Arsenite Oxidation in Ancylobacter dichloromethanicus As3-1b Strain: Detection of Genes Involved in Arsenite Oxidation and CO2 Fixation

Synthetic bacteria designed using ars operons: a promising solution for arsenic biosensing and bioremediation

The global concern over arsenic contamination in water due to its natural occurrence and human activities has led to the development of innovative solutions for its detection and remediation. Microbial metabolism and mobilization play crucial roles in the global cycle of arsenic. Many microbial arsenic-resistance systems, especially the ars operons, prevalent in bacterial plasmids and genomes, play vital roles in arsenic resistance and are utilized as templates for designing synthetic bacteria. This review novelty focuses on the use of these tailored bacteria, engineered with ars operons, for arsenic biosensing and bioremediation. We discuss the advantages and disadvantages of using synthetic bacteria in arsenic pollution treatment. We highlight the importance of genetic circuit design, reporter development, and chassis cell optimization to improve biosensors' performance. Bacterial arsenic resistances involving several processes, such as uptake, transformation, and methylation, engineered in customized bacteria have been summarized for arsenic bioaccumulation, detoxification, and biosorption. In this review, we present recent insights on the use of synthetic bacteria designed with ars operons for developing tailored bacteria for controlling arsenic pollution, offering a promising avenue for future research and application in environmental protection.

The Composition and Function of the Rhizosphere Bacterial Community of Paeonia lactiflora Varies with the Cultivar

The composition and diversity of the rhizosphere microbial community maintain the stability of the root microclimate, and several studies have focused on this aspect of rhizosphere microorganisms. However, how these communities vary with cultivars of a species is not completely understood. Paeonia lactiflora-a perennial herb species of the family Paeoniaceae-includes a wide variety of cultivars, with rich rhizosphere microbial resources. Hence, we studied the differences in rhizosphere bacterial communities associated with eight P. lactiflora cultivars. We noted that Actinobacteria, Proteobacteria, Acidobacteria, Bacteroidetes, Firmicutes, Verrucomicrobia, Planctomycetes and Chloroflexi were the dominant phyla associated with the cultivars. The composition of rhizosphere bacterial community of different cultivars was highly similar at taxonomic levels, but there were slightly differences in the relative abundance. LEfSe analysis showed that the cultivars "Sheng Tao Hua" and "Zi Lou Xian Jin" exhibited the most biomarkers. Differential ASV analysis revealed the maximum difference in ASV abundance between "Lian Tai" and "Zi Hong Zheng Hui", as well as between "Sheng Tao Hua" and "Tao Hua Fei Xue", and the maximum similarity between "Duo Ye Zi" and "Xue Feng". Co-occurrence network analysis revealed that rhizosphere bacteria in most cultivars maintain homeostasis by cooperation, wherein Actinobacteria and Proteobacteria played a vital role. In addition, microbial resources related to cultivars like bioremediation, organic degradation and resistance to diseases are found. This study revealed the structures of the rhizosphere bacterial communities associated with different cultivars of P. lactiflora and explored their stress resistance potential, which can be used to guide future agricultural practices.

Microbial biochemical pathways of arsenic biotransformation and their application for bioremediation

Arsenic is a ubiquitous toxic metalloid, the concentration of which is beyond WHO safe drinking water standards in many areas of the world, owing to many natural and anthropogenic activities. Long-term exposure to arsenic proves lethal for plants, humans, animals, and even microbial communities in the environment. Various sustainable strategies have been developed to mitigate the harmful effects of arsenic which include several chemical and physical methods, however, bioremediation has proved to be an eco-friendly and inexpensive technique with promising results. Many microbes and plant species are known for arsenic biotransformation and detoxification. Arsenic bioremediation involves different pathways such as uptake, accumulation, reduction, oxidation, methylation, and demethylation. Each of these pathways has a certain set of genes and proteins to carry out the mechanism of arsenic biotransformation. Based on these mechanisms, various studies have been conducted for arsenic detoxification and removal. Genes specific for these pathways have also been cloned in several microorganisms to enhance arsenic bioremediation. This review discusses different biochemical pathways and the associated genes which play important roles in arsenic redox reactions, resistance, methylation/demethylation, and accumulation. Based on these mechanisms, new methods can be developed for effective arsenic bioremediation.

Ancylobacter radicis sp. nov., a novel aerobic methylotrophic bacteria associated with plants

