Bacterial metabolism of environmental arsenic--mechanisms and biotechnological applications

Arsenic Contamination of Groundwater Is Determined by Complex Interactions between Various Chemical and Biological Processes

At a great many locations worldwide, the safety of drinking water is not assured due to pollution with arsenic. Arsenic toxicity is a matter of both systems chemistry and systems biology: it is determined by complex and intertwined networks of chemical reactions in the inanimate environment, in microbes in that environment, and in the human body. We here review what is known about these networks and their interconnections. We then discuss how consideration of the systems aspects of arsenic levels in groundwater may open up new avenues towards the realization of safer drinking water. Along such avenues, both geochemical and microbiological conditions can optimize groundwater microbial ecology vis-à-vis reduced arsenic toxicity.

Biosensing of arsenic by whole-cell bacterial bioreporter immobilized on polycaprolactone (PCL) electrospun fiber

In recent years, heavy metals derived from several anthropogenic sources have both direct and indirect detrimental effects on the health of the environment and living organisms. Whole-cell bioreporters (WCBs) that can be used to monitor the levels of heavy metals in drinking and natural spring waters are important. In this study, whole-cell arsenic bacterial bioreporters were immobilized using polycaprolactone (PCL) electrospun fibers as the support material. The aim is to determine the properties of this immobilized bioreporter system by evaluating its performance in arsenic detection. Within the scope of the study, different growth media and fiber immobilization times were tested to determine the parameters affecting the fluorescent signals emitted by the immobilized bioreporter system in the presence of two dominant forms of arsenic, namely arsenite (As(III)) and arsenate (As(V)). In addition, the sensitivity, selectivity, response time, and shelf-life of the developed bioreporter system were evaluated. As far as the literature is concerned, this is the first study to investigate the potential of using PCL-electrospun fiber-immobilized fluorescent bacterial bioreporter for arsenic detection. This study will open new avenues in environmental arsenic monitoring.

The effects of arsenic on dechlorination of trichloroethene by consortium DH: Microbial response and resistance

Inorganic arsenic and organochlorines are frequently co-occurring contaminants in anoxic groundwater environments, and the bioremediation of their composite pollution has long been a rigorous predicament. Currently, the dechlorination behaviors and stress responses of microbial dechlorination consortia to arsenic are not yet fully understood. This study assessed the reductive dechlorination performance of a Dehalococcoides-bearing microcosm DH under gradient concentrations of arsenate [As(V)] or arsenite [As(III)] and investigated the response patterns of different functional microorganisms. Our results demonstrated that although the dechlorination rates declined with increasing arsenic concentrations in both As(III/V) scenarios, the inhibitory impact was more pronounced in As(III)-amended groups compared to As(V)-amended groups. Moreover, the vinyl chloride (VC)-to-ethene step was more susceptible to arsenic exposure compared to the trichloroethene (TCE)-to-dichloroethane (DCE) step, while high levels of arsenic exposure [e.g. As(III) > 75 μM] can induce significant accumulation of VC. Functional gene variations and microbial community analyses revealed that As(III/V) affected reductive dechlorination by directly inhibiting organohalide-respiring bacteria (OHRB) and indirectly inhibiting synergistic populations such as acetogens. Metagenomic results indicated that arsenic metabolic and efflux mechanisms were identical among different Dhc strains, and variations in arsenic uptake pathways were possibly responsible for their differential responses to arsenic exposures. By comparison, fermentative bacteria showed high potential for arsenic resistance due to their inherent advantages in arsenic detoxification and efflux mechanisms. Collectively, our findings expanded the understanding of the response patterns of different functional populations to arsenic stress in the dechlorinating consortium and provided insights into modifying bioremediation strategies at co-contaminated sites for furtherance.

Unraveling the Role of Metals and Organic Acids in Bacterial Antimicrobial Resistance in the Food Chain

Antimicrobial resistance (AMR) has a significant impact on human, animal, and environmental health, being spread in diverse settings. Antibiotic misuse and overuse in the food chain are widely recognized as primary drivers of antibiotic-resistant bacteria. However, other antimicrobials, such as metals and organic acids, commonly present in agri-food environments (e.g., in feed, biocides, or as long-term pollutants), may also contribute to this global public health problem, although this remains a debatable topic owing to limited data. This review aims to provide insights into the current role of metals (i.e., copper, arsenic, and mercury) and organic acids in the emergence and spread of AMR in the food chain. Based on a thorough literature review, this study adopts a unique integrative approach, analyzing in detail the known antimicrobial mechanisms of metals and organic acids, as well as the molecular adaptive tolerance strategies developed by diverse bacteria to overcome their action. Additionally, the interplay between the tolerance to metals or organic acids and AMR is explored, with particular focus on co-selection events. Through a comprehensive analysis, this review highlights potential silent drivers of AMR within the food chain and the need for further research at molecular and epidemiological levels across different food contexts worldwide.

Structural-functional analysis and molecular characterization of arsenate reductase from Enterobacter cloacae RSC3 for arsenic biotransformation

Enterobacter cloacae RSC3 isolated from an industrial pesticide site transformed arsenate into arsenite. The arsenate is transported by membrane-bound phosphate transporter and transformed to arsenite by arsenate reductase (arsC). E. cloacae RSC3 produced an arsenate reductase enzyme with a maximum activity of 354 U after 72 h of incubation. Arsenate reductase was found to be active and stable at a wide range of temperatures (20 and 45 °C) and pH (5-10), with maximum activity at 35 °C and pH 7.0. The arsenate reductase protein was further characterised molecularly using different bioinformatics tools. The 3D structure of ArsC protein was predicted by homology modelling and validated by the Ramachandran plot with 91.9% residues in the most favoured region. ArsC protein of E. cloacae RSC3 revealed structural homology with ArsC from PDB ID: 1S3C. The gene ontology results also showed that the ArsC protein had a molecular functionality of the arsenate reductase (glutaredoxin) activity and the biological function of cellular response to DNA damage stimulus. Molecular docking analysis of 3D structures using AutoDock vina-1.5.7 server predicted four ligand binding active site residues at Gln70, Asp68, Leu68, and Leu63. Strong ArsC-arsenate ion interaction was observed with binding energy -1.03 kcal/mol, indicating significant arsenate reductase activity and specificity of ArsC protein. On the basis of molecular dynamics simulation analysis, the RMSD and RMSF values revealed the stability of ArsC protein from E. cloacae RSC3.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-023-03730-9.

Role of microbial microbes in arsenic bioaccumulation and biotransformation in mice

Although gut microbes can affect the accumulation and metabolism of arsenic (As), the microbes contributing to these processes remain largely unknown. Therefore, this study aimed to investigate the bioaccumulation and biotransformation of arsenate [As(V)] and arsenobetaine (AsB) in mice with a disordered gut microbiome. We used cefoperazone (Cef) to construct a mouse model of gut microbiome disruption along with 16S rRNA sequencing to elucidate the effect of gut microbiome destruction on the biotransformation and bioaccumulation of As(V) and AsB. This revealed the role of specific bacteria in As metabolism. Gut microbiome destruction increased the bioaccumulation of As(V) and AsB in various organs and reduced the excretion of As(V) and AsB in the feces. Further, gut microbiome destruction was found to be important for the biotransformation of As(V). Interference with Cef can significantly decrease Blautia and Lactobacillus while increasing Enterococcus, leading to increase As accumulation in mice and enhanced methylation. We also identified Lachnoclostridium, Erysipelatoclostridium, Blautia, Lactobacillus, and Enterococcus as biomarkers involved in As bioaccumulation and biotransformation. In conclusion, specific microbes can increase As accumulation in the host, exacerbating its potential health risks.

