,O-diethyl thiophosphate (DETP), 2,3,5-trichloro-6-methoxypyridine (TMP) and O,O,S-trimethyl phosphorothioate were identified as the biodegradation products. This work suggests that MMI is a promising technology for separating functional microorganisms from complex microbiota, with deeper insight into their ecological functions, providing fundamental knowledge on soil bioremediation strategies.

Oxidative dissolution of Sb2O3 mediated by surface Mn redox cycling in oxic aquatic systems

Antimony trioxide (Sb₂O₃) is one of the primary forms of Sb in the environment, and its dissolution significantly impacts the migration and bioavailability of Sb. However, the dissolution of Sb₂O₃ coupled with abiotic redox of Mn processes is unclear. Here, we investigated the kinetics of Sb₂O₃ dissolution in the presence of the ubiquitous Mn(II) by kinetic experiments, spectroscopies, density functional theory calculations and the chemical kinetic modeling. The oxidative dissolution of Sb₂O₃ was catalyzed by Mn(II) through the in-situ generated amorphous Mn oxides (MnO_x) under oxic conditions, during which the generation of Mn(III) is a critical step in Sb(V) release. The released Sb(V) was partially retained on MnO_x through bidentate-binuclear (corner-sharing) complexes as revealed by extended X-ray absorption fine structure analysis. The coexistent morphological forms of Sb₂O₃, i.e., senarmontite and valentinite exhibited distinct dissolution patterns. Valentinite showed higher activity in catalyzing Mn(II) oxidation and faster oxidative dissolution than senarmontite, due to its higher surface energy and lower conduction band minimum of its exposed facets. These abiotic processes can extrapolate to other metal(loid)s (hydr)oxides, further supplying for the comprehensive understanding of the redox transformation of Mn.

Antimony redox processes in the environment: A critical review of associated oxidants and reductants

The environmental behavior of antimony (Sb) has recently received greater attention due to the increasing global use of Sb in a range of industrial applications. Although present at trace levels in most natural systems, elevated Sb concentrations in aquatic and terrestrial environments may result from anthropogenic activities. The mobility and toxicity of Sb largely depend on its speciation, which is dependent to a large extent on its oxidation state. To a certain extent, our understanding of the environmental behavior of Sb has been informed by studies of the environmental behavior of arsenic (As), as Sb and As have somewhat similar chemical properties. However, recently it has become evident that the speciation of Sb and As, especially in the context of redox reactions, may be fundamentally different. Therefore, it is crucial to study the biogeochemical processes impacting Sb redox transformations to understand the behavior of Sb in natural and engineered environments. Currently, there is a growing body of literature involving the speciation, mobility, toxicity, and remediation of Sb, and several reviews on these general topics are available; however, a comprehensive review focused on Sb environmental redox chemistry is lacking. This paper provides a review of research conducted within the past two decades examining the redox chemistry of Sb in aquatic and terrestrial environments and identifies knowledge gaps that need to be addressed to develop a better understanding of Sb biogeochemistry for improved management of Sb in natural and engineered systems.

Hydrous ferric oxides (HFO's) precipitated from contaminated waters at several abandoned Sb deposits - Interdisciplinary assessment