The two novel bacterial strains, designated as VT^T and ML, were isolated from roots of cinquefoil (Potentilla sp.) and leaves of meadow-grass (Poa sp.) on the flooded bank of lake, respectively. These isolates were Gram-negative, non-spore-forming, non-motile, rod-shaped cells, utilized methanol, methylamine, and polycarbon compounds as carbon and energy sources. In the whole-cell fatty acid pattern of strains prevailed C_18:1ω7c and C_19:0cyc. Based on the phylogenetic analysis of 16S rRNA gene sequences, strains VT^T and ML were closely related to the representatives of the genus Ancylobacter (98.3-98.5%). The assembled genome of strain VT^T has a total length of 4.22 Mbp, and a G + C content is 67.3%. The average nucleotide identity (ANI), average amino acid identity (AAI) and digital DNA-DNA hybridization (dDDH) values between strain VT^T and closely related type strains of genus Ancylobacter were 78.0-80.6%, 73.8-78.3% and 22.1-24.0%, respectively, that clearly lower than proposed thresholds for species. On the basis of the phylogenetic, phenotypic, and chemotaxonomic analysis, isolates VT^T and ML represent a novel species of the genus Ancylobacter, for which the name Ancylobacter radicis sp. nov. is proposed. The type strain is VT^T (= VKM B-3255^T = CCUG 72400^T). In addition, novel strains were able to dissolve insoluble phosphates, to produce siderophores and plant hormones (auxin biosynthesis). According to genome analysis genes involved in the biosynthesis of siderophores, polyhydroxybutyrate, exopolysaccharides and phosphorus metabolism, as well as the genes involved in the assimilation of C₁-compounds (natural products of plant metabolism) were found in the genome of type strain VT^T.

Ancylobacter moscoviensis sp. nov., novel facultatively methylotrophic bacteria from activated sludge and the reclassification of Starkeya novella (Starkey 1934) Kelly et al. 2000 as Ancylobacter novellus comb. nov., Starkeya koreensis Im et al. 2006 as Ancylobacter koreensis comb.nov., Angulomicrobium tetraedrale Vasil'eva et al. 1986 as Ancylobacter tetraedralis comb. nov., Angulomicrobium amanitiforme Fritz et al. 2004 as Ancylobacter amanitiformis comb. nov., and Methylorhabdus multivorans Doronina et al. 1996 as Ancylobacter multivorans comb. nov., and emended description of the genus Ancylobacter

Three novel facultatively methylotrophic bacteria, strains 3C^T, 1A, 8P, were isolated from activated sludges. The isolates were aerobic, Gram-stain-negative, non-motile, non-spore forming rods multiplying by binary fission. The predominant polar lipids were phosphatidylcholine, phosphatidylglycerol, phosphatidylethylethanolamine, phosphatidylmonomethylethanolamine, and diphosphatidylglycerol. The major fatty acids of cells were С_18:1ω7c, C_19:0ω8c cyclo and C_16:0. Levels of 16S rRNA gene similarity indicates that the closely relatives are representatives of the genera Starkeya, Ancylobacter, Angulomicrobium and Methylorhabdus (96.4-99.4%). Genomic comparisons of 3C^T and its closest relatives, S. novella DSM 506^T and S. koreensis Jip08^T, shared 87.3 and 86.8% nucleotide identity and 28.3 and 26.8% digital DNA-DNA hybridization values, respectively. The average amino acid identities between the strain 3C^T and representatives of Starkeya, Ancylobacter and Angulomicrobium were in the range of 75.6-84.3%, which combines these strains into a single genus and gives rise to their reclassification. Based on polyphasic analyses, the strains 3C^T, 1A, 8P represents a novel species of the genus Ancylobacter, for which the name Ancylobacter moscoviensis sp. nov. is proposed. The type strain is 3C^T (= VKM B-3218^T = KCTC 62336^T). Furthermore, we also suggested the reclassification of Starkeya novella as Ancylobacter novellus comb. nov., Starkeya koreensis as Ancylobacter koreensis comb. nov., Angulomicrobium tetraedrale as Ancylobacter tetraedralis comb. nov., Angulomicrobium amanitiforme as Ancylobacter amanitiformis comb. nov. and Methylorhabdus multivorans as Ancylobacter multivorans comb. nov. with the emended description of the genus Ancylobacter.

Development of a rapid startup method of direct electron transfer-dominant methanogenic microbial electrosynthesis

The rapid startup of carbon dioxide reduction-methanogenic microbial electrosynthesis is crucial for its industrial application, and the development of cathode biofilm is the key to its industrialization. Based on the new discovery that biofilm formed by placing graphite felt in an anaerobic reactor was electroactive, with strong direct electron transfer and methanogenesis ability (24.52 mL/L/d), a new startup method was developed. The startup time was shortened by at least 20 days and charge transfer resistance was reduced by 4.45-10.78 times than common startup methods (inoculating cathode effluent or granular sludge into the cathode chamber). The new method enriched electroactive bacteria. Methanobacterium and Methanosaeta accounted for 62.04% and 34.96%, respectively. The common methods inoculating cathode effluent or granular sludge enriched hydrogenotrophic microorganisms (>95%) or Methanosaeta (54.10%) due to the local environments of cathode. This new rapid and easy startup method may support the scale-up of microbial electrosynthesis.