Indigenous cyanobacteria enhances remediation of arsenic-contaminated soils by regulating physicochemical properties, microbial community structure and function in soil microenvironment

Biocrust was widely used for the immobilization and removal of arsenic (As) in drainage systems of rice fields and mining areas. In this study, the role of an indigenous cyanobacteria (Leptolyngbya sp. XZMQ) was explored in the bioremediation of As-contaminated farmland and tailing soil. After 80 d of inoculation with cyanobacteria, total As (As(T)) accumulated in the cyanobacterial crust of farmland and tailing soil was 279.89 mg kg^-1 and 269.57 mg kg^-1, respectively, and non-EDTA exchangeable fraction was the major fraction of it. The As(T) in farmland and tailing soil of micro-environment decreased by 10.76% and 12.73%, respectively. Meanwhile, the available As (As(a)) decreased by 21.25% and 27.65%, respectively. The XRD results showed that hematite and SiO₂ existed in cyanobacterial crust of farmland and tailing soil. FTIR spectra indicated that the adsorption of As in cyanobacterial crust was mediated by OH and CO. After inoculation of Leptolyngbya sp. XZMQ, in subcrust soil, As biotransformation gene aioA was the most abundant, followed by arsM. The dominant phyla of soil biota were Proteobacteria, Cyanobacteria, Actinobacteria, and Bacteroiota, which could play critical roles in shaping aioA and arsM harboring microbe communities in soil. Redundancy analysis (RDA) showed that soil organic carbon (OC), pH, and chlorophyll a (Chl a) were the most important environmental factors in altering soil bacterial communities. Correlation analysis showed the Leptolyngbya had a positive correlation with Chl a, effective nitrogen (N(a)), electrical conductivity (EC), OC, pH in the soil, respectively, while it had a significant negative correlation with As(a), As(III) and As(T). These results emphasized on the significance of cyanobacteria in the behavior of As in mine soils and offered a promising strategy for bioremediation of As-contaminated soil in the mining area.

Genome-wide signatures of adaptation to extreme environments in red algae

The high temperature, acidity, and heavy metal-rich environments associated with hot springs have a major impact on biological processes in resident cells. One group of photosynthetic eukaryotes, the Cyanidiophyceae (Rhodophyta), has successfully thrived in hot springs and associated sites worldwide for more than 1 billion years. Here, we analyze chromosome-level assemblies from three representative Cyanidiophyceae species to study environmental adaptation at the genomic level. We find that subtelomeric gene duplication of functional genes and loss of canonical eukaryotic traits played a major role in environmental adaptation, in addition to horizontal gene transfer events. Shared responses to environmental stress exist in Cyanidiales and Galdieriales, however, most of the adaptive genes (e.g., for arsenic detoxification) evolved independently in these lineages. Our results underline the power of local selection to shape eukaryotic genomes that may face vastly different stresses in adjacent, extreme microhabitats.

The fecal arsenic excretion, tissue arsenic accumulation, and metabolomics analysis in sub-chronic arsenic-exposed mice after in situ arsenic-induced fecal microbiota transplantation

Arsenic can be specifically enriched by rice, and the health hazards caused by high arsenic rice are gradually attracting attention. This study aimed to explore the potential of microbial detoxification via gut microbiome in the treatment of sub-chronic arsenic poisoning. We first exposed mice to high-dose arsenic feed (30 mg/kg, rice arsenic composition) for 60 days to promote arsenic-induced microbes in situ in the gastrointestinal tract, then transplanted their fecal microbiota (FMT) into another batch of healthy recipient mice, and dynamically monitored the microbial colonization by 16S rRNA sequencing and ITS sequencing. The results showed that in situ arsenic-induced fecal microbiome can stably colonized and interact with indigenous microbes in the recipient mice in two weeks, and established a more stable network of gut microbiome. Then, the recipient mice continued to receive high-dose arsenic exposure for 52 days. After above sub-chronic arsenic exposure, compared with the non-FMT group, fecal arsenic excretion, liver and plasma arsenic accumulation were significantly lower (P < 0.05), and that in kidney, hair, and thighbone present no significant differences. Metabolomics of feces- plasma-brain axis were also disturbed, some up-regulated metabolites in feces, plasma, and cerebral cortex may play positive roles for the host. Therefore, microbial detoxification has potential in the treatment of sub-chronic arsenic poisoning. However, gut flora is an extremely complex community with different microorganisms have different arsenic metabolizing abilities, and various microbial metabolites. Coupled with the matrix effects, these factors will have various effects on the efflux and accumulation of arsenic. The definite effects (detoxification or non-detoxification) could be not assured based on the current study, and more systematic and rigorous studies are needed in the future.

As(III) oxidation and kinetic analysis by Herminiimonas arsenicoxydans-loaded electrospinning activated carbon fiber biofilms

In this study, an integrated and assembled recyclable biofilm material was prepared by loading Herminiimonas arsenicoxydans (H. arsenicoxydans) onto electrospun biomass-activated carbon nanofibers (denoted as H. arsenicoxydans-BACFs films). The H. arsenicoxydans-BACFs biofilms showed an approximately 50% increase in As(III) removal rate for 50 mg/L during a 48-h incubation. Furthermore, the biofilms demonstrated satisfactory biocompatibility, ideal catalytic As(III) oxidation and excellent recyclability in cyclic reactions (at least 5 runs). The improved catalytic efficiency is mainly due to a large amount of biomass accumulation and biofilms formation on the surface of the BACF films. More important, the BACF films as an electron transport medium from an oxidized state to a reduced state promote the electron transfer of As(III) oxidation of H. arsenicoxydans. The dual factors can synergistically promote As(III) oxidation efficiency. The oxidation process of As(III) in the H. arsenicoxydans-BACFs composite biofilm reactor was more in line with the first-order kinetic equation, and the oxidation rate of As(III) by H. arsenicoxydans-BACF_0.4 was the fastest. The H. arsenicoxydans-BACF films outperformed conventional catalytic materials and could represent biomaterials for the remediation of As(III)-contaminated wastewater.

Towards Understanding Factors Affecting Arsenic, Chromium, and Vanadium Mobility in the Subsurface

Arsenic (As), chromium (Cr), and vanadium (V) are naturally occurring, redox-active elements that can become human health hazards when they are released from aquifer substrates into groundwater that may be used as domestic or irrigation source. As such, there is a need to develop incisive conceptual and quantitative models of the geochemistry and transport of potentially hazardous elements to assess risk and facilitate interventions. However, understanding the complexity and heterogeneous subsurface environment requires knowledge of solid-phase minerals, hydrologic movement, aerobic and anaerobic environments, microbial interactions, and complicated chemical kinetics. Here, we examine the relevant geochemical and hydrological information about the release and transport of potentially hazardous geogenic contaminants, specifically As, Cr, and V, as well as the potential challenges in developing a robust understanding of their behavior in the subsurface. We explore the development of geochemical models, illustrate how they can be utilized, and describe the gaps in knowledge that exist in translating subsurface conditions into numerical models, as well as provide an outlook on future research needs and developments.

