Thioarsenate transformation by filamentous microbial mats thriving in an alkaline, sulfidic hot spring

Coupled redox cycling of arsenic and sulfur regulates thioarsenate enrichment in groundwater

High‑arsenic groundwater is influenced by a combination of processes: reductive dissolution of iron minerals and formation of secondary minerals, metal complexation and redox reactions of organic matter (OM), and formation of more migratory thioarsenate, which together can lead to significant increases in arsenic concentration in groundwater. This study was conducted in a typical sulfur- and arsenic-rich groundwater site within the Datong Basin to explore the conditions of thioarsenate formation and its influence on arsenic enrichment in groundwater using HPLC-ICPMS, hydrogeochemical modeling, and fluorescence spectroscopy. The shallow aquifer exhibited a highly reducing environment, marked by elevated sulfide levels, low concentrations of Fe(II), and the highest proportion of thioarsenate. In the middle aquifer, an optimal ∑S/∑As led to the presence of significant quantities of thioarsenate. In contrast, the deep aquifer exhibited low sulfide and high Fe(II) concentration, with arsenic primarily originating from dissolved iron minerals. Redox fluctuations in the sediment driven by sulfur‑iron minerals generated reduced sulfur, thereby facilitating thioarsenate formation. OM played a crucial role as an electron donor for microbial activities, promoting iron and sulfate reduction processes and creating conditions conducive to thioarsenate formation in reduced and high‑sulfur environments. Understanding the process of thioarsenate formation and the influencing factors is of paramount importance for comprehending the migration and redistribution of arsenic in groundwater systems.

Metabolic coupling of arsenic, carbon, nitrogen, and sulfur in high arsenic geothermal groundwater: Evidence from molecular mechanisms to community ecology

Groundwater arsenic (As) poses a global environmental problem and is regulated by complex biogeochemical processes. However, the As biogeochemistry and its metabolic coupling with carbon (C), nitrogen (N), and sulfur (S) in high As geothermal groundwater remain unclear. Here, we reported significant shifts in the geothermal groundwater microbiome and its functional ecological clusters along the flow path with increased As levels and dynamic As-C-N-S biogeochemical cycle from the Guide Basin, China. Strong associations among As(III), NH4+, HCO3-, and corresponding functional microbial taxa suggest that microbe-mediated As transformation, ammonification, and organic carbon biodegradation potentially contributed to the As mobilization in the discharge area. And As oxidizers (coupling with denitrification or carbon fixation) and S oxidizers were closely linked to the transformation of As(III) to immobile As(V) in the recharge area. Our study provides a comprehensive insight into the complex microbial As-C-N-S coupling network and its potential role in groundwater As mobilization under hydrological disturbances.

Exploring the potential: Can arsenic (As) resistant silicate-solubilizing bacteria manage the dual effects of silicon on As accumulation in rice?

Rice (Oryza sativa L.) cultivation in regions marked by elevated arsenic (As) concentrations poses significant health concerns due to As uptake by the plant and its subsequent entry into the human food chain. With rice serving as a staple crop for a substantial share of the global population, addressing this issue is critical for food security. In flooded paddy soils, where As availability is pronounced, innovative strategies to reduce As uptake and enhance agricultural sustainability are mandatory. Silicon (Si) and Si nanoparticles have emerged as potential candidates to mitigate As accumulation in rice. However, their effects on As uptake exhibit complexity, influenced by initial Si levels in the soil and the amount of Si introduced through fertilization. While low Si additions may inadvertently increase As uptake, higher Si concentrations may alleviate As uptake and toxicity. The interplay among existing Si and As availability, Si supplementation, and soil biogeochemistry collectively shapes the outcome. Adding water-soluble Si fertilizers (e.g., Na2SiO3 and K2SiO3) has demonstrated efficacy in mitigating As toxicity stress in rice. Nonetheless, the expense associated with these fertilizers underscores the necessity for low cost innovative solutions. Silicate-solubilizing bacteria (SSB) resilient to As hold promise by enhancing Si availability by accelerating mineral dissolution within the rhizosphere, thereby regulating the Si biogeochemical cycle in paddy soils. Promoting SSB could make cost-effective Si sources more soluble and, consequently, managing the intricate interplay of Si's dual effects on As accumulation in rice. This review paper offers a comprehensive exploration of Si's nuanced role in modulating As uptake by rice, emphasizing the potential synergy between As-resistant SSB and Si availability enhancement. By shedding light on this interplay, we aspire to shed light on an innovative attempt for reducing As accumulation in rice while advancing agricultural sustainability.

