Genetic mechanisms of arsenic detoxification and metabolism in bacteria

Characterization and identification of Pseudomonas sp. AW4, an arsenic-resistant and plant growth-promoting bacteria isolated from the soybean (Glycine max L.) rhizosphere

Pseudomonas sp. AW4 is a highly arsenic (As) resistant bacterium with plant growth promoting properties, originally isolated from the soybean (Glycine max L.) rhizosphere. In order to safely use this isolate in diverse bioformulations, its characterization needs to be completed and a reliable identification must be provided. In the present work, we analyzed the morpho-physiological, biochemical and genomic characteristics of Pseudomonas sp. AW4. Identification of the isolate varied according to the parameters analyzed, mainly biochemical and physiological tests or individual genes and phylogenetic analyses. In this regard, we performed massive sequencing of its genome, in order to consistently complete its characterization and identification. Pseudomonas sp. AW4 formed a monophyletic clade with P. urmiensis SWRI10, presenting 3.08 % of unique genes against this reference isolate. More than 70 % of AW4 genes were also shared with P. oryziphila strain 1257 NZ and with P. reidholzensis strain CCOS 865. The search for genes related to As resistance evidenced the presence of the operon arsHRBC. Taken together, results of the present work allow identification of this bacterium as Pseudomonas urmiensis AW4 and open up a number of opportunities to study this strain and understand the mechanisms of arsenic resistance and plant growth promotion.

Source-oriented risks of heavy metals and their effects on resistance genes in natural biofilms

Heavy metal (HM) introduction from various land-use patterns can be a major source of metal resistance genes (MRGs) entering river environments. This influx can trigger the occurrence of other resistomes, such as antibiotic resistance genes (ARGs), by improving co-resistant conjugative transfer. Biofilms, which form at water-solid interfaces, could serve as potential hotspots for HMs and resistance genes. However, the enrichment of HMs from various sources within biofilms and their effect on resistomes remain undocumented. This study aims to investigate the physicochemical properties of biofilm samples collected from the Heihui River, a tributary of the Lancang River, and to analyze the concentrations of nine HMs (As, Cd, Co, Cr, Cu, Ni, Pb, V, and Zn) within these biofilms. The 16S rRNA gene and metagenomic high-throughput sequencing techniques were integrated to uncover the association between HM accumulation levels in biofilms and ecological and health risks, considering the presence of two resistance genes. Natural sources (Co, Cr), industrial (As, Cu, V), agricultural (Cd, Ni), and transportation activities (Pb, Zn) markedly contributed to HM presence within biofilms, with industrial activities posing higher noncarcinogenic and carcinogenic risks than other sources. The network-correlation analyses revealed higher levels of ARG-MRG coexistence in biofilms, with the ecological and health risk index of HMs in biofilms closely associated with the abundance of both resistance genes. Furthermore, the biofilm As concentration markedly affected the abundance and expression of ARGs and MRGs, with elevated As levels within biofilms significantly and positively influencing all four functional categories of MRGs. Water pH also indirectly impacted these functional types by modulating the ionic form of HMs within the biofilm matrix. Our findings underscore the significance of integrating biofilms into environmental management practices and standards for assessing environmental quality.

Assessing the impact of arsenic on symbiotic and free-living PGPB: plant growth promoting traits, bacterial compatibility and adhesion on soybean seed

Arsenic (As) contamination in agricultural groundwater and soil is a significant economic and health problem worldwide. It inhibits soybean (Glycine max (L.) Merr.) nodulation and biological nitrogen fixation in symbiosis with Bradyrhizobium japonicum E109 (E109), a commonly used rhizobial strain for commercial biofertiliser formulation in Argentina. In the context of sustainable and climate-smart agriculture promoted by FAO, co-inoculating legumes with As-tolerant plant growth-promoting bacteria (PGPB) is suggested as a superior alternative to single inoculation. This study aimed to evaluate the impact of As on plant growth-promoting (PGP) traits -siderophore and indole acetic acid production, phosphate solubilisation, diazotrophic activity and hydrolytic enzymes activity- in E109 and three other PGPB strains: Pseudomonas sp. AW4 (AW4), Bacillus pumilus SF5 (SF5) and Bacillus toyonensis SFC 500-1E (Bt). In addition, bacterial compatibility and adhesion on soybean seed were evaluated. Arsenic significantly reduced PGP traits of E109 even at low concentrations, AW4's traits remained unchanged, while those of SF5 and Bt traits were affected (positively or negatively) only at the highest concentrations tested (500 µM arsenate, 250 µM arsenite). All PGPB strains were compatible with E109 under both control and As-stress conditions. Soybean seed adhesion was reduced for E109, only under As stress. Findings suggest that the effect of As on PGP traits is highly strain-dependent and influenced by As concentration and speciation. AW4, SF5, and Bt strains show promise for co-inoculation with E109 in soybean cultivation.