Dissimilatory and Cytoplasmic Antimonate Reductions in a Hydrogen-Based Membrane Biofilm Reactor

A hydrogen-based membrane biofilm reactor (H₂-MBfR) was operated to investigate the bioreduction of antimonate [Sb(V)] in terms of Sb(V) removal, the fate of Sb, and the pathways of reduction metabolism. The MBfR achieved up to 80% Sb(V) removal and an Sb(V) removal flux of 0.55 g/m²·day. Sb(V) was reduced to Sb(III), which mainly formed Sb₂O₃ precipitates in the biofilm matrix, although some Sb(III) was retained intracellularly. High Sb(V) loading caused stress that deteriorated performance that was not recovered when the high Sb(V) loading was removed. The biofilm community consisted of DSbRB (dissimilatory Sb-reduction bacteria), SbRB (Sb-resistant bacteria), and DIRB (dissimilatory iron-reducing bacteria). Dissimilatory antimonate reduction, mediated by the respiratory arsenate reductase ArrAB, was the main reduction route, but respiratory reduction coexisted with cytoplasmic Sb(V)-reduction mediated by arsenate reductase ArsC. Increasing Sb(V) loading caused stress that led to increases in the expression of arsC gene and intracellular accumulation of Sb(III). By illuminating the roles of the dissimilatory and cytoplasmic Sb(V) reduction mechanism in the biofilms of the H₂-MBfR, this study reveals that the Sb(V) loading should be controlled to avoid stress that deteriorates Sb(V) reduction.

Arsenic Metabolism, Toxicity and Accumulation in the White Button Mushroom Agaricus bisporus

Mushrooms have unique properties in arsenic metabolism. In many commercial and wild-grown mushrooms, arsenobetaine (AsB), a non-toxic arsenical, was found as the dominant arsenic species. The AsB biosynthesis remains unknown, so we designed experiments to study conditions for AsB formation in the white button mushroom, Agaricus bisporus. The mushrooms were treated with various arsenic species including arsenite (As(III)), arsenate (As(V)), methylarsenate (MAs(V)), dimethylarsenate (DMAs(V)) and trimethylarsine oxide (TMAsO), and their accumulation and metabolism were determined using inductively coupled mass spectrometer (ICP-MS) and high-pressure liquid chromatography coupled with ICP-MS (HPLC-ICP-MS), respectively. Our results showed that mycelia have a higher accumulation for inorganic arsenicals while fruiting bodies showed higher accumulation for methylated arsenic species. Two major arsenic metabolites were produced in fruiting bodies: DMAs(V) and AsB. Among tested arsenicals, only MAs(V) was methylated to DMAs(V). Surprisingly, AsB was only detected as the major arsenic product when TMAsO was supplied. Additionally, AsB was only detected in the fruiting body, but not mycelium, suggesting that methylated products were transported to the fruiting body for arsenobetaine formation. Overall, our results support that methylation and AsB formation are two connected pathways where trimethylated arsenic is the optimal precursor for AsB formation.

Heavy metals distribution and ecological risk assessment including arsenic resistant PGPR in tidal mangrove ecosystem

Heavy metals (HM) are the major proximate drivers of pollution in the mangrove ecosystem. Therefore, ecological risk (ER) due to HM distribution/concentration in core-sediment of Puzi mangrove region (Taiwan) was examined with tidal influence (TI) along with indigenous rhizospheric bacteria (IRB). The HM concentration was observed higher at active-tidal-sediment compared to partially-active-sediment. Geo-accumulation index (I_geo) and contamination factor (CF) indicated the tidal-sediment was highly contaminated with arsenic (As) and moderately contaminated with Lead (Pb) and Zinc (Zn). However, the pollution loading index (PLI) and degree of contamination (C_d) exhibited 'no pollution' and 'low-moderate degree of contamination', in the studied region respectively. The isolated IRB (Priestia megaterium, Bacillus safenis, Bacillus aerius, Bacillus subtilis, Bacillus velenzenesis, Bacillus lichenoformis, Kocuria palustris, Enterobacter hormaechei, Pseudomonus fulva, and Paenibacillus favisporus; accession number OM979069-OM979078) exhibited the arsenic resistant behavior with plant-growth-promoting characters (IAA, NH₃, and P-solubilization), which can be used in mangrove reforestation and bioremediation of HM.

Sb(III) resistance mechanism and oxidation characteristics of Klebsiella aerogenes X

Resistant bacteria are potential natural materials for the bioremediation of soil metalloid pollution. A strain isolated from farmland soil chronically exposed to Sb was identified as K. aerogenes X with high antimonite [Sb(III)] tolerance and oxidation ability. The resistance mechanism of K. aerogenes X and its extracellular polymeric substances (EPS), antioxidant enzymes, and oxidation characteristics in Sb(III) stress were investigated in this study by stress incubation experiments and FTIR. The biotoxicity of Sb was limited by the binding of the organic compounds in EPS, and the anionic functional groups (e.g., amino, carboxyl and hydroxyl groups, etc.) present in the cell envelope were the components primarily responsible for the metalloid-binding capability of K. aerogenes X. The K. aerogenes X can oxidize Sb(III), and its metabolites induce changes in reactive oxygen species (ROS), catalase (CAT), total superoxide dismutase (SOD) and glutathione s-transferase (GSH-S) activity, indicating that the resistance mechanisms of K. aerogenes X are mediated by oxidative stress, EPS restriction and cell damage. Oxidation of Sb(III) is driven by interactions in intracellular oxidation, cell electron transport, extracellular metabolism including proteins and low molecular weight components (LMWs). LMWs (molecular weight <3 kDa) are the main driving factor of Sb(III) oxidation. In addition, Sb resistance genes arsA, arsB, arsC, arsD and acr3 and potential oxidation gene arsH were identified in K. aerogenes X. Owing to its natural origin, high tolerance and oxidation ability, K. aerogenes X could serve as a potential bioremediation material for the mitigation of Sb(III) in contaminated areas.

Significance of Shewanella Species for the Phytoavailability and Toxicity of Arsenic-A Review

Simple Summary

The availability of some toxic heavy metals, such as arsenic (As), is related to increased human and natural activities. This type of metal availability in the environment is associated with various health and environmental issues. Such problems may arise due to direct contact with or consumption of plant products containing this metal in some of their parts. A microbial approach that employs a group of bacteria (Shewanella species) is proposed to reduce the negative consequences of the availability of this metal (As) in the environment. This innovative strategy can reduce As mobility, its spread, and uptake by plants in the environment. The benefits of this approach include its low cost and the possibility of not exposing other components of the environment to unfavourable consequences.

Abstract

The distribution of arsenic continues due to natural and anthropogenic activities, with varying degrees of impact on plants, animals, and the entire ecosystem. Interactions between iron (Fe) oxides, bacteria, and arsenic are significantly linked to changes in the mobility, toxicity, and availability of arsenic species in aquatic and terrestrial habitats. As a result of these changes, toxic As species become available, posing a range of threats to the entire ecosystem. This review elaborates on arsenic toxicity, the mechanisms of its bioavailability, and selected remediation strategies. The article further describes how the detoxification and methylation mechanisms used by Shewanella species could serve as a potential tool for decreasing phytoavailable As and lessening its contamination in the environment. If taken into account, this approach will provide a globally sustainable and cost-effective strategy for As remediation and more information to the literature on the unique role of this bacterial species in As remediation as opposed to conventional perception of its role as a mobiliser of As.