The presented paper represents a comprehensive analysis of ochre sediments precipitated from Fe rich drainage waters contaminated by arsenic and antimony. Ochre samples from three abandoned Sb deposits were collected in three different seasons and were characterized from the mineralogical, geochemical, and microbiological point of view. They were formed mainly by poorly crystallized 2-line ferrihydrite, with the content of arsenic in samples ranging from 7 g·kg^-1 to 130 g·kg^-1 and content of antimony ranging from 0.25 g·kg^-1 up to 12 g·kg^-1. Next-generation sequencing approach with 16S RNA, 18S RNA and ITS markers was used to characterize bacterial, fungal, algal, metazoal and protozoal communities occurring in the HFOs. In the 16S RNA, the analysis dominated bacteria (96.2%) were mainly Proteobacteria (68.8%) and Bacteroidetes (10.2%) and to less extent also Acidobacteria, Actinobacteria, Cyanobacteria, Firmicutes, Nitrosprae and Chloroflexi. Alpha and beta diversity analysis revealed that the bacterial communities of individual sites do not differ significantly, and only subtle seasonal changes were observed. In this As and Sb rich, circumneutral microenvironment, rich in iron, sulfates and carbonates, methylotrophic bacteria (Methylobacter, Methylotenera), metal/reducing bacteria (Geobacter, Rhodoferax), metal-oxidizing and denitrifying bacteria (Gallionella, Azospira, Sphingopyxis, Leptothrix and Dechloromonas), sulfur-oxidizing bacteria (Sulfuricurvum, Desulphobulbaceae) and nitrifying bacteria (Nitrospira, Nitrosospira) accounted for the most dominant ecological groups and their impact over Fe, As, Sb, sulfur and nitrogen geocycles is discussed. This study provides evidence of diverse microbial communities that exist in drainage waters and are highly important in the process of mobilization or immobilization of the potentially toxic elements.

Reduction of antimony mobility from Sb-rich smelting slag by Shewanella oneidensis: Integrated biosorption and precipitation

The dissimilatory Fe(III)-reducing bacteria play a significant role in the mobility of antimony (Sb) under reducing environment. Sb-rich smelting slag is iron (Fe)-containing antimonic mine waste, which is one of the main sources of antimony pollution. In this study, the soluble antimony reacted with Fe(III) by S. oneidensis (Shewanella oneidensis strain MR-1) was performed in reduction condition, then the dissolution behavior of the Sb-rich smelting slag with S. oneidensis was investigated. The results showed that the released Sb was immobilized by S. oneidensis and the strain adsorbed Sb(III) preferentially. Sb(V) can be reduced by S. oneidensis without aqueous Fe. In the presence of Fe(III), S. oneidensis mediated Sb bio-adsorption and the chemical redox of Sb-Fe occurred simultaneously. Sb was co-precipitated with Fe to form the Sb(V)-O-Fe(III) secondary mineral, which was identified as the bidentate mononuclear edge-sharing structure by extended X-ray absorption fine structure (EXAFS) analysis. These results suggest that S. oneidensis has a positive effect on the immobilization and minimizing toxicity of antimony in anoxic soil and groundwater, which provides a theoretical basis for the treatment of antimony contamination.

Removal of Antimony in Wastewater by Antimony Tolerant Sulfate-Reducing Bacteria Isolated from Municipal Sludge

Antimony (Sb), a global and priority controlled pollutant, causes severe environmental issues. Bioremediation by microbial communities containing sulfate-reducing bacteria (SRB) is considered to be among the safest, economical, and environmentally friendly methods to remove Sb from wastewater. However, the roles of SRB species in these communities remain uncertain, and pure cultures of bacteria that may be highly efficient have not yet been developed for Sb removal. In this study, an Sb tolerant community was enriched from municipal sludge, and molecular ecological analysis showed that Escherichia (40%) and Desulfovibrio (15%) were the dominant bacteria. Further isolation and identification showed that the enriched SRB strains were closely related to Cupidesulfovibrio oxamicus, based on the molecular analyses of 16S rRNA and dsrB genes. Among them, a strain named SRB49 exhibited the highest activity in removal of Sb(V). SRB49 was able to remove 95% of Sb(V) at a concentration of 100 mg/L within 48 h under optimum conditions: a temperature of 37–40 °C, an initial pH value of 8, 4 mM of sulfate, and an initial redox potential of 145–229 mV. SEM-EDX analysis showed that SRB49 did not adsorb Sb(V) but reduced and precipitated Sb(V) via the formation of Sb₂S₃. The results demonstrated the potential roles that pure cultures of SRB species may play in Sb removal and the use of Sb-tolerant SRB strains for Sb remediation.