Biofilm formation of Ancylobacter sp. TS-1 on different granular materials and its ability for chemolithoautotrophic As(III)-oxidation at high concentrations

The oxidation of arsenic (As) is a key step in its removal from water, and biological oxidation may provide a cost-effective and sustainable method. The biofilm-formation ability of Ancylobacter sp. TS-1, a novel chemolithoautotrophic As oxidizer, was studied for four materials: polypropylene, graphite, sand, and zeolite. After seven days under batch mixotrophic conditions, with high concentrations of As(III) (225 mg·L^-1), biofilm formation was detected on all materials except for polypropylene. The results demonstrate As(III)-oxidation of TS-1 biofilms and suggest that the number of active cells was similar for graphite, sand, and zeolite. However, the biofilm biomass follows the specific surface area of each material: 7.0, 2.4, and 0.4 mg VSS·cm^-3 for zeolite, sand, and graphite, respectively. Therefore, the observed biofilm-biomass differences were probably associated with different amounts of EPS and inert biomass. Lastly, As(III)-oxidation kinetics were assessed for the biofilms formed on graphite and zeolite under chemolithoautotrophic conditions. The normalized oxidation rate for biofilms formed on these materials was 3.6 and 1.0 mg·L^-1·h^-1·cm^-3, resulting among the highest reported values for As(III)-oxidizing biofilms operated at high-As(III) concentrations. Our findings suggest that biofilm reactors based on Ancylobacter sp. TS-1 are highly promising for their utilization in As(III)-oxidation pre-treatment of high-As(III) polluted waters.

Bioaccumulation and detoxification of trivalent arsenic by Achromobacter xylosoxidans BHW-15 and electrochemical detection of its transformation efficiency

Arsenotrophic bacteria play an essential role in lowering arsenic contamination by converting toxic arsenite [As (III)] to less toxic and less bio-accumulative arsenate [As (V)]. The current study focused on the qualitative and electrocatalytic detection of the arsenite oxidation potential of an arsenite-oxidizing bacteria A. xylosoxidans BHW-15 (retrieved from As-contaminated tube well water), which could significantly contribute to arsenic detoxification, accumulation, and immobilization while also providing a scientific foundation for future electrochemical sensor development. The minimum inhibitory concentration (MIC) value for the bacteria was 15 mM As (III). Scanning Electron Microscopy (SEM) investigation validated its intracellular As uptake capacity and demonstrated a substantial association with the MIC value. During the stationary phase, the strain’s As (III) transformation efficiency was 0.0224 mM/h. Molecular analysis by real-time qPCR showed arsenite oxidase (aioA) gene expression increased 1.6-fold in the presence of As (III) compared to the untreated cells. The immobilized whole-cell also showed As (III) conversion up to 18 days. To analyze the electrochemical oxidation in water, we developed a modified GCE/P-Arg/ErGO-AuNPs electrode, which successfully sensed and quantified conversion of As (III) into As (V) by accepting electrons; implying a functional As oxidase enzyme activity in the cells. To the best of our knowledge, this is the first report on the electrochemical observation of the As-transformation mechanism with Achromobactersp. Furthermore, the current work highlighted that our isolate might be employed as a promising candidate for arsenic bioremediation, and information acquired from this study may be helpful to open a new window for the development of a cost-effective, eco-friendly biosensor for arsenic species detection in the future.

Unraveling the Underlying Heavy Metal Detoxification Mechanisms of Bacillus Species

The rise of anthropogenic activities has resulted in the increasing release of various contaminants into the environment, jeopardizing fragile ecosystems in the process. Heavy metals are one of the major pollutants that contribute to the escalating problem of environmental pollution, being primarily introduced in sensitive ecological habitats through industrial effluents, wastewater, as well as sewage of various industries. Where heavy metals like zinc, copper, manganese, and nickel serve key roles in regulating different biological processes in living systems, many heavy metals can be toxic even at low concentrations, such as mercury, arsenic, cadmium, chromium, and lead, and can accumulate in intricate food chains resulting in health concerns. Over the years, many physical and chemical methods of heavy metal removal have essentially been investigated, but their disadvantages like the generation of chemical waste, complex downstream processing, and the uneconomical cost of both methods, have rendered them inefficient,. Since then, microbial bioremediation, particularly the use of bacteria, has gained attention due to the feasibility and efficiency of using them in removing heavy metals from contaminated environments. Bacteria have several methods of processing heavy metals through general resistance mechanisms, biosorption, adsorption, and efflux mechanisms. Bacillus spp. are model Gram-positive bacteria that have been studied extensively for their biosorption abilities and molecular mechanisms that enable their survival as well as their ability to remove and detoxify heavy metals. This review aims to highlight the molecular methods of Bacillus spp. in removing various heavy metals ions from contaminated environments.

Assessing the Diversity and Metabolic Potential of Psychrotolerant Arsenic-Metabolizing Microorganisms From a Subarctic Peatland Used for Treatment of Mining-Affected Waters by Culture-Dependent and -Independent Techniques