Bacterial Arsenic Metabolism and Its Role in Arsenic Bioremediation

Arsenic contaminations, often adversely influencing the living organisms, including plants, animals, and the microbial communities, are of grave apprehension. Many physical, chemical, and biological techniques are now being explored to minimize the adverse affects of arsenic toxicity. Bioremediation of arsenic species using arsenic loving bacteria has drawn much attention. Arsenate and arsenite are mostly uptaken by bacteria through aquaglycoporins and phosphate transporters. After entering arsenic inside bacterial cell arsenic get metabolized (e.g., reduction, oxidation, methylation, etc.) into different forms. Arsenite is sequentially methylated into monomethyl arsenic acid (MMA) and dimethyl arsenic acid (DMA), followed by a transformation of less toxic, volatile trimethyl arsenic acid (TMA). Passive remediation techniques, including adsorption, biomineralization, bioaccumulation, bioleaching, and so on are exploited by bacteria. Rhizospheric bacterial association with some specific plants enhances phytoextraction process. Arsenic-resistant rhizospheric bacteria have immense role in enhancement of crop plant growth and development, but their applications are not well studied till date. Emerging techniques like phytosuction separation (PS-S) have a promising future, but still light to be focused on these techniques. Plant-associated bioremediation processes like phytoextraction and phytosuction separation (PS-S) techniques might be modified by treating with potent bacteria for furtherance.

Isolation and molecular identification of native As-resistant bacteria: As(III) and As(V) removal capacity and possible mechanism of detoxification

The study of arsenic (As)-resistant microorganisms with high As removal capacity is fundamental for the development of economically sustainable technologies used for the treatment of water contaminated with metalloid. In the current study, four bacterial strains were isolated from As-contaminated water samples of the Xichu region, Mexico. Based on 16S rRNA gene sequencing and phylogenetic analysis of the isolated strains, Rhodococcus gordoniae, Microbacterium hydrocarbonoxydans, Exiguobacterium indicum, and Pseudomonas kribbensis were identified as potential As removal strains. R. gordoniae shows the highest growth capacity in both As(III) and As(V). R. gordoniae, M. hydrocarbonoxydans, and E. indicum removed approximately 81.6, 79.9, and 61.7% of As(III), as well as 77.2, 68.9, and 74.8% of As(V), respectively. P. kribbensis removed only about 80.2% of As(V). This study contributes to the possible detoxification mechanisms employed by these bacteria. Such insight could be crucial in the successful implementation of in situ bioremediation programs using these little-known bacteria.

Water and soil contaminated by arsenic: the use of microorganisms and plants in bioremediation

Owing to their roles in the arsenic (As) biogeochemical cycle, microorganisms and plants offer significant potential for developing innovative biotechnological applications able to remediate As pollutions. This possible use in bioremediation processes and phytomanagement is based on their ability to catalyse various biotransformation reactions leading to, e.g. the precipitation, dissolution, and sequestration of As, stabilisation in the root zone and shoot As removal. On the one hand, genomic studies of microorganisms and their communities are useful in understanding their metabolic activities and their interaction with As. On the other hand, our knowledge of molecular mechanisms and fate of As in plants has been improved by laboratory and field experiments. Such studies pave new avenues for developing environmentally friendly bioprocessing options targeting As, which worldwide represents a major risk to many ecosystems and human health.

Simulations to Aid in the Design of Microbes for Synthesis of Metallic Nanomaterials

Microbes are champions of nanomaterial synthesis. By virtue of their incredible native range─from thermal vents to radioactive soil─microbes evolved tools to thrive on inorganic material, and, in their normal course of living, forge nanomaterials. In recent decades, synthetic biologists have engineered a vast array of functional nanomaterials using genetic tools that control the natural ability of bacteria to perform complex redox chemistry, maintain steep chemical gradients, and express biomolecular scaffolds. Leveraging microbial biology can lead to intricate nanomaterial architectures whose design and assembly exists beyond the ken of inorganic methods. Theories enumerating microbial nanomaterial synthesis are spare, however, despite the advantage they could offer. Here, we describe a theoretical approach to simulating biogenic nanomaterial synthesis that incorporates key features and parameters of Gram-negative bacteria. By adapting previously verified inorganic theories of nanoparticle synthesis, we recapitulate past biogenic experiments, such as the ability to localize nanoparticle synthesis or regulate nucleation of specific nanomaterials. Moreover, the simulation offers direction in the design of future experiments. Our results demonstrate the promise of marrying experimental and theoretical approaches to microbial nanomaterial synthesis.

Arsenic contamination, impact and mitigation strategies in rice agro-environment: An inclusive insight

Arsenic (As) contamination and its adverse consequences on rice agroecosystem are well known. Rice has the credit to feed more than 50% of the world population but concurrently, rice accumulates a substantial amount of As, thereby compromising food security. The gravity of the situation lays in the fact that the population in theAs uncontaminated areas may be accidentally exposed to toxic levels of As from rice consumption. In this review, we are trying to summarize the documents on the impact of As contamination and phytotoxicity in past two decades. The unique feature of this attempt is wide spectrum coverages of topics, and that makes it truly an interdisciplinary review. Aprat from the behaviour of As in rice field soil, we have documented the cellular and molecular response of rice plant upon exposure to As. The potential of various mitigation strategies with particular emphasis on using biochar, seed priming technology, irrigation management, transgenic variety development and other agronomic methods have been critically explored. The review attempts to give a comprehensive and multidiciplinary insight into the behaviour of As in Paddy -Water - Soil - Plate prospective from molecular to post-harvest phase. From the comprehensive literature review, we may conclude that considerable emphasis on rice grain, nutritional and anti-nutritional components, and grain quality traits under arsenic stress condition is yet to be given. Besides these, some emerging mitigation options like seed priming technology, adoption of nanotechnological strategies, applications of biochar should be fortified in large scale without interfering with the proper use of biodiversity.

Interactions with Arsenic: Mechanisms of Toxicity and Cellular Resistance in Eukaryotic Microorganisms

Arsenic (As) is quite an abundant metalloid, with ancient origin and ubiquitous distribution, which represents a severe environmental risk and a global problem for public health. Microbial exposure to As compounds in the environment has happened since the beginning of time. Selective pressure has induced the evolution of various genetic systems conferring useful capacities in many microorganisms to detoxify and even use arsenic, as an energy source. This review summarizes the microbial impact of the As biogeochemical cycle. Moreover, the poorly known adverse effects of this element on eukaryotic microbes, as well as the As uptake and detoxification mechanisms developed by yeast and protists, are discussed. Finally, an outlook of As microbial remediation makes evident the knowledge gaps and the necessity of new approaches to mitigate this environmental challenge.

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.

Pollutant Degrading Enzyme: Catalytic Mechanisms and Their Expanded Applications

The treatment of environmental pollution by microorganisms and their enzymes is an innovative and socially acceptable alternative to traditional remediation approaches. Microbial biodegradation is often characterized with high efficiency as this process is catalyzed via degrading enzymes. Various naturally isolated microorganisms were demonstrated to have considerable ability to mitigate many environmental pollutants without external intervention. However, only a small fraction of these strains are studied in detail to reveal the mechanisms at the enzyme level, which strictly limited the enhancement of the degradation efficiency. Accordingly, this review will comprehensively summarize the function of various degrading enzymes with an emphasis on catalytic mechanisms. We also inspect the expanded applications of these pollutant-degrading enzymes in industrial processes. An in-depth understanding of the catalytic mechanism of enzymes will be beneficial for exploring and exploiting more degrading enzyme resources and thus ameliorate concerns associated with the ineffective biodegradation of recalcitrant and xenobiotic contaminants with the help of gene-editing technology and synthetic biology.