Impact of Arsenic Stress on the Antioxidant System and Photosystem of Arthrospira platensis

Arthrospira platensis exhibits high tolerance to arsenic; however, the mechanisms underlying its response to the arsenic stress have not been fully elucidated. This study investigated the growth and resistance mechanisms of A. platensis under As3+ stress by measuring physiological and biochemical indices, conducting transcriptome sequencing, and validating the results through qPCR. The findings show that arsenic stress affected the antioxidant system and photosynthetic pigment synthesis in A. platensis. The algae mitigated arsenic-induced oxidative stress by increasing cellular metabolic rates, enhancing cell wall stability, and reducing membrane lipid peroxidation. Transcriptome analysis revealed that pathways related to oxidative phosphorylation and chlorophyll degradation were upregulated under arsenic stress, while the expression of membrane transporters was significantly downregulated. Additionally, the algae alleviated arsenic stress by producing hydrogen and polyamine compounds. This study provides insights into the mechanisms of A. platensis response to arsenic stress and elucidates the molecular pathways involved in the stress response to As3+.

Biotransformations of arsenic in marine sediments across marginal slope to hadal zone

Arsenic compounds are accumulating in deep ocean, but their ecological impacts on deep-sea ecosystem remain elusive. We studied 32 sediment cores (101 layers for metagenomes, along with 41 global reference sediment metagenomes) collected from the South China Sea and the Mariana Trench (MT), characterized with high arsenic accumulation in MT. In these metagenomes we revealed a significantly positive correlation between relative abundance of arsenite methyltransferase gene (arsM) and sampling depth, which suggests that arsenic methylation is the most prevalent arsenic biotransformation process in the deep sea. Lower relative abundance of arsenic efflux gene, compared with arsM, indicates that microbes in deep-sea sediments were prone to methylate arsenite and retain it rather than efflux it. Phylogenetic analysis identified seven clades of ArsM proteins, including two new clades derived primarily from deep-sea microorganisms. Five metagenome-assembled genomes containing aioA for arsenite oxidation also harbor carbon fixation genes in the deep-sea sediment layers, suggesting previously unnoticed contribution of arsenite-oxidizing autotrophic bacteria to the carbon cycle. Therefore, deep-sea microorganisms adopt different detoxification and transformation strategies in response to arsenic compounds, which renews our understanding of arsenic in their ecological impacts and potential contribution in deep ocean.

Arsenite tolerance and removal potential of the indigenous halophilic bacterium, Halomonas elongata SEK2

The indigenous halophilic arsenite-resistant bacterium Halomonas elongata strain SEK2 isolated from the high saline soil of Malek Mohammad hole, Lut Desert, Iran, could tolerate high concentrations of arsenate (As5+) and arsenite (As3+) up to 800 and 40 mM in the SW-10 agar medium, respectively. The isolated strain was able to tolerate considerable concentrations of other toxic heavy metals and oxyanions, including Cadmium (Cd2+), Chromate (Cr6+), lead (Pb2+), and selenite (Se4+), regarding the high salinity of the culture media (with a total salt concentration of 10% (w/v)), the tolerance potential of the isolate SEK2 was unprecedented. The bioremoval potential of the isolate SEK2 was examined through the Silver diethyldithiocarbamate (SDDC) method and demonstrated that the strain SEK2 could remove 60% of arsenite from arsenite-containing growth medium after 48 h of incubation without converting it to arsenate. The arsenite adsorption or uptake by the halophilic bacterium was investigated and substantiated through Fourier-transform infrared spectroscopy (FTIR), Scanning Electron Microscope (SEM), and Energy Dispersive X-ray (EDX) analyses. Furthermore, Transmission electron microscope (TEM) analysis revealed ultra-structural alterations in the presence of arsenite that could be attributed to intracellular accumulation of arsenite by the bacterial cell. Genome sequencing analysis revealed the presence of arsenite resistance as well as other heavy metals/oxyanion resistance genes in the genome of this bacterial strain. Therefore, Halomonas elongata strain SEK2 was identified as an arsenite-resistant halophilic bacterium for the first time that could be used for arsenite bioremediation in saline arsenite-polluted environments.