Arsenic contamination in water by natural causes or industrial activities is a major environmental concern, and treatment of contaminated waters is needed to protect water resources and minimize the risk for human health. In mining environments, treatment peatlands are used in the polishing phase of water treatment to remove arsenic (among other contaminants), and peat microorganisms play a crucial role in arsenic removal. The present study assessed culture-independent diversity obtained through metagenomic and metatranscriptomic sequencing and culture-dependent diversity obtained by isolating psychrotolerant arsenic-tolerant, arsenite-oxidizing, and arsenate-respiring microorganisms from a peatland treating mine effluent waters of a gold mine in Finnish Lapland using a dilution-to-extinction technique. Low diversity enrichments obtained after several transfers were dominated by the genera Pseudomonas, Polaromonas, Aeromonas, Brevundimonas, Ancylobacter, and Rhodoferax. Even though maximal growth and physiological activity (i.e., arsenite oxidation or arsenate reduction) were observed at temperatures between 20 and 28°C, most enrichments also showed substantial growth/activity at 2–5°C, indicating the successful enrichments of psychrotolerant microorganisms. After additional purification, eight arsenic-tolerant, five arsenite-oxidizing, and three arsenate-respiring strains were obtained in pure culture and identified as Pseudomonas, Rhodococcus, Microbacterium, and Cadophora. Some of the enriched and isolated genera are not known to metabolize arsenic, and valuable insights on arsenic turnover pathways may be gained by their further characterization. Comparison with phylogenetic and functional data from the metagenome indicated that the enriched and isolated strains did not belong to the most abundant genera, indicating that culture-dependent and -independent methods capture different fractions of the microbial community involved in arsenic turnover. Rare biosphere microorganisms that are present in low abundance often play an important role in ecosystem functioning, and the enriched/isolated strains might thus contribute substantially to arsenic turnover in the treatment peatland. Psychrotolerant pure cultures of arsenic-metabolizing microorganisms from peatlands are needed to close the knowledge gaps pertaining to microbial arsenic turnover in peatlands located in cold climate regions, and the isolates and enrichments obtained in this study are a good starting point to establish model systems. Improved understanding of their metabolism could moreover lead to their use in biotechnological applications intended for bioremediation of arsenic-contaminated waters.

The biotransformation of arsenic by spent mushroom compost - An effective bioremediation agent

Spent mushroom compost (SMC) is a lignocellulose-rich waste material commonly used in the passive treatment of heavy metal-contaminated environments. In this study, we investigated the bioremediation potential of SMC against an inorganic form of arsenic, examining the individual abiotic and biotic transformations carried out by SMC. We demonstrated, that key SMC physiological groups of bacteria (denitrifying, cellulolytic, sulfate-reducing, and heterotrophic) are resistant to arsenites and arsenates, while the microbial community in SMC is also able to oxidize As(III) and reduce As(V) in respiratory metabolisms, although the SMC did not contain any As. We showed, that cooperation between arsenate and sulfate-reducing bacteria led to the precipitation of As_xS_y. We also found evidence of the significant role organic acids may play in arsenic complexation, and we demonstrated the occurrence of As-binding proteins in the SMC. Furthermore, we confirmed, that biofilm produced by the microbial community in SMC was able to trap As(V) ions. We postulated, that the above-mentioned transformations are responsible for the sorption efficiency of As(V) (up to 25%) and As(III) (up to 16%), as well as the excellent buffering properties of SMC observed in the sorption experiments.

Effects of Arsenic and Iron on the Community and Abundance of Arsenite-Oxidizing Bacteria in an Arsenic-Affected Groundwater Aquifer

Arsenic (As) contamination of groundwater aquifers is a global environmental problem, especially in South and Southeast Asian regions, and poses a risk to human health. Arsenite-oxidizing bacteria that transform As(III) to less toxic As(V) can be potentially used as a groundwater As remediation strategy. This study aimed to examine the community and abundance of arsenite-oxidizing bacteria in groundwater with various As concentrations from Rayong Province, Thailand using PCR-cloning-sequencing and quantitative PCR (qPCR) of catalytic subunit of arsenite oxidase gene (aioA). Key factors influencing their community and abundance were also identified. The results demonstrated that arsenite-oxidizing bacteria retrieved from groundwater were phylogenetically related to Betaproteobacteria and Alphaproteobacteria. The aioA gene abundances ranged from 8.6 × 10¹ to 1.1 × 10⁴ copies per ng of genomic DNA, accounting for 0.16-1.37% of the total 16S rRNA bacterial gene copies. Although the abundance of arsenite-oxidizing bacteria in groundwater was low, groundwater with As(III) dominance likely promoted their abundance which possibly played an important role in chemolithoautotrophic oxidation of As(III) to As(V). Fe and As(III) were the major environmental factors influencing the community and abundance of arsenite-oxidizing bacteria. The knowledge gained from this study can be used to further contribute to the development of bioremediation strategies for As removal from groundwater resources.

Characterization of Arsenite-Oxidizing Bacteria Isolated from Arsenic-Rich Sediments, Atacama Desert, Chile

Arsenic (As), a semimetal toxic for humans, is commonly associated with serious health problems. The most common form of massive and chronic exposure to As is through consumption of contaminated drinking water. This study aimed to isolate an As resistant bacterial strain to characterize its ability to oxidize As (III) when immobilized in an activated carbon batch bioreactor and to evaluate its potential to be used in biological treatments to remediate As contaminated waters. The diversity of bacterial communities from sediments of the As-rich Camarones River, Atacama Desert, Chile, was evaluated by Illumina sequencing. Dominant taxonomic groups (>1%) isolated were affiliated with Proteobacteria and Firmicutes. A high As-resistant bacterium was selected (Pseudomonas migulae VC-19 strain) and the presence of aio gene in it was investigated. Arsenite detoxification activity by this bacterial strain was determined by HPLC/HG/AAS. Particularly when immobilized on activated carbon, P. migulae VC-19 showed high rates of As(III) conversion (100% oxidized after 36 h of incubation). To the best of our knowledge, this is the first report of a P. migulae arsenite oxidizing strain that is promising for biotechnological application in the treatment of arsenic contaminated waters.