Differential protein expression during growth on model and commercial mixtures of naphthenic acids in Pseudomonas fluorescens Pf-5

Naphthenic acids (NAs) are carboxylic acids with the formula (Cn H2n+Z O2 ) and are among the most toxic, persistent constituents of oil sands process-affected waters (OSPW), produced during oil sands extraction. Currently, the proteins and mechanisms involved in NA biodegradation are unknown. Using LC-MS/MS shotgun proteomics, we identified proteins overexpressed during the growth of Pseudomonas fluorescens Pf-5 on a model NA (4'-n-butylphenyl)-4-butanoic acid (n-BPBA) and commercial NA mixture (Acros). By day 11, >95% of n-BPBA was degraded. With Acros, a 17% reduction in intensity occurred with 10-18 carbon compounds of the Z family -2 to -14 (major NA species in this mixture). A total of 554 proteins (n-BPBA) and 631 proteins (Acros) were overexpressed during growth on NAs, including several transporters (e.g., ABC transporters), suggesting a cellular protective response from NA toxicity. Several proteins associated with fatty acid, lipid, and amino acid metabolism were also overexpressed, including acyl-CoA dehydrogenase and acyl-CoA thioesterase II, which catalyze part of the fatty acid beta-oxidation pathway. Indeed, multiple enzymes involved in the fatty acid oxidation pathway were upregulated. Given the presumed structural similarity between alkyl-carboxylic acid side chains and fatty acids, we postulate that P. fluorescens Pf-5 was using existing fatty acid catabolic pathways (among others) during NA degradation.

Unraveling the role of plant growth-promoting rhizobacteria in the alleviation of arsenic phytotoxicity: A review

The toxic metalloid arsenic (As), is a major pollutant of soil and water, imposing severe health concerns on human lives. It enters the food chain mainly through As-contaminated crops. The uptake, translocation and accumulation of As in plant tissue are often controlled by certain soil-inhabiting microbial communities. Among them, indigenous, free-living As-resistant plant growth-promoting rhizobacteria (PGPR) plays a pivotal role in As-immobilization. Besides, the plant's inability to withstand As after a threshold level is actively managed by these PGPR increasing As-tolerance in host plants by a synergistic plant-microbe interaction. The dual functionality of As-resistant PGPR i.e., phytostimulation and minimization of As-induced phytotoxic damages are one of the main focal points of this review article. It is known that such PGPR having the functional arsenic-resistant genes (in ars operon) including As-transporters, As-transforming genes contributed to the As accumulation and detoxification/transformation respectively. Apart from assisting in nutrient acquisition and modulating phytohormone levels, As-resistant PGPR also influences the antioxidative defense system in plants by maneuvering multiple enzymatic and non-enzymatic antioxidants. Furthermore, they are effective in reducing membrane damage and electrolyte leakage in plant cells. As-induced photosynthetic damage is also found to be salvaged by As-resistant PGPR. Briefly, the eco-physiological, biochemical and molecular mechanisms of As-resistant PGPR are thus elaborated here with regard to the As-exposed crops.

Antibiotic exposure decreases soil arsenic oral bioavailability in mice by disrupting ileal microbiota and metabolic profile

Oral bioavailability of arsenic (As) determines levels of As exposure via ingestion of As-contaminated soil, however, the role of gut microbiota in As bioavailability has not evaluated in vivo although some in vitro studies have investigated this. Here, we made a comparison in As relative bioavailability (RBA) estimates for a contaminated soil (3913 mg As kg^-1) using a mouse model with and without penicillin perturbing gut microbiota and metabolites. Compared to soil exposure alone (2% w/w soil in diets), addition of penicillin (100 or 1000 mg kg^-1) reduced probiotic Lactobacillus and sulfate-reducing bacteria Desulfovibrio, enriched penicillin-resistant Enterobacter and Bacteroides, and decreased amino acid concentrations in ileum. With perturbed gut microbiota and metabolic profile, penicillin and soil co-exposed mice accumulated 2.81-3.81-fold less As in kidneys, excreted 1.02-1.35-fold less As in urine, and showed lower As-RBA (25.7-29.0%) compared to mice receiving diets amended with soil alone (56 ± 9.63%). One mechanism accounted for this is the decreased concentrations of amino acids arising from the gut microbiota shift which resulted in elevated iron (Fe) and As co-precipitation, leading to reduced As solubilization in the intestine. Another mechanism was conversion of bioavailable inorganic As to less bioavailable monomethylarsonic acid (MMA^V) and dimethylarsinic acid (DMA^V) by the antibiotic perturbed microflora. Based on in vivo mouse model, we demonstrated the important role of gut microbiota and gut metabolites in participating soil As solubilization and speciation transformation then affecting As oral bioavailability. Results are useful to better understand the role of gut bacteria in affecting As metabolism and the health risks of As-contaminated soils.

Biological characterization of Bacillus flexus strain SSAI1 transforming highly toxic arsenite to less toxic arsenate mediated by periplasmic arsenite oxidase enzyme encoded by aioAB genes

Bacillus flexus strain SSAI1 isolated from agro-industry waste, Tuem, Goa, India displayed high arsenite resistance as minimal inhibitory concentration was 25 mM in mineral salts medium. This bacterial strain exposed to 10 mM arsenite demonstrated rapid arsenite oxidation and internalization of 7 mM arsenate within 24 h. The Fourier transformed infrared (FTIR) spectroscopy of cells exposed to arsenite revealed important functional groups on the cell surface interacting with arsenite. Furthermore, scanning electron microscopy combined with electron dispersive X-ray spectroscopy (SEM-EDAX) of cells exposed to arsenite revealed clumping of cells with no surface adsorption of arsenite. Transmission electron microscopy coupled with electron dispersive X-ray spectroscopic (TEM-EDAX) analysis of arsenite exposed cells clearly demonstrated ultra-structural changes and intracellular accumulation of arsenic. Whole-genome sequence analysis of this bacterial strain interestingly revealed the presence of large number of metal(loid) resistance genes, including aioAB genes encoding arsenite oxidase responsible for the oxidation of highly toxic arsenite to less toxic arsenate. Enzyme assay further confirmed that arsenite oxidase is a periplasmic enzyme. The genome of strain SSAI1 also carried glpF, aioS and aioE genes conferring resistance to arsenite. Therefore, multi-metal(loid) resistant arsenite oxidizing Bacillus flexus strain SSAI1 has potential to bioremediate arsenite contaminated environmental sites and is the first report of its kind.

Introducing the ArsR-Regulated Arsenic Stimulon

The microbial ars operon encodes the primary bacterial defense response to the environmental toxicant, arsenic. An important component of this operon is the arsR gene, which encodes ArsR, a member of the family of proteins categorized as DNA-binding transcriptional repressors. As currently documented, ArsR regulates its own expression as well as other genes in the same ars operon. This study examined the roles of four ArsR proteins in the well-developed model Gram-negative bacterium Agrobacterium tumefaciens 5A. RNASeq was used to compare and characterize gene expression profiles in ± arsenite-treated cells of the wild-type strain and in four different arsR mutants. We report that ArsR-controlled transcription regulation is truly global, extending well beyond the current ars operon model, and includes both repression as well as apparent activation effects. Many cellular functions are significantly influenced, including arsenic resistance, phosphate acquisition/metabolism, sugar transport, chemotaxis, copper tolerance, iron homeostasis, and many others. While there is evidence of some regulatory overlap, each ArsR exhibits its own regulatory profile. Furthermore, evidence of a regulatory hierarchy was observed; i.e. ArsR1 represses arsR4, ArsR4 activates arsR2, and ArsR2 represses arsR3. Additionally and unexpectedly, aioB (arsenite oxidase small subunit) expression was shown to be under partial positive control by ArsR2 and ArsR4. Summarizing, this study demonstrates the regulatory portfolio of arsenite-activated ArsR proteins and includes essentially all major cellular functions. The broad bandwidth of arsenic effects on microbial metabolism assists in explaining and understanding the full impact of arsenic in natural ecosystems, including the mammalian gut.