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

Hot springs represent a major source of arsenic release into the environment. Speciation is typically reported to be dominated by arsenite, arsenate, and inorganic thiolated arsenates. Much less is known about the relevance and formation of methylated thioarsenates, a group with species of high mobility and toxicity. In hot spring samples taken from the Tengchong volcanic region in China, methylated thioarsenates contributed up to 13% to total arsenic. Enrichment cultures were obtained from the corresponding sediment samples and incubated to assess their capability to convert arsenite into methylated thioarsenates over time and in the presence of different microbial inhibitors. In contrast to observations in other environmental systems (e.g., paddy soils), there was no solid evidence, supporting that the sulfate-reducing bacteria contributed to the arsenic methylation. Methanosarcina, the sole genus of methanogens detected in the enrichment cultures, as well as Methanosarcina thermophila TM-1, a pure strain within the genus, did methylate arsenic. We propose that methylated thioarsenates in a typical sulfide-rich hot spring environment like Tengchong form via a combination of biotic arsenic methylation driven by thermophilic methanogens and arsenic thiolation with either geogenic sulfide or sulfide produced by sulfate-reducing bacteria.

Dissolved thiolated arsenic formed by weathering of mine wastes

Aqueous thiolated arsenic (As) species play an important role in the biogeochemical cycling of As in wetlands and hydrothermal systems. Although mine wastes such as tailings ponds and waste rock piles may harbor similarly sub-oxic and neutral to alkaline conditions that favor the formation and mobility of thio-As species, quantitative data on their existence in these systems is lacking. We conducted laboratory column experiments under contrasting redox conditions with waste rock from the Antamina mine, Peru, and processed tailings from Montague, Nova Scotia, Canada. Dissolved As concentrations between 1 and 7000 μg/L were recorded in drainages across these mine waste types, with up to 13 μg/L As present in thiolated form, predominantly monothioarsenate. Higher percentages of thio-As species (up to 5%) were observed in drainages from enargite-rich materials compared to arsenopyrite-bearing materials (<0.5%). The lower abundance of dissolved thio-As in the arsenopyrite-rich mine waste samples is attributed to their partially oxidized nature and reduced mineral reactivity under the experimental circumneutral drainage pH, the difference in S [-II/0]-to-As molar ratios compared to the enargite-rich mine waste samples, as well as the oxidation of di- and tri-thiolated As species by dissolved Fe. Overall, our results demonstrate that aqueous thiolated As species may occur in mine wastes with different As-bearing minerals and could play an important role in governing the mobility and fate of As in these systems.

Different sulfide to arsenic ratios driving arsenic speciation and microbial community interactions in two alkaline hot springs

Arsenic (As) is ubiquitous in geothermal fluids, which threatens both water supply safety and local ecology. The co-occurrence of sulfur (S) and As increases the complexity of As migration and transformation in hot springs. Microorganisms play important roles in As-S transformation processes. In the present study, two Tibetan alkaline hot springs (designated Gulu [GL] and Daba [DB]) with different total As concentrations (0.88 mg/L and 12.42 mg/L, respectively) and different sulfide/As ratios (3.97 and 0.008, respectively) were selected for investigating interactions between As-S geochemistry and microbial communities along the outflow channels. The results showed that As-S transformation processes were similar, although concentrations and percentages of As and S species differed between the two hot springs. Thioarsenates were detected at the vents of the hot springs (18% and 0.32%, respectively), and were desulfurized to arsenite along the drainage channel. Arsenite was finally oxidized to arsenate (532 μg/L and 12,700 μg/L, respectively). Monothioarsenate, total As, and sulfate were the key factors shaping the changes in microbial communities with geochemical gradients. The relative abundances of sulfur reduction genes (dsrAB) and arsenate reduction genes (arsC) were higher in upstream portions of GL explaining high thiolation. Arsenite oxidation genes (aoxAB) were relatively abundant in downstream parts of GL and at the vent of DB explaining low thiolation. Sulfur oxidation genes (soxABXYZ) were abundant in GL and DB. Putative sulfate-reducing bacteria (SRB), such as Desulfuromusa and Clostridium, might be involved in forming thioarsenates by producing reduced S for chemical reactions with arsenite. Sulfur-oxidizing bacteria (SOB), such as Elioraea, Pseudoxanthomonas and Pseudomonas, and arsenite-oxidizing bacteria (AsOB) such as Thermus, Sulfurihydrogenibium and Hydrogenophaga, may be responsible for the oxidation of As-bound S, thereby desulfurizing thioarsenates, forming arsenite and, by further abiotic or microbial oxidation, arsenate. This study improves our understanding of As and S biogeochemistry in hot springs.