Exploring Biodiversity and Arsenic Metabolism of Microbiota Inhabiting Arsenic-Rich Groundwaters in Northern Italy

Arsenic contamination of groundwater aquifers is an issue of global concern. Among the affected sites, in several Italian groundwater aquifers arsenic levels above the WHO limits for drinking water are present, with consequent issues of public concern. In this study, for the first time, the role of microbial communities in metalloid cycling in groundwater samples from Northern Italy lying on Pleistocene sediments deriving from Alps mountains has been investigated combining environmental genomics and cultivation approaches. 16S rRNA gene libraries revealed a high number of yet uncultured species, which in some of the study sites accounted for more of the 50% of the total community. Sequences related to arsenic-resistant bacteria (arsenate-reducing and arsenite-oxidizing) were abundant in most of the sites, while arsenate-respiring bacteria were negligible. In some of the sites, sulfur-oxidizing bacteria of the genus Sulfuricurvum accounted for more than 50% of the microbial community, whereas iron-cycling bacteria were less represented. In some aquifers, arsenotrophy, growth coupled to autotrophic arsenite oxidation, was suggested by detection of arsenite monooxygenase (aioA) and 1,5-ribulose bisphosphate carboxylase (RuBisCO) cbbL genes of microorganisms belonging to Rhizobiales and Burkholderiales. Enrichment cultures established from sampled groundwaters in laboratory conditions with 1.5 mmol L^-1 of arsenite as sole electron donor were able to oxidize up to 100% of arsenite, suggesting that this metabolism is active in groundwaters. The presence of heterotrophic arsenic resistant bacteria was confirmed by enrichment cultures in most of the sites. The overall results provided a first overview of the microorganisms inhabiting arsenic-contaminated aquifers in Northern Italy and suggested the importance of sulfur-cycling bacteria in the biogeochemistry of arsenic in these ecosystems. The presence of active arsenite-oxidizing bacteria indicates that biological oxidation of arsenite, in combination with arsenate-adsorbing materials, could be employed for metalloid removal.

Arsenic-tolerant microbial consortia from sediments of Copahue geothermal system with potential applications in bioremediation

Although arsenic (As) is recognized as a toxic element for living species, some microorganisms have the ability to tolerate and transform it; recent studies have proposed to take advantage of such capacity to develop sustainable bioremediation strategies. In this study, we evaluated the adaptation to increasing concentrations of As(III) and As(V) of three metabolically different microbial cultures (heterotrophic, autotrophic-acidophilic, and anaerobic) obtained from a sample with low-soluble As content from the Copahue geothermal system. At the end of the adaptation process, the heterotrophic culture was able to grow at 20 mM and 450 mM of As(III) and As(V), respectively; the autotrophic-acidophilic culture showed tolerance to 15 mM of As(III) and 150 mM of As(V), whereas the anaerobic culture only developed in As(V) at concentrations up to 50 mM. The most tolerant consortia were characterized by their growth performance, complexity, and the presence of genes related to As metabolism and resistance. Regarding the consortia complexity, the predominant genera identified were: Paenibacillus in both heterotrophic consortia, Acidithiobacillus in the autotrophic-acidophilic consortium tolerant to As(III), Acidiphilium in the autotrophic-acidophilic consortium tolerant to As(V), and Thiomonas and Clostridium in the anaerobic consortium. This study is the first report of As tolerance microorganisms obtained from Copahue and reasserts the versatility and flexibility of the community of this natural extreme environment; also, it opens the door to the study of possible uses of these consortia in the design of biotechnological processes where the As concentration may fluctuate.

Effects of arsenic on the biofilm formations of arsenite-oxidizing bacteria

Arsenite-oxidizing bacteria (AOB) play a key role in the biogeochemical cycle of arsenic in the environment, and are used for the bioremediation of As contaminated groundwater; however, it is not yet known about how arsenic affects biofilm formations of AOB, and how biofilm formations affect bacterial arsenite-oxidizing activities. To address these issues, we isolated seven novel AOB strains from the arsenic-contaminated soils. They can completely oxidize 1.0 mM As(III) in 22-60 h. Their arsenite oxidase sequences show 43-99% identities to those of other known AOB. Strains Cug1, Cug2, Cug3, Cug4, and Cug6 are able to form biofilms with thickness of 15-95 µm, whereas Cug8 and Cug9 cannot form biofilms. It is interesting to see that arsenite inhibited the biofilm formations of heterotrophic AOB strains, but promoted the biofilm formations of autotrophic strains in a concentration-dependent manner. The arsenite-oxidizing rates of Cug1 and Cug4 biofilms are 31.6% and 27.6% lower than those of their suspension cultures, whereas the biofilm activities of other strains are similar to those of their suspension cultures. The biofilm formation significantly promoted the bacterial resistance to arsenic. This work is the first report on the complex correlations among environmental arsenic, bacterial biofilm formations and bacterial arsenite-oxidizing activities. The data highlight the diverse lifestyle of different AOB under arsenic stress, and provide essential knowledge for the screening of efficient AOB strains used for constructions of bioreactors.