Organoarsenic feed additives in biological wastewater treatment processes: Removal, biotransformation, and associated impacts

Aromatic organoarsenicals are widely used in animal feeding operations and cause arsenic contamination on livestock wastewater and manure, thereby raising the risk of surface water pollution. Biological wastewater treatment processes are often used for livestock wastewater treatment. Organoarsenic removal and biotransformation under aerobic and anaerobic conditions, and the associated impacts have received extensive attention due to the potential threat to water security. The removal efficiency and biotransformation of organoarsenicals in biological treatment processes are reviewed. The underlying mechanisms are discussed in terms of functional microorganisms and genes. The impacts associated with organoarsenicals and their degradation products on microbial activity and performance of bioreactors are also documented. Based on the current research advancement, knowledge gaps and potential research in this field are discussed. Overall, this work delivers a comprehensive understanding on organoarsenic behaviors in biological wastewater treatment processes, and provides valuable information on the control of arsenic contamination from the degradation of organoarsenicals in biological wastewater treatment processes.

Advanced application of nano-technological and biological processes as well as mitigation options for arsenic removal

Arsenic (As) removal is a huge challenge, since several million people are potentially exposed (>10 μg/L World Health Organization guideline limit) through As contaminated drinking water worldwide. Review attempts to address the present situation of As removal, considering key topics on nano-technological and biological process and current progress and future perspectives of possible mitigation options have been evaluated. Different physical, chemical and biological methods are available to remove As from contaminated water/soil/wastes, where removal efficiency mainly depends on absorbent type, initial adsorbate concentration, speciation and interfering species. Oxidation is an important pretreatment step in As removal, which is generally achieved by several media such as O₂/O₃, HClO, KMnO₄ and H₂O₂. The Fe-based-nanomaterials (α/β/γ-FeOOH, Fe₂O₃/Fe₃O₄-γ-Fe₂O₃), Fe-based-composite-compounds, activated-Al₂O₃, HFO, Fe-Al₂O₃, Fe₂O₃-impregnated-graphene-aerogel, iron-doped-TiO₂, aerogel-based- CeTiO₂, and iron-oxide-coated-manganese are effective to remove As from contaminated water. Biological processes (phytoremediation/microbiological) are effective and ecofriendly for As removal from water and/or soil environment. Microorganisms remove As from water, sediments and soil by metabolism, detoxification, oxidation-reduction, bio-adsorption, bio-precipitation, and volatilization processes. Ecofriendly As mitigation options can be achieved by utilizing an alternative As-safe-aquifer, surface-water or rainwater-harvesting. Application of hybrid (biological with chemical and physical process) and Best-Available-Technologies (BAT) can be the most effective As removal strategy to remediate As contaminated environments.

Arsenic removal by iron-oxidizing bacteria in a fixed-bed coconut husk column: Experimental study and numerical modeling

Groundwater in several parts of the world, particularly in developing countries, has been contaminated with Arsenic (As). In search of low-cost As removal methods, the biological oxidation of As(III) and Fe(II) followed by co-precipitation requires detailed investigation for the practical implementation of this technology. The present study investigated the biological oxidation of As(III) and Fe(II) through a combination of laboratory experiments and reactive transport modeling. Batch experiments were conducted to evaluate the As(III) oxidation by Fe-oxidizing bacteria, mainly Leptothrix spp. A fixed-bed down-flow biological column containing inexpensive and readily available coconut husk support media was used to evaluate the combined removal of As(III) and Fe(II) from synthetic groundwater. Oxidation and co-precipitation processes effectively reduced the concentration of As(III) from 500 μg/L to < 10 μg/L with a hydraulic retention time of 120 min. A one-dimensional reactive transport model was developed based on the microbially mediated biochemical reactions of As(III) and Fe(II). The model successfully reproduced the observed As(III) and Fe(II) removal trends in the column experiments. The modeling results showed that the top 20 cm aerobic layer of the column played a primary role in the microbial oxidation of Fe(II) and As(III). The model calibration identified the hydraulic residence time as the most significant process parameter for the removal of Fe and As in the column. The developed model can effectively predict As concentrations in the effluent and provide design guidelines for the biological treatment of As. The model would also be useful for understanding the biogeochemical behavior of Fe and As under aerobic conditions.

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.

Isolation and characterization of arsenic-binding siderophores from Rhodococcus erythropolis S43: role of heterobactin B and other heterobactin variants

Rhodococcus erythropolis S43 is an arsenic-tolerant actinobacterium isolated from an arsenic contaminated soil. It has been shown to produce siderophores when exposed to iron-depleting conditions. In this work, strain S43 was shown to have the putative heterobactin production cluster htbABCDEFGHIJ(K). To induce siderophore production, the strain was cultured in iron-depleted medium in presence and absence of sodium arsenite. The metabolites produced by S43 in the colorimetric CAS and As-_mCAS assays, respectively, showed iron- and arsenic-binding properties reaching a chelating activity equivalent to 1.6 mM of desferroxamine B in the supernatant of the culture without arsenite. By solid-phase extraction and two subsequent HPLC separations from both cultures, several fractions were obtained, which contained CAS and As-_mCAS activity and which were submitted to LC-MS analyses including fragmentation of the major peaks. The mixed-type siderophore heterobactin B occurred in all analyzed fractions, and the mass of the "Carrano heterobactin A" was detected as well. In addition, generation of a molecular network based on fragment spectra revealed the occurrence of several other compounds with heterobactin-like structures, among them a heterobactin B variant with an additional CH₂O moiety. ¹H NMR analyses obtained for preparations from the first HPLC step showed signals of heterobactin B and of "Carrano heterobactin A" with different relative amounts in all three samples. In summary, our results reveal that in R. erythropolis S43, a pool of heterobactin variants is responsible for the iron- and arsenic-binding activities. KEY POINTS: • Several heterobactin variants are the arsenic-binding compounds in Rhodococcus erythropolis S43. • Heterobactin B and the compound designated heterobactin A by Carrano are of importance. • In addition, other heterobactins with ornithine in the backbone exist, e.g., the new heterobactin C.

Microbiome-environment interactions in antimony-contaminated rice paddies and the correlation of core microbiome with arsenic and antimony contamination

Mining activities of antimony (Sb) and arsenic (As) typically result in severe environmental contamination. These contaminants accumulate in rice and thus threaten the health of local residents, who consume Sb- and As-enriched rice grains. Microorganisms play a critical role in the transformation and transportation of Sb and As in paddy soil. Thus, an understanding of the microbiology of contaminated sites would promote the production of safe agricultural products. In this study, six Sb- and As-contaminated rice fields near an active Sb-mining area were investigated. The Sb and As concentrations of all samples were elevated compared to the background level in China. Nitrate, total As, total Sb, and Fe(III) were the major determinants of the microbial community structure. Seven bacterial taxa (i.e. Bradyrhizobium, Bryobacter, Candidatus Solibacter, Geobacter, Gemmatimonas, Halingium, and Sphingomonas) were identified as the core microbiome. These taxa were strongly correlated with the As and Sb contaminant fractions and likely to metabolize As and Sb. Results imply that many soil microbes can survival in the Sb/As contaminated sites.