New Mobilization Pathway of Antimonite: Thiolation and Oxidation by Dissimilatory Metal-Reducing Bacteria via Elemental Sulfur Respiration

Antimony (Sb) mobilization is widely explored with dissimilatory metal-reducing bacteria (DMRB) via microbial iron(III)-reduction. Here, our study found a previously unknown pathway whereby DMRB release adsorbed antimonite (Sb^III-O) from goethite via elemental sulfur (S⁰) respiratory reduction under mild alkaline conditions. We incubated Sb^III-O-loaded goethite with Shewanella oneidensis MR-1 in the presence of S⁰ at pH 8.5. The incubation results showed that MR-1 reduced S⁰ instead of goethite, and biogenic sulfide induced the formation of thioantimonite (Sb^III-S). Sb^III-S was then oxidized by S⁰ to mobile thioantimonate (Sb^V-S), resulting in over fourfold greater Sb release to water compared with the abiotic control. Sb^IV-S was identified as the intermediate during the oxidation process by Fourier transform ion cyclotron resonance mass spectrometry and electron spin resonance analysis. The existence of Sb^IV-S reveals that the oxidation of Sb^III-S to Sb^V-S follows a two-step consecutive one-electron transfer from Sb to S atoms. Sb^V-S then links with Sb^III-S by sharing S atoms and inhibits Sb^III-S polymerization and Sb^III₂S₃ precipitation like a "capping agent". This study clarifies the thiolation and oxidation pathway of Sb^III-O to Sb^V-S by S⁰ respiration and expands the role of DMRB in the fate of Sb.

Identification of Antimonate Reducing Bacteria and Their Potential Metabolic Traits by the Combination of Stable Isotope Probing and Metagenomic-Pangenomic Analysis

Microorganisms play an important role in altering antimony (Sb) speciation, mobility, and bioavailability, but the understanding of the microorganisms responsible for Sb(V) reduction has been limited. In this study, DNA-stable isotope probing (DNA-SIP) and metagenomics analysis were combined to identify potential Sb(V)-reducing bacteria (SbRB) and predict their metabolic pathways for Sb(V) reduction. Soil slurry cultures inoculated with Sb-contaminated paddy soils from two Sb-contaminated sites demonstrated the capability to reduce Sb(V). DNA-SIP identified bacteria belonging to the genera Pseudomonas and Geobacter as putative SbRB in these two Sb-contaminated sites. In addition, bacteria such as Lysinibacillus and Dechloromonas may potentially participate in Sb(V) reduction. Nearly complete draft genomes of putative SbRB (i.e., Pseudomonas and Geobacter) were obtained, and the genes potentially responsible for arsenic (As) and Sb reduction (i.e., respiratory arsenate reductase (arrA) and antimonate reductase (anrA)) were examined. Notably, bins affiliated with Geobacter contained arrA and anrA genes, supporting our hypothesis that they are putative SbRB. Further, pangenomic analysis indicated that various Geobacter-associated genomes obtained from diverse habitats also contained arrA and anrA genes. In contrast, Pseudomonas may use a predicted DMSO reductase closely related to sbrA (Sb(V) reductase gene) clade II to reduce Sb(V), which may need further experiments to verify. This current work represents a demonstration of using DNA-SIP and metagenomic-binning to identify SbRB and their key genes involved in Sb(V) reduction and provides valuable data sets to link bacterial identities with Sb(V) reduction.

Complete Genome Sequence of Geobacter sp. Strain SVR, an Antimonate-Reducing Bacterium Isolated from Antimony-Rich Mine Soil

We report here the complete genome sequence of Geobacter sp. strain SVR, isolated from antimony mine soil in Nakase Mine, Hyogo Prefecture, Japan. SVR strains proliferate using antimonate [Sb(V)] as an electron acceptor, providing insights into the antimony reduction mechanism.