Nitrate reduced arsenic redox transformation and transfer in flooded paddy soil-rice system

Inhibition of reductive transformation of arsenic (As) in flooded paddy soils is of fundamental importance for mitigating As transfer into food chain. Anaerobic arsenite (As(III)) oxidizers maintain As in less mobile fraction under nitrate-reducing conditions. In this study, we explored the dynamic profile of As speciation in porewater and As distribution among the pools of differential bioavailability in soil solid phase with and without nitrate treatment. In parallel, the abundance and diversity of As(III) oxidase gene (aioA) in flooded paddy soil with nitrate amendment was examined by quantitative PCR and aioA gene clone library. Furthermore, the impact of nitrate on As accumulation and speciation in rice seedlings was unraveled. With nitrate addition (25 mmol NO₃^- kg^-1 soil), porewater As(III) was maintained at a consistently negligible concentration in the flooded paddy soil and the reductive dissolution of As-bearing Fe oxides/hydroxides was significantly restrained. Specifically, nitrate amendment kept 81% of total soil As in the nonlabile fraction with arsenate (As(V)) dominating after 30 days of flooding, compared to only 61% in the unamended control. Nitrate treatment induced 4-fold higher abundance of aioA gene, which belonged to domains of bacteria and archaea under the classes α-Proteobacteria (6%), ß-Proteobacteria (90%), ɣ-Proteobacteria (2%), and Thermoprotei (2%). By nitrate addition, As accumulation in rice seedlings was decreased by 85% with simultaneously elevated As(V) ratio in rice plant relative to control after 22 days of growth under flooded conditions. These results highlight that nitrate application can serve an efficient method to inhibit reductive dissolution of As in flooded paddy soils, and hence diminish As uptake by rice under anaerobic growing conditions.

Arsenite biotransformation and bioaccumulation by Klebsiella pneumoniae strain SSSW7 possessing arsenite oxidase (aioA) gene

Arsenite oxidizing Klebsiella pneumoniae strain SSSW7 isolated from shipyard waste Goa, India showed a minimum inhibitory concentration of 21 mM in mineral salts medium. The strain possessed a small supercoiled plasmid and PCR amplification of arsenite oxidase gene (aioA) was observed on plasmid as well as chromosomal DNA. It was confirmed that arsenite oxidase enzyme was a periplasmic protein with a 47% increase in arsenite oxidase activity at 1 mM sodium arsenite. Scanning electron microscopy coupled with electron dispersive X-ray spectroscopic (SEM-EDS) analysis of 15 mM arsenite exposed cells revealed long chains of cells with no surface adsorption of arsenic. Transmission electron microscopy combined with electron dispersive X-ray spectroscopic (TEM-EDS) analysis demonstrated plasma membrane disruption, cytoplasmic condensation and periplasmic accumulation of arsenic. The bacterial strain oxidized 10 mM of highly toxic arsenite to less toxic arsenate after 24 h of incubation. Fourier transformed infrared (FTIR) spectroscopy confirmed the interaction of arsenite with functional groups present on the bacterial cell surface. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis of 5 mM arsenite exposed cells demonstrated over-expression of 87 kDa and 14 kDa proteins of two subunits aioA and aioB of heterodimer arsenite oxidase enzyme as compared to control cells. Therefore, this bacterial strain might be employed as a potential candidate for bioremediation of arsenite contaminated environmental sites.

Influence of the Chemical Form of Antimony on Soil Microbial Community Structure and Arsenite Oxidation Activity

In the present study, the influence of the co-contamination with various chemical forms of antimony (Sb) with arsenite (As[III]) on soil microbial communities was investigated. The oxidation of As(III) to As(V) was monitored in soil columns amended with As(III) and three different chemical forms of Sb: antimony potassium tartrate (Sb[III]-tar), antimony(III) oxide (Sb₂O₃), and potassium antimonate (Sb[V]). Soil microbial communities were examined qualitatively and quantitatively using 16S rDNA- and arsenite oxidase gene (aioA)-targeted analyses. Microbial As(III) oxidation was detected in all soil columns and 90–100% of added As(III) (200 μmol L⁻¹) was oxidized to As(V) in 9 d, except in the Sb(III)-tar co-amendments that only oxidized 30%. 16S rDNA- and aioA-targeted analyses showed that the presence of different Sb chemical forms significantly affected the selection of distinct As(III)-oxidizing bacterial populations. Most of the 16S rRNA genes detected in soil columns belonged to Betaproteobacteria and Gammaproteobacteria, and some sequences were closely related to those of known As(III) oxidizers. Co-amendments with Sb(III)-tar and high concentrations of Sb₂O₃ significantly increased the ratios of aioA-possessing bacterial populations, indicating the enrichment of As(III) oxidizers resistant to As and Sb toxicity. Under Sb co-amendment conditions, there was no correlation between aioA gene abundance and the rates of As(III) oxidation. Collectively, these results demonstrated that the presence of different Sb chemical forms imposed a strong selective pressure on the soil bacterial community and, thus, the co-existing metalloid is an important factor affecting the redox transformation of arsenic in natural environments.