Arsenic efflux in Enterobacter cloacae RSN3 isolated from arsenic-rich soil

In the present study, bacterial isolates were screened for arsenic resistance efficiency. Environmental isolates were isolated from arsenic-rich soil samples (i.e., from Rajnandgaon district of Chhattisgarh state, India). Amplification and sequencing of 16S rRNA gene revealed that the isolates were of Bacillus firmus RSN1, Brevibacterium senegalense RSN2, Enterobacter cloacae RSN3, Stenotrophomonas pavanii RSN6, Achromobacter mucicolens RSN7, and Ochrobactrum intermedium RSN10. Arsenite efflux gene (arsB) was successfully amplified in E. cloacae RSN3. Atomic absorption spectroscopy (AAS) analysis showed an absorption of 32.22% arsenic by the RSN3 strain. Furthermore, results of scanning electron microscopy (SEM) for morphological variations revealed an initial increase in the cell size at 1 mM sodium arsenate; however, it was decreased at 10 mM concentration in comparison to control. This change of the cell size in different metal concentrations was due to the uptake and expulsion of the metal from the cell, which also confirmed the arsenite efflux system.

Effect of arsenite and growth in biofilm conditions on the evolution of Thiomonas sp. CB2

Thiomonas bacteria are ubiquitous at acid mine drainage sites and play key roles in the remediation of water at these locations by oxidizing arsenite to arsenate, favouring the sorption of arsenic by iron oxides and their coprecipitation. Understanding the adaptive capacities of these bacteria is crucial to revealing how they persist and remain active in such extreme conditions. Interestingly, it was previously observed that after exposure to arsenite, when grown in a biofilm, some strains of Thiomonas bacteria develop variants that are more resistant to arsenic. Here, we identified the mechanisms involved in the emergence of such variants in biofilms. We found that the percentage of variants generated increased in the presence of high concentrations of arsenite (5.33 mM), especially in the detached cells after growth under biofilm-forming conditions. Analysis of gene expression in the parent strain CB2 revealed that genes involved in DNA repair were upregulated in the conditions where variants were observed. Finally, we assessed the phenotypes and genomes of the subsequent variants generated to evaluate the number of mutations compared to the parent strain. We determined that multiple point mutations accumulated after exposure to arsenite when cells were grown under biofilm conditions. Some of these mutations were found in what is referred to as ICE19, a genomic island (GI) carrying arsenic-resistance genes, also harbouring characteristics of an integrative and conjugative element (ICE). The mutations likely favoured the excision and duplication of this GI. This research aids in understanding how Thiomonas bacteria adapt to highly toxic environments, and, more generally, provides a window to bacterial genome evolution in extreme environments.

Arsenic and arsenic speciation in mushrooms from China: A review

Arsenic (As) is a natural environmental contaminant to which humans are usually exposed in water, air, soil, and food. China is a typical high-As region, and also a great contributor of the world production of cultivated edible mushrooms and a region abundant in wild growing edible mushrooms. Mushrooms can accumulate different amounts of As and different As compounds, so potential health risk of As intake may exist to people who use mushrooms with elevated As contents as food or medicine. A systematic literature search was carried out for studies on As and As compounds in mushrooms from China. We compiled existing data from published sources in English or Chinese and provide an updated review of the findings on As in mushrooms associated with environments and health risks. Future perspectives for studies on As in mushrooms have also been discussed.

Arsenic transformation mediated by gut microbiota affects the fecundity of Caenorhabditis elegans

Arsenic biotransformation has been discovered in guts of soil invertebrates. Reproduction of invertebrates is sensitive to arsenic contamination in soils. However, little is known about the impact of gut microbe-mediated arsenic biotransformation on the fecundity of invertebrates. Here, Caenorhabditis elegans was firstly pre-fed with Escherichia coli BL21 possessing the capability of reducing arsenate [As(V)] or BL21M having the ability to reduce As(V) and methylate arsenite [As(III)], then inoculated worms were transferred to inactive E. coli AW3110 (harboring no arsenic transformation gene)-seeded plates treated with As(V) at different concentrations. Quantification of gut microbes showed that both E. coli BL21 and BL21M stably colonized in the guts after worms were cultured on inactive E. coli AW3110-seeded plates for 72 h. The analysis of arsenic species indicated that there was As(III) in C. elegans guts colonized with E. coli BL21, As(III) and dimethylarsinic acid [DMAs(V)] in C. elegans guts with E. coli BL21M exposed to As(V) for 6 h. After treatment of 100 μM As(V), decrease in brood sizes was observed for worms that were colonized with E. coli BL21 or BL21M compared to that with AW3110 in the guts. The levels of vitellogenin (VTG), glutathione S-transferases (GST) and superoxide dismutase (SOD), closely linked to reproduction and antioxidation-linked indicators, were the highest in worms whose guts colonized with E. coli BL21, followed by worms colonized with E. coli BL21M and worms colonized with inactive E. coli AW3110 exposed to As(V). Our results indicated the toxic impact of As(III) and DMAs(V) produced by gut microbes on reproduction of C. elegans. The work provides novel insight into the interplay between arsenic biotransformation mediated by gut microbes and the host fecundity in soils.

Integrating high-throughput sequencing and metagenome analysis to reveal the characteristic and resistance mechanism of microbial community in metal contaminated sediments

Some metallic tailings from closed mines were scattered in upstream of the Miyun Reservoir, Beijing, threatening the ecological environment of rivers due to trace metals. The Liuli River, one of the main rivers affected, was investigated as a typical model in this work. In this study, we selected eight sites to assess interactions among the various geochemical factors especially between trace metals and sediment microbiota. Random forest predicted that low concentrations of Cu, Cd, Cr and Ni (lower than 61.8 mg/kg, 3.2 mg/kg, 173.2 mg/kg and 34.1 mg/kg, respectively) were able to enhance community diversity but generally, trace metals contamination impaired microbial diversity. Environmental factor correlation analysis showed that As, pH and available P were the major factors that shifted the distribution of the microbial communities. Metagenome sequencing revealed that Proteobacteria harbored the vast majority of heavy metal resistance genes followed by Actinobacteria and Bacteroidetes. Metal tolerance of Proteobacteria were achieved by exportation of metals by the corresponding transporters, by pumps and ion channels, or by their reduction via redox reactions. In addition, Proteobacteria harbored a greater ability to repair DNA damage via DNA recombination.

Co-selection and stability of bacterial antibiotic resistance by arsenic pollution accidents in source water

Frequent heavy-metal pollution accidents severely deteriorated the source water quality of drinking water treatment plants (DWTP). Limited data have explicitly addressed the impact of these incidents on bacterial antibiotic resistance (BAR). In present study, we investigated the shift of antibiotic resistome caused by heavy metal pollution incidents via simulating an arsenic shock loading [As (III)], along with the associated risks imposed on drinking water systems. The results indicated that a quick co-selection of antibiotic resistant bacteria (ARB) was achieved after exposure to 0.2-1 mg/L As (III) for only 6 h, meanwhile, there was an increase of relative abundance of antibiotic resistance genes (ARGs) and mobile genetic elements. Most of the co-selected BAR could be maintained for at least 4 days in the absence of As (III) and antibiotics, implying that the pollution in source water possibly contributed to the preservation and proliferation of antibiotic resistance determinants in the subsequent DWTP. Bacterial community structure analysis showed a strong correlation between bacterial community shift and BAR promotion, and enrichment of opportunistic bacteria (e.g. Escherichia-Shigella, Empedobacter sp. and Elizabethkingia sp.). The results indicated a potential epidemiological threat to the public due to accident-level arsenic contamination in the source water. This study gave insight into understanding the source water pollution accidents from the perspective of bio-hazard and biological risks, and highlighted a neglected important source of BAR in drinking water systems.