Arsenic redox disequilibrium in geogenic contaminated groundwater: Bioenergetic insights from organic molecular characterization and gene-informed modeling

Biotransformation of arsenic (As) influences its speciation and mobility, obscuring mechanistic comprehension on spatiotemporal variation of As concentration in geogenic contaminated groundwater. In particular, unresolved processes underlying As redox disequilibrium in comparison to major redox couples discourage practical efforts to rehabilitate the As-contaminated groundwater. Here, quantitative metagenomic sequencing and ultrahigh-resolution mass spectrometry (FT-ICR-MS) were jointly applied to reveal the links between vertical distribution of As metabolic gene assemblages and that of free energy density of dissolved organic matter (DOM) in As-contaminated groundwater of Datong Basin. Observed small excess of Gibbs free energy available by DOM relative to that required for As(V)-to-As(III) reduction exerts thermodynamic constraint on metabolism-mediated redox transformation of As. Accordingly, the vertical distribution of dissolved As(V)/As(III) ratio correlated significantly with that of ars+acr3 and arr encoding As(V) reduction and aio encoding As(III) oxidation in the moderately/strongly reducing groundwater. Further gene-informed biogeochemical modeling suggests that a net effect of these kinetics-restricted bidirectional metabolic pathways leads to co-preservation of As(V) and As(III) even at relatively high rates of ars+acr3 encoded As(V) reduction. This study therefore provides new insights into bioenergetic constraints on As hydrobiogeochemical behavior, with implications for other redox-sensitive contaminants in the groundwater systems.

The role of microbiomes in cooperative detoxification mechanisms of arsenate reduction and arsenic methylation in surface agricultural soil

Microbial arsenic (As) transformations play a vital role in both driving the global arsenic biogeochemical cycle and determining the mobility and toxicity of arsenic in soils. Due to the complexity of soils, variations in soil characteristics, and the presence and condition of overlying vegetation, soil microbiomes and their functional pathways vary from site to site. Consequently, key arsenic-transforming mechanisms in soil are not well characterized. This study utilized a combination of high-throughput amplicon sequencing and shotgun metagenomics to identify arsenic-transforming pathways in surface agricultural soils. The temporal and successional variations of the soil microbiome and arsenic-transforming bacteria in agricultural soils were examined during tropical monsoonal dry and wet seasons, with a six-month interval. Soil microbiomes of both dry and wet seasons were relatively consistent, particularly the relative abundance of Chloroflexi, Gemmatimonadota, and Bacteroidota. Common bacterial taxa present at high abundance, and potentially capable of arsenic transformations, were Bacillus, Streptomyces, and Microvirga. The resulting shotgun metagenome indicated that among the four key arsenic-functional genes, the arsC gene exhibited the highest relative abundance, followed by the arsM, aioA, and arrA genes, in declining sequence. Gene sequencing data based on 16S rRNA predicted only the arsC and aioA genes. Overall, this study proposed that a cooperative mechanism involving detoxification through arsenate reduction and arsenic methylation was a key arsenic transformation in surface agricultural soils with low arsenic concentration (7.60 to 10.28 mg/kg). This study significantly advances our knowledge of arsenic-transforming mechanisms interconnected with microbial communities in agricultural soil, enhancing pollution control measures, mitigating risks, and promoting sustainable soil management practices.

Advances in mechanism for the microbial transformation of heavy metals: implications for bioremediation strategies

Heavy metals are extensively discharged through various anthropogenic activities, resulting in an environmental risk on a global scale. In this case, microorganisms can survive in an extreme heavy metal-contaminated environment via detoxification or resistance, playing a pivotal role in the speciation, bioavailability