Mechanism of Arsenic Partitioning During Sulfidation of As-Sorbed Ferrihydrite Nanoparticles

Knowledge of how arsenic (As) partitions among various phases in Fe-rich sulfidic environments is critical for understanding the fate and mobility of As in such environments. We studied the reaction of arsenite and arsenate sorbed on ferrihydrite nanoparticle surfaces with dissolved sulfide at varying S/Fe ratios (0.1-2.0) to understand the fate and transformation mechanism of As during sulfidation of ferrihydrite. By using aqueous As speciation analysis by IC-ICP-MS and solid-phase As speciation analysis by synchrotron-based X-ray absorption spectroscopy (XAS), we were able to discern the mechanism and pathways of As partitioning and thio-arsenic species formation. Our results provide a mechanistic understanding of the fate and transformation of arsenic during the codiagenesis of As, Fe, and S in reducing environments. Our aqueous-phase As speciation data, combined with solid-phase speciation data, indicate that sulfidation of As-sorbed ferrihydrite nanoparticles results in their transformation to trithioarsenate and arsenite, independent of the initial arsenic species used. The nature and extent of transformation and the thioarsenate species formed were controlled by S/Fe ratios in our experiments. However, arsenate was reduced to arsenite before transformation to trithioarsenate.

Sorption of Monothioarsenate to the Natural Sediments and Its Competition with Arsenite and Arsenate

Monothioarsenate (MTAs^V) is one of the major arsenic species in sulfur- or iron-rich groundwater, and the sediment adsorption of MTAs^V plays an important role in arsenic cycling in the subsurface environment. In this study, batch experiments and characterization are conducted to investigate the sorption characteristic and mechanism of MTAs^V on natural sediments and the influences of arsenite and arsenate. Results show that MTAs^V adsorption on natural sediments is similar to arsenate and arsenite, manifested by a rapid early increasing stage, a slowly increasing stage at an intermediate time until 8 h, before finally approaching an asymptote. The sediment sorption for MTAs^V mainly occurs on localized sites with high contents of Fe and Al, where MTAs^V forms a monolayer on the surface of natural sediments via a chemisorption mechanism and meanwhile the adsorbed MTAs^V mainly transforms into other As species, such as AlAs, Al-As-O, and Fe-As-O compounds. At low concentration, MTAs^V sorption isotherm by natural sediments becomes the Freundlich isotherm model, while at high concentration of MTAs^V, its sorption isotherm becomes the Langmuir isotherm model. The best-fitted maximum adsorption capacity for MTAs^V adsorption is about 362.22 μg/g. Furthermore, there is a competitive effect between MTAs^V and arsenate adsorption, and MTAs^V and arsenite adsorption on natural sediments. More specifically, the presence of arsenite greatly decreases MTAs^V sorption, while the presence of MTAs^V causes a certain degree of reduction of arsenite adsorption on the sediments before 4 h, and this effect becomes weaker when approaching the equilibrium state. The presence of arsenate greatly decreases MTAs^V sorption and the presence of MTAs^V also greatly decreases arsenate sorption. These competitive effects may greatly affect MTAs^V transport in groundwater systems and need more attention in the future.

Arsenic and the gastrointestinal tract microbiome

Arsenic is a toxin, ranking first on the Agency for Toxic Substances and Disease Registry and the Environmental Protection Agency Priority List of Hazardous Substances. Chronic exposure increases the risk of a broad range of human illnesses, most notably cancer; however, there is significant variability in arsenic-induced disease among exposed individuals. Human genetics is a known component, but it alone cannot account for the large inter-individual variability in the presentation of arsenicosis symptoms. Each part of the gastrointestinal tract (GIT) may be considered as a unique environment with characteristic pH, oxygen concentration, and microbiome. Given the well-established arsenic redox transformation activities of microorganisms, it is reasonable to imagine how the GIT microbiome composition variability among individuals could play a significant role in determining the fate, mobility and toxicity of arsenic, whether inhaled or ingested. This is a relatively new field of research that would benefit from early dialogue aimed at summarizing what is known and identifying reasonable research targets and concepts. Herein, we strive to initiate this dialogue by reviewing known aspects of microbe-arsenic interactions and placing it in the context of potential for influencing host exposure and health risks. We finish by considering future experimental approaches that might be of value.

Transcriptomic Analysis of Two Thioalkalivibrio Species Under Arsenite Stress Revealed a Potential Candidate Gene for an Alternative Arsenite Oxidation Pathway