ArsM-mediated arsenite volatilization is limited by efflux catalyzed by As efflux transporters

Arsenic (As) methylation is regarded as an efficient strategy for As contamination remediation by As volatilization. However, most microorganisms display low As volatilization efficiency, which is possibly linked to As efflux transporters competing for cytoplasmic As(III) as a substrate. Here, we developed two types of As biosensors in Escherichia coli to compare the As efflux rate of three efflux transporters and to further investigate the correlation between As efflux rates and As volatilization. The engineered As-sensitive E. coli AW3110 expressing arsB_RP, acr3_RP or arsB_EC displayed a higher As resistance compared to the control. The fluorescence intensity was in a linear correlation in the range of 0-2.0 μmol/L of As(III). The intracellular As(III) concentration was negatively related to As efflux activity of As efflux transporter, which was consistent with the As resistance assays. Moreover, arsM derived from R. palustris CGA009 was subsequently introduced to construct an E. coli AW3110 co-expressing arsB/acr3 and arsM, which exhibited higher As(III) resistance, lower fluorescence intensity and intracellular As concentration compared to the engineered E. coli AW3110 expressing only arsB/acr3. The As volatilization efficiency was negatively related to As efflux activity of efflux transporters, the recombinants without arsB/acr3 displayed the highest rate of As volatilization. This study provided new insights into parameters affecting As volatilization with As efflux being the main limiting factor for As methylation and subsequent volatilization in many microorganisms.

Biochemical and molecular characterization of arsenic response from Azospirillum brasilense Cd, a bacterial strain used as plant inoculant

Azospirillum brasilense Cd is a bacterial strain widely used as an inoculant of several crops due to its plant growth promoting properties. However, its beneficial effects depend on its viability and functionality under adverse environmental conditions, including the presence of arsenic (As) in agricultural soils. Therefore, the aim of this work was to evaluate the response of A. brasilense Cd to arsenate (AsV) and arsenite (AsIII). This bacterium was tolerant to As concentrations frequently found in soils. Moreover, properties related to roots colonization (motility, biofilm, and exopolymers) and plant growth promotion (auxin, siderophore production, and N₂ fixation) were not significantly affected by the metalloid. In order to deepen the understanding on As responses of A. brasilense Cd, As resistance genes were sequenced and characterized for the first time in this work. These genes could mediate the redox As transformation and its extrusion outside the cell, so they could have direct association with the As tolerance observed. In addition, its As oxidation/reduction capacity could contribute to change the AsV/AsIII ratio in the environment. In conclusion, the results allowed to elucidate the As response of A. brasilense Cd and generate interest for its potential use in polluted environments.

Isolation and Identification of Arsenic-resistant Bacteria for Possible Application in Arsenic Bioremediation

BACKGROUND AND OBJECTIVE

North Sulawesi is rich in minerals, among them gold is also present. The gold mining in the Buyat area produces heavy metal waste which can pollute the environment, among others is arsenic. Arsenic is a heavy metal that is very toxic to humans, so an agent is needed for the remediation process. The aim of this study was to isolate and identify arsenic-resistant bacteria from the Buyat estuary and beach to analyze the isolates' ability to detoxify arsenic.

MATERIALS AND METHODS

Soil sediment samples were obtained from Buyat estuary and beach in North Sulawesi. Isolation of arsenic-resistant bacteria was carried out by growing the samples in LB broth media containing 100, 500 and 1000 ppm arsenite. Indentification of arsenic-resistant bacteria was carried out by microbiological, biochemical and biomolecular analysis. The ability to detoxify arsenite was analyzed by CVAFS.

RESULTS

The study showed that there were 4 isolates of arsenic-resistant bacteria isolated from the soil samples. All isolates are rod-shaped, Gram-negative and non-motile bacteria. BLAST results showed that isolates A was Stenotrophomonas sp., isolate B was Stenotrophomonas maltophilia, isolate C was Pseudomonas sp. and isolate D was Pseudomonas putida. All isolates reduced the levels of arsenic in media by almost 100% within 72 h.

CONCLUSION

The study suggested that Stenotrophomonas sp., S. maltophilia, Pseudomonas sp. and P. putida had the potentials to be used in the bioremediation of arsenic.

Screening of plant growth promoting attributes and arsenic remediation efficacy of bacteria isolated from agricultural soils of Chhattisgarh

Arsenic (As) resistant indigenous bacteria with discrete minimum inhibitory concentration values for arsenate [As(V)] and arsenite [As(III)] were isolated from the paddy fields of different regions of Chhattisgarh, India, following enrichment culture technique. Evaluation of the plant growth promoting (PGP) properties of the isolates revealed that two rod-shaped Gram-positive bacteria viz., ARP2 and ART2 acquired various PGP traits, including phosphate solubilization, production of siderophore, indole acetic acid, ammonia, and exopolysaccharide. Both the isolates significantly increased (40-80%) the root length of Oryza sativa L. even under As-exposure. Sequencing of 16S rRNA gene identified these isolates as Bacillus nealsonii strain ARP2 and Bacillus tequilensis strain ART2, respectively. Isolate ARP2 exhibited arsenate reductase activity thereby rapidly reduced As(V) into As(III), achieving a reduction rate of 37.5 μM min^-1. Alike, strain ART2 was capable of oxidizing As(III) into As(V) via arsenite oxidase enzyme, and revealed the oxidation rate of 21.8 μM min^-1. Quantitative estimation of As through atomic absorption spectrophotometer revealed that the isolates ARP2 and ART2 removed 93 ± 0.2% and 77 ± 0.14% of As(V) and As(III), respectively, from As-containing culture media. The FTIR analysis showed the interaction of As with the cell membrane and was further confirmed by SEM and TEM techniques, which marked the increase in cell volume owing to successive accumulation of As. The As-resistant and PGP properties of above two isolates demonstrates their potentiality for sustainable bioremediation of As, and establishment of flora in As-rich environment.

Arsenic Response of Three Altiplanic Exiguobacterium Strains With Different Tolerance Levels Against the Metalloid Species: A Proteomics Study

Exiguobacterium is a polyextremophile bacterial genus with a physiology that allows it to develop in different adverse environments. The Salar de Huasco is one of these environments due to its altitude, atmospheric pressure, solar radiation, temperature variations, pH, salinity, and the presence of toxic compounds such as arsenic. However, the physiological and/or molecular mechanisms that enable them to prosper in these environments have not yet been described. Our research group has isolated several strains of Exiguobacterium genus from different sites of Salar de Huasco, which show different resistance levels to As(III) and As(V). In this work, we compare the protein expression patterns of the three strains in response to arsenic by a proteomic approach; strains were grown in absence of the metalloid and in presence of As(III) and As(V) sublethal concentrations and the protein separation was carried out in 2D electrophoresis gels (2D-GE). In total, 999 spots were detected, between 77 and 173 of which showed significant changes for As(III) among the three strains, and between 90 and 143 for As(V), respectively, compared to the corresponding control condition. Twenty-seven of those were identified by mass spectrometry (MS). Among these identified proteins, the ArsA [ATPase from the As(III) efflux pump] was found to be up-regulated in response to both arsenic conditions in the three strains, as well as the Co-enzyme A disulfide reductase (Cdr) in the two more resistant strains. Interestingly, in this genus the gene that codifies for Cdr is found within the genic context of the ars operon. We suggest that this protein could be restoring antioxidants molecules, necessary for the As(V) reduction. Additionally, among the proteins that change their expression against As, we found several with functions relevant to stress response, e.g., Hpf, LuxS, GLpX, GlnE, and Fur. This study allowed us to shed light into the physiology necessary for these bacteria to be able to tolerate the toxicity and stress generated by the presence of arsenic in their niche.