Effect of the natural arsenic gradient on the diversity and arsenic resistance of bacterial communities of the sediments of Camarones River (Atacama Desert, Chile)

Arsenic (As), a highly toxic metalloid, naturally present in Camarones River (Atacama Desert, Chile) is a great health concern for the local population and authorities. In this study, the taxonomic and functional characterization of bacterial communities associated to metal-rich sediments from three sites of the river (sites M1, M2 and M3), showing different arsenic concentrations, were evaluated using a combination of approaches. Diversity of bacterial communities was evaluated by Illumina sequencing. Strains resistant to arsenic concentrations varying from 0.5 to 100 mM arsenite or arsenate were isolated and the presence of genes coding for enzymes involved in arsenic oxidation (aio) or reduction (arsC) investigated. Bacterial communities showed a moderate diversity which increased as arsenic concentrations decreased along the river. Sequences of the dominant taxonomic groups (abundances ≥1%) present in all three sites were affiliated to Proteobacteria (range 40.3–47.2%), Firmicutes (8.4–24.8%), Acidobacteria (10.4–17.1%), Actinobacteria (5.4–8.1%), Chloroflexi (3.9–7.5%), Planctomycetes (1.2–5.3%), Gemmatimonadetes (1.2–1.5%), and Nitrospirae (1.1–1.2%). Bacterial communities from sites M2 and M3 showed no significant differences in diversity between each other (p = 0.9753) but they were significantly more diverse than M1 (p<0.001 and p<0.001, respectively). Sequences affiliated with Proteobacteria, Firmicutes, Acidobacteria, Chloroflexi and Actinobacteria at M1 accounted for more than 89% of the total classified bacterial sequences present but these phyla were present in lesser proportions in M2 and M3 sites. Strains isolated from the sediment of sample M1, having the greatest arsenic concentration (498 mg kg^-1), showed the largest percentages of arsenic oxidation and reduction. Genes aio were more frequently detected in isolates from M1 (54%), whereas arsC genes were present in almost all isolates from all three sediments, suggesting that bacterial communities play an important role in the arsenic biogeochemical cycle and detoxification of arsenical compounds. Overall, results provide further knowledge on the microbial diversity of arsenic contaminated fresh-water sediments.

Genetic identification of arsenate reductase and arsenite oxidase in redox transformations carried out by arsenic metabolising prokaryotes - A comprehensive review

Arsenic (As) contamination in water is a cause of major concern to human population worldwide, especially in Bangladesh and West Bengal, India. Arsenite (As(III)) and arsenate (As(V)) are the two common forms in which arsenic exists in soil and groundwater, the former being more mobile and toxic. A large number of arsenic metabolising microorganisms play a crucial role in microbial transformation of arsenic between its different states, thus playing a key role in remediation of arsenic contaminated water. This review focuses on advances in biochemical, molecular and genomic developments in the field of arsenic metabolising bacteria - covering recent developments in the understanding of structure of arsenate reductase and arsenite oxidase enzymes, their gene and operon structures and their mechanism of action. The genetic and molecular studies of these microbes and their proteins may lead to evolution of successful strategies for effective implementation of bioremediation programs.

Isolation and characterization of arsenic-resistant bacteria and possible application in bioremediation

•

Isolation of two rod-shaped Gram-positive bacteria.

•

Isolates tolerate up to 4500 ppm and 550 ppm concentration of arsenate and arsenite.

•

Bacteria mediated arsenic bioremediation.

Ground water arsenic contamination is a widespread problem in many developing countries including Bangladesh and India. In recent years development of modern innovative technologies for the removal of arsenic from aqueous system has become an interesting topic for research. In this present study, two rod shaped Gram-positive bacteria are being reported, isolated from arsenic affected ground water of Purbasthali block of Burdwan, West Bengal, India, which can tolerate arsenate concentration up to 4500 ppm and 550 ppm of arsenite concentration. From biochemical analysis and 16S rRNA sequencing, they were identified as Bacillus sp. and Aneurinibacillus aneurinilyticus respectively. The isolates SW2 and SW4 can remove 51.45% and 51.99% of arsenite and 53.29% and 50.37% of arsenate, respectively from arsenic containing culture media. Both of the isolate can oxidize arsenite to less toxic arsenate. These two arsenic resistant bacteria can be used as a novel pathway for the bioremediation of arsenic.

Cellular Response of Sinorhizobium sp. Strain A2 during Arsenite Oxidation

Arsenic (As) is a widely distributed toxic element in the environment and microorganisms have developed resistance mechanisms in order to tolerate it. The cellular response of the chemoorganotrophic arsenite (As[III])-oxidizing α-Proteobacteria, Sinorhizobium sp. strain A2, to arsenic was examined in the present study. Several proteins associated with arsenite oxidase and As resistance were shown to be accumulated in the presence of As(III). A shift in central carbon metabolism from the tricarboxylic acid pathway to glyoxylate pathway was also observed in response to oxidative stress. Our results revealed the strategy of the As(III)-oxidizing Sinorhizobium strain to mitigate arsenic toxicity and oxidative damage by multiple metabolic adaptations.