The genus Thioalkalivibrio includes haloalkaliphilic chemolithoautotrophic sulfur-oxidizing bacteria isolated from various soda lakes worldwide. Some of these lakes possess in addition to their extreme haloalkaline environment also other harsh conditions, to which Thioalkalivibrio needs to adapt. An example is arsenic in soda lakes in eastern California, which is found there in concentrations up to 3000 μM. Arsenic is a widespread element that can be an environmental issue, as it is highly toxic to most organisms. However, resistance mechanisms in the form of detoxification are widespread and some prokaryotes can even use arsenic as an energy source. We first screened the genomes of 76 Thioalkalivibrio strains for the presence of known arsenic oxidoreductases and found 15 putative ArxA (arsenite oxidase) and two putative ArrA (arsenate reductase). Subsequently, we studied the resistance to arsenite in detail in Thioalkalivibrio jannaschii ALM2^T, and Thioalkalivibrio thiocyanoxidans ARh2^T by comparative genomics and by growing them at different arsenite concentrations followed by arsenic species and transcriptomic analysis. Tv. jannaschii ALM2^T, which has been isolated from Mono Lake, an arsenic-rich soda lake, could resist up to 5 mM arsenite, whereas Tv. thiocyanoxidans ARh2^T, which was isolated from a Kenyan soda lake, could only grow up to 0.1 mM arsenite. Interestingly, both species oxidized arsenite to arsenate under aerobic conditions, although Tv. thiocyanoxidans ARh2^T does not contain any known arsenite oxidases, and in Tv. jannaschii ALM2^T, only arxB2 was clearly upregulated. However, we found the expression of a SoeABC-like gene, which we assume might have been involved in arsenite oxidation. Other arsenite stress responses for both strains were the upregulation of the vitamin B₁₂ synthesis pathway, which can be linked to antioxidant activity, and the up- and downregulation of different DsrE/F-like genes whose roles are still unclear. Moreover, Tv. jannaschii ALM2^T induced the ars gene operon and the Pst system, and Tv. thiocanoxidans ARh2^T upregulated the sox and apr genes as well as different heat shock proteins. Our findings for Thioalkalivibrio confirm previously observed adaptations to arsenic, but also provide new insights into the arsenic stress response and the connection between the arsenic and the sulfur cycle.

Monothioarsenate Uptake, Transformation, and Translocation in Rice Plants

Thioarsenates form under sulfur-reducing conditions in paddy soil pore waters. Sulfur fertilization, recently promoted for decreasing total arsenic (As) grain concentrations, could enhance their formation. Yet, to date, thioarsenate toxicity, uptake, transformation, and translocation in rice are unknown. Our growth inhibition experiments showed that the toxicity of monothioarsenate (MTA) was similar to that of arsenate but lower than that of arsenite. Higher toxicity of MTA with lower phosphate availability might imply uptake through phosphate transporters similar to arsenate. To demonstrate direct uptake of MTA by rice plants, a species-preserving extraction method for plant samples was developed. When plants were exposed to 10 μM MTA for 72 h, up to 19% and 4% of total As accumulated in roots and shoots, respectively, was MTA. Monothioarsenate was detected in xylem sap and root exudates, and its reduction to arsenite in rice roots and shoots was shown. Total As uptake was lower upon exposure to MTA compared to arsenate, but root to shoot translocation was higher, resulting in comparable As shoot concentrations. Thus, before promoting sulfur fertilization, uptake and detoxifying mechanisms of thioarsenates as well as potential contribution to grain As accumulation need to be better understood.

Thiolated arsenic in natural systems: What is current, what is new and what needs to be known

Thiolated arsenic compounds are the sulfur analogous substructures of oxo-arsenicals as the arsinoyl (As = O) is substituted by an arsinothioyl (As = S) group. Relatively brief history of thioarsenic research, mostly in the current decade has endeavored to understand their consequences in the natural environment. However, thioarsenic related aspects have by far not attached much research concern on global scale compared to other arsenic species. This review attempts to provide a critical overview for the first time on formation mechanisms of thioarsenicals, their chemistry, speciation and analytical methodologies in order to provide a rational assessment of what is new, what is current, what needs to be known or what should be done in future research. Thioarsenic compounds play a vital role in determining the biogeochemistry of arsenic in sulfidic environments under reducing conditions. Thioarsenic species are widely immobilized by naturally occurring processes such as the adsorption on iron (oxyhydr)oxides and precipitation on iron sulfide minerals. Accurate measurement of thioarsenic species is a challenging task due to their instability upon pH, temperature, redox potential, and concentrations of oxygen, sulfur and iron. Assessment of direct and indirect effects of toxic thioarsenic species on global population those who frequently get exposed to high levels of arsenic is an urgent necessity. Dimethylmonothioarsinic acid (DMMTA^V) is the most cytotoxic arsenic metabolite having similar toxicological effects as dimethylarsinous acid (DMA^III) in human and animal tissues. The formation and chemical analysis of thioarsenicals in soil and sediments are highly unknown. Therefore, future research needs to be more inclined towards in determining the molecular structure of unknown thioarsenic complexes in various environmental suites. Contemporary approaches hyphenated to existing technologies would pave the way to overcome critical challenges of thioarsenic speciation such as standards synthesis, structural determination, quantification and sample preservation in future research.

Phylogenetic Structure and Metabolic Properties of Microbial Communities in Arsenic-Rich Waters of Geothermal Origin

Arsenic (As) is a toxic element released in aquatic environments by geogenic processes or anthropic activities. To counteract its toxicity, several microorganisms have developed mechanisms to tolerate and utilize it for respiratory metabolism. However, still little is known about identity and physiological properties of microorganisms exposed to natural high levels of As and the role they play in As transformation and mobilization processes. This work aims to explore the phylogenetic composition and functional properties of aquatic microbial communities in As-rich freshwater environments of geothermal origin and to elucidate the key microbial functional groups that directly or indirectly may influence As-transformations across a natural range of geogenic arsenic contamination. Distinct bacterial communities in terms of composition and metabolisms were found. Members of Proteobacteria, affiliated to Alpha- and Betaproteobacteria were mainly retrieved in groundwaters and surface waters, whereas Gammaproteobacteria were the main component in thermal waters. Most of the OTUs from thermal waters were only distantly related to 16S rRNA gene sequences of known taxa, indicating the occurrence of bacterial biodiversity so far unexplored. Nitrate and sulfate reduction and heterotrophic As(III)-oxidization were found as main metabolic traits of the microbial cultivable fraction in such environments. No growth of autotrophic As(III)-oxidizers, autotrophic and heterotrophic As(V)-reducers, Fe-reducers and oxidizers, Mn-reducers and sulfide oxidizers was observed. The ars genes, involved in As(V) detoxifying reduction, were found in all samples whereas aioA [As(III) oxidase] and arrA genes [As(V) respiratory reductase] were not found. Overall, we found that As detoxification processes prevailed over As metabolic processes, concomitantly with the intriguing occurrence of novel thermophiles able to tolerate high levels of As.

Thioarsenate Formation Coupled with Anaerobic Arsenite Oxidation by a Sulfate-Reducing Bacterium Isolated from a Hot Spring

Thioarsenates are common arsenic species in sulfidic geothermal waters, yet little is known about their biogeochemical traits. In the present study, a novel sulfate-reducing bacterial strain Desulfotomaculum TC-1 was isolated from a sulfidic hot spring in Tengchong geothermal area, Yunnan Province, China. The arxA gene, encoding anaerobic arsenite oxidase, was successfully amplified from the genome of strain TC-1, indicating it has a potential ability to oxidize arsenite under anaerobic condition. In anaerobic arsenite oxidation experiments inoculated with strain TC-1, a small amount of arsenate was detected in the beginning but became undetectable over longer time. Thioarsenates (AsO4-xSx2- with x = 1-4) formed with mono-, di- and tri-thioarsenates being dominant forms. Tetrathioarsenate was only detectable at the end of the experiment. These results suggest that thermophilic microbes might be involved in the formation of thioarsenates and provide a possible explanation for the widespread distribution of thioarsenates in terrestrial geothermal environments.

Linking Genes to Microbial Biogeochemical Cycling: Lessons from Arsenic

The biotransformation of arsenic is highly relevant to the arsenic biogeochemical cycle. Identification of the molecular details of microbial pathways of arsenic biotransformation coupled with analyses of microbial communities by meta-omics can provide insights into detailed aspects of the complexities of this biocycle. Arsenic transformations couple to other biogeochemical cycles, and to the fate of both nutrients and other toxic environmental contaminants. Microbial redox metabolism of iron, carbon, sulfur, and nitrogen affects the redox and bioavailability of arsenic species. In this critical review we illustrate the biogeochemical processes and genes involved in arsenic biotransformations. We discuss how current and future metagenomic-, metatranscriptomic-, metaproteomic-, and metabolomic-based methods will help to decipher individual microbial arsenic transformation processes, and their connections to other biogeochemical cycle. These insights will allow future use of microbial metabolic capabilities for new biotechnological solutions to environmental problems. To understand the complex nature of inorganic and organic arsenic species and the fate of environmental arsenic will require integrating systematic approaches with biogeochemical modeling. Finally, from the lessons learned from these studies of arsenic biogeochemistry, we will be able to predict how the environment changes arsenic, and, in response, how arsenic biotransformations change the environment.

Thioarsenate Toxicity and Tolerance in the Model System Arabidopsis thaliana

Thioarsenates form from arsenite under sulfate-reducing conditions, e.g., in rice paddy soils, and are structural analogues of arsenate. Even though rice is one of the most important sources of human arsenic intake, nothing is published about uptake, toxicity, or tolerance of thioarsenates in plants. Experiments using the model system Arabidopsis thaliana showed that monothioarsenate is less toxic than arsenite, but more toxic than arsenate at concentrations ≥25 μM As, reflected in stronger seedling growth inhibition on agar plates. Despite higher toxicity, total As accumulation in roots was lower upon exposure to monothioarsenate compared to arsenate, and a higher root efflux was confirmed. Root-shoot translocation was higher for monothioarsenate than for arsenate. Compared to the wild type (Col-0), both arsenate and monothioarsenate induced higher toxicity in phytochelatin (PC)-deficient mutants (cad1-3) as well as in glutathione biosynthesis (cad2) and PC transport (abcc12) mutants, demonstrating the important role of the PC pathway, not only for arsenate, but also for monothioarsenate detoxification. In Col-0, monothioarsenate induced relatively higher accumulation of PCs than arsenate. The observed differences in plant uptake, toxicity, and tolerance of thioarsenate vs oxyarsenate show that studying the effects of As on plants should include experiments with thiolated As species.

Sponge-associated bacteria mineralize arsenic and barium on intracellular vesicles

Arsenic and barium are ubiquitous environmental toxins that accumulate in higher trophic-level organisms. Whereas metazoans have detoxifying organs to cope with toxic metals, sponges lack organs but harbour a symbiotic microbiome performing various functions. Here we examine the potential roles of microorganisms in arsenic and barium cycles in the sponge Theonella swinhoei, known to accumulate high levels of these metals. We show that a single sponge symbiotic bacterium, Entotheonella sp., constitutes the arsenic- and barium-accumulating entity within the host. These bacteria mineralize both arsenic and barium on intracellular vesicles. Our results indicate that Entotheonella sp. may act as a detoxifying organ for its host.

The marine sponge Theonella swinhoei accumulates toxic arsenic and barium. Here the authors show that these toxic elements are actually accumulated and mineralized within vesicles inside bacteria that live within the sponge tissues.

Organoarsenical Biotransformations by Shewanella putrefaciens

Microbes play a critical role in the global arsenic biogeocycle. Most studies have focused on redox cycling of inorganic arsenic in bacteria and archaea. The parallel cycles of organoarsenical biotransformations are less well characterized. Here we describe organoarsenical biotransformations in the environmental microbe Shewanella putrefaciens. Under aerobic growth conditions, S. putrefaciens reduced the herbicide MSMA (methylarsenate or MAs(V)) to methylarsenite (MAs(III)). Even though it does not contain an arsI gene, which encodes the ArsI C-As lyase, S. putrefaciens demethylated MAs(III) to As(III). It cleaved the C-As bond in aromatic arsenicals such as the trivalent forms of the antimicrobial agents roxarsone (Rox(III)), nitarsone (Nit(III)) and phenylarsenite (PhAs(III)), which have been used as growth promoters for poultry and swine. S. putrefaciens thiolated methylated arsenicals, converting MAs(V) into the more toxic metabolite monomethyl monothioarsenate (MMMTAs(V)), and transformed dimethylarsenate (DMAs(V)) into dimethylmonothioarsenate (DMMTAs(V)). It also reduced the nitro groups of Nit(V), forming p-aminophenyl arsenate (p-arsanilic acid or p-AsA(V)), and Rox(III), forming 3-amino-4-hydroxybenzylarsonate (3A4HBzAs(V)). Elucidation of organoarsenical biotransformations by S. putrefaciens provides a holistic appreciation of how these environmental pollutants are degraded.

Microbial Sulfate Reduction Enhances Arsenic Mobility Downstream of Zerovalent-Iron-Based Permeable Reactive Barrier

We assessed the potential of zerovalent-iron- (Fe(0)) based permeable reactive barrier (PRB) systems for arsenic (As) remediation in the presence or absence of microbial sulfate reduction. We conducted long-term (200 day) flow-through column experiments to investigate the mechanisms of As transformation and mobility in aquifer sediment (in particular, the PRB downstream linkage). Changes in As speciation in the aqueous phase were monitored continuously. Speciation in the solid phase was determined at the end of the experiment using X-ray absorption near-edge structure (XANES) spectroscopy analysis. We identified thio-As species in solution and AsS in solid phase, which suggests that the As(V) was reduced to As(III) and precipitated as AsS under sulfate-reducing conditions and remained as As(V) under abiotic conditions, even with low redox potential and high Fe(II) content (4.5 mM). Our results suggest that the microbial sulfate reduction plays a key role in the mobilization of As from Fe-rich aquifer sediment under anoxic conditions. Furthermore, they illustrate that the upstream-downstream linkage of PRB affects the speciation and mobility of As in downstream aquifer sediment, where up to 47% of total As initially present in the sediment was leached out in the form of mobile thio-As species.

Microbial communities and arsenic biogeochemistry at the outflow of an alkaline sulfide-rich hot spring

Alkaline sulfide-rich hot springs provide a unique environment for microbial community and arsenic (As) biogeochemistry. In this study, a representative alkaline sulfide-rich hot spring, Zimeiquan in the Tengchong geothermal area, was chosen to study arsenic geochemistry and microbial community using Illumina MiSeq sequencing. Over 0.26 million 16S rRNA sequence reads were obtained from 5-paired parallel water and sediment samples along the hot spring’s outflow channel. High ratios of As(V)/As_Sum (total combined arsenate and arsenite concentrations) (0.59–0.78), coupled with high sulfide (up to 5.87 mg/L), were present in the hot spring’s pools, which suggested As(III) oxidation occurred. Along the outflow channel, As_Sum increased from 5.45 to 13.86 μmol/L, and the combined sulfide and sulfate concentrations increased from 292.02 to 364.28 μmol/L. These increases were primarily attributed to thioarsenic transformation. Temperature, sulfide, As and dissolved oxygen significantly shaped the microbial communities between not only the pools and downstream samples, but also water and sediment samples. Results implied that the upstream Thermocrinis was responsible for the transformation of thioarsenic to As(III) and the downstream Thermus contributed to derived As(III) oxidation. This study improves our understanding of microbially-mediated As transformation in alkaline sulfide-rich hot springs.

Pyrobaculum yellowstonensis Strain WP30 Respires on Elemental Sulfur and/or Arsenate in Circumneutral Sulfidic Geothermal Sediments of Yellowstone National Park

Thermoproteales (phylum Crenarchaeota) populations are abundant in high-temperature (>70°C) environments of Yellowstone National Park (YNP) and are important in mediating the biogeochemical cycles of sulfur, arsenic, and carbon. The objectives of this study were to determine the specific physiological attributes of the isolate Pyrobaculum yellowstonensis strain WP30, which was obtained from an elemental sulfur sediment (Joseph's Coat Hot Spring [JCHS], 80°C, pH 6.1, 135 μM As) and relate this organism to geochemical processes occurring in situ. Strain WP30 is a chemoorganoheterotroph and requires elemental sulfur and/or arsenate as an electron acceptor. Growth in the presence of elemental sulfur and arsenate resulted in the formation of thioarsenates and polysulfides. The complete genome of this organism was sequenced (1.99 Mb, 58% G+C content), revealing numerous metabolic pathways for the degradation of carbohydrates, amino acids, and lipids. Multiple dimethyl sulfoxide-molybdopterin (DMSO-MPT) oxidoreductase genes, which are implicated in the reduction of sulfur and arsenic, were identified. Pathways for the de novo synthesis of nearly all required cofactors and metabolites were identified. The comparative genomics of P. yellowstonensis and the assembled metagenome sequence from JCHS showed that this organism is highly related (∼95% average nucleotide sequence identity) to in situ populations. The physiological attributes and metabolic capabilities of P. yellowstonensis provide an important foundation for developing an understanding of the distribution and function of these populations in YNP.

Anaerobic Chemolithotrophic Growth of the Haloalkaliphilic Bacterium Strain MLMS-1 by Disproportionation of Monothioarsenate

A novel chemolithotrophic metabolism based on a mixed arsenic-sulfur species has been discovered for the anaerobic deltaproteobacterium, strain MLMS-1, a haloalkaliphile isolated from Mono Lake, California, U.S. Strain MLMS-1 is the first reported obligate arsenate-respiring chemoautotroph which grows by coupling arsenate reduction to arsenite with the oxidation of sulfide to sulfate. In that pathway the formation of a mixed arsenic-sulfur species was reported. That species was assumed to be monothioarsenite ([H2As(III)S(-II)O2](-)), formed as an intermediate by abiotic reaction of arsenite with sulfide. We now report that this species is monothioarsenate ([HAs(V)S(-II)O3](2-)) as revealed by X-ray absorption spectroscopy. Monothioarsenate forms by abiotic reaction of arsenite with zerovalent sulfur. Monothioarsenate is kinetically stable under a wide range of pH and redox conditions. However, it was metabolized rapidly by strain MLMS-1 when incubated with arsenate. Incubations using monothioarsenate confirmed that strain MLMS-1 was able to grow (μ = 0.017 h(-1)) on this substrate via a disproportionation reaction by oxidizing the thio-group-sulfur (S(-II)) to zerovalent sulfur or sulfate while concurrently reducing the central arsenic atom (As(V)) to arsenite. Monothioarsenate disproportionation could be widespread in nature beyond the already studied arsenic and sulfide rich hot springs and soda lakes where it was discovered.

Effects of nutrient and sulfate additions on As mobility in contaminated soils: a laboratory column study

The effect of nutrient and sulfate additions on As mobility in contaminated soils was investigated under advective-flow anoxic columns in this study. The mobility of As in surface contaminated soils was investigated by the leaching of de-ionized water (DI-water), artificial ground water (AG-water) and AG-water+sulfate (Sulfate). After 144 d of experiments, compared to the DI-water column, the total As exported from the columns AG-water and Sulfate was enhanced by seven and eightfold, respectively. The results indicated that the nutrient and sulfate addition significantly enhanced the As mobility in contaminated soils. In low-sulfate soils (DI-water and AG-water systems), As mobilization was primarily attributed to As reduction and to the transformation of amorphous Fe(III) (oxy)hydroxides. In soil with sulfate addition (Sulfate system), besides As reduction and Fe(III) (oxy)hydroxides transformation, the dissolution of As sulfides and the formation of thioarsenic species under sulfidogenic condition were possibly important processes accelerating As release. In conclusion, the addition of the nutrient solution and sulfates may increase the mobility of As in contaminated soils, posing a potential threat to groundwater.

Transformation of monothioarsenate by haloalkaliphilic, anoxygenic photosynthetic purple sulfur bacteria

Thioarsenates are the dominant arsenic species in arsenic-rich, alkaline, and sulfidic waters, but bacterial interactions with these compounds have only recently been examined. Previous studies have shown that microorganisms play a role in the transformation of monothioarsenate to arsenate, including use of monothioarsenate as a chemolithotrophic electron donor coupled with oxygen as an electron acceptor. We obtained enrichment cultures from two saline, alkaline lakes (Mono Lake, CA and Big Soda Lake, NV) that are able to use monothioarsenate as the sole electron donor for anoxygenic photosynthesis. These anoxic cultures were able to convert a 1 mM mixture of thioarsenates completely to arsenate in c. 13 days and 4 mM monothioarsenate to arsenate in c. 17 days. This conversion was light dependent; thus, monothioarsenate can be used as the sole electron donor for anoxygenic photosynthesis. Both of the Mono Lake and Big Soda Lake enrichment cultures were dominated by an organism closely related to Ectothiorhodospira species. We tested additional strains of purple sulfur bacteria and found widespread ability to use monothioarsenate as an electron donor. The ability of bacteria to transform thioarsenates directly via anoxygenic photosynthesis adds a new perspective to the well-studied arsenic and sulfur cycles.

Effect of sulfide on the cytotoxicity of arsenite and arsenate in human hepatocytes (HepG2) and human urothelial cells (UROtsa)

Arsenic, a common poison, is known to react with sulfide in vivo, forming thioarsenates. The acute toxicity of the inorganic thioarsenates is currently unknown. Our experiments showed that a fourfold sulfide excess reduced acute arsenite cytotoxicity in human hepatocytes (HepG2) and urothelial cells (UROtsa) significantly, but had little effect on arsenate toxicity. Speciation analysis showed immediate formation of thioarsenates (up to 73 % of total arsenic) in case of arsenite, but no speciation changes for arsenate. Testing acute toxicity of mono- and trithioarsenate individually, both thioarsenates were found to be more toxic than their structural analogue arsenate, but less toxic than arsenite. Toxicity increased with the number of thio groups. The amount of cellular arsenic uptake after 24 h corresponded to the order of toxicity of the four compounds tested. The dominant to almost exclusive intracellular arsenic species was arsenite. The results imply that thiolation is a detoxification process for arsenite in sulfidic milieus. The mechanism could either be that thioarsenates regulate the amount of free arsenite available for cellular uptake without entering the cells themselves, or, based on their chemical similarity to arsenate, they could be taken up by similar transporters and reduced rapidly intracellularly to arsenite.

Arsenic geochemistry in a biostimulated aquifer: an aqueous speciation study

Stimulating microbial growth through the use of acetate injection wells at the former uranium mill site in Rifle, Colorado, USA, has been shown to decrease dissolved uranium (VI) concentrations through bacterial reduction to immobile uranium (IV). Bioreduction also changed the redox chemistry of site groundwater, altering the mobility of several other redox-sensitive elements present in the subsurface, including iron, sulfur, and arsenic. Following acetate amendment at the site, elevated concentrations of arsenic in the groundwater were observed. Ion chromatography-inductively coupled plasma-mass spectrometry was used to determine the aqueous arsenic speciation. Upgradient samples, unexposed to acetate, showed low levels of arsenic (≈1 μM), with greater than 90% as arsenate (As[V]) and a small amount of arsenite (As[III]). Downgradient acetate-stimulated water samples had much higher levels of arsenic (up to 8 μM), and 4 additional thioarsenic species were present under sulfate-reducing conditions. These thioarsenic species demonstrate a strong correlation between arsenic release and sulfide concentrations in groundwater, and their formation may explain the elevated total arsenic concentrations. An alternative remediation approach, enhanced flushing of uranium, was accomplished by addition of bicarbonate and did not result in highly elevated arsenic concentrations.