Identification of anaerobic arsenite-oxidizing and arsenate-reducing bacteria associated with an alkaline saline lake in Khovsgol, Mongolia

Microbial arsenic transformation pathways associated with a saline lake located in northern Mongolia were examined using molecular biological and culturing approaches. Bacterial 16S rRNA gene sequences recovered from saline lake sediments and soils were affiliated with haloalkaliphiles, including Bacillus and Halomonas spp. Diverse sequences of arsenate respiratory reductase (arrA) and a new group of arsenite oxidase (arxA) genes were also identified. Pure cultures of arsenate-reducing Nitrincola strain and anaerobic arsenite-oxidizing Halomonas strain were isolated. The chemoorganotrophic Halomonas strain contains arxA gene similar to that of a chemoautotrophic arsenite-oxidizing Alkalilimnicola ehrlichii strain MLHE-1. These results revealed the diversity of arsenic transformation pathways associated with a geographically distinct saline system and the potential contribution of arx-dependent arsenite oxidation by heterotrophic bacteria.

Microbiology of inorganic arsenic: From metabolism to bioremediation

Arsenic (As) contamination of drinking water and soils poses a threat to a large number of people worldwide, especially in Southeast Asia. The predominant forms of As in soils and aquifers are inorganic arsenate [As(V)] and arsenite [As(III)], with the latter being more mobile and toxic. Thus, redox transformations of As are of great importance to predict its fate in the environment, as well as to achieve remediation of As-contaminated water and soils. Although As has been recognized as a toxic element, a wide variety of microorganisms, mainly bacteria, can use it as an electron donor for autotrophic growth or as an electron acceptor for anaerobic respiration. In addition, As detoxification systems in which As is oxidized to the less toxic form or reduced for subsequent excretion are distributed widely in microorganisms. This review describes current development of physiology, biochemistry, and genomics of arsenic-transforming bacteria. Potential application of such bacteria to removal of As from soils and water is also highlighted.

A novel arsenate reductase from the bacterium Thermus thermophilus HB27: its role in arsenic detoxification

Microorganisms living in arsenic-rich geothermal environments act on arsenic with different biochemical strategies, but the molecular mechanisms responsible for the resistance to the harmful effects of the metalloid have only partially been examined. In this study, we investigated the mechanisms of arsenic resistance in the thermophilic bacterium Thermus thermophilus HB27. This strain, originally isolated from a Japanese hot spring, exhibited tolerance to concentrations of arsenate and arsenite up to 20mM and 15mM, respectively; it owns in its genome a putative chromosomal arsenate reductase (TtarsC) gene encoding a protein homologous to the one well characterized from the plasmid pI258 of the Gram+bacterium Staphylococcus aureus. Differently from the majority of microorganisms, TtarsC is part of an operon including genes not related to arsenic resistance; qRT-PCR showed that its expression was four-fold increased when arsenate was added to the growth medium. The gene cloning and expression in Escherichia coli, followed by purification of the recombinant protein, proved that TtArsC was indeed a thioredoxin-coupled arsenate reductase with a kcat/KM value of 1.2×10(4)M(-1)s(-1). It also exhibited weak phosphatase activity with a kcat/KM value of 2.7×10(-4)M(-1)s(-1). The catalytic role of the first cysteine (Cys7) was ascertained by site-directed mutagenesis. These results identify TtArsC as an important component in the arsenic resistance in T. thermophilus giving the first structural-functional characterization of a thermophilic arsenate reductase.

Microbial transformations of arsenic: perspectives for biological removal of arsenic from water

Arsenic is present in many environments and is released by various natural processes and anthropogenic actions. Although arsenic is recognized to cause a wide range of adverse health effects in humans, diverse bacteria can metabolize it by detoxification and energy conservation reactions. This review highlights the current understanding of the ecology, biochemistry and genomics of these bacteria, and their potential application in the treatment of arsenic-polluted water.

Identification of antimony- and arsenic-oxidizing bacteria associated with antimony mine tailing

Antimony (Sb) is a naturally occurring toxic element commonly associated with arsenic (As) in the environment and both elements have similar chemistry and toxicity. Increasing numbers of studies have focused on microbial As transformations, while microbial Sb interactions are still not well understood. To gain insight into microbial roles in the geochemical cycling of Sb and As, soils from Sb mine tailing were examined for the presence of Sb- and As-oxidizing bacteria. After aerobic enrichment culturing with As^III (10 mM) or Sb^III (100 μM), pure cultures of Pseudomonas- and Stenotrophomonas-related isolates with Sb^III oxidation activities and a Sinorhizobium-related isolate capable of As^III oxidation were obtained. The As^III-oxidizing Sinorhizobium isolate possessed the aerobic arsenite oxidase gene (aioA), the expression of which was induced in the presence of As^III or Sb^III. However, no Sb^III oxidation activity was detected from the Sinorhizobium-related isolate, suggesting the involvement of different mechanisms for Sb and As oxidation. These results demonstrate that indigenous microorganisms associated with Sb mine soils are capable of Sb and As oxidation, and potentially contribute to the speciation and mobility of Sb and As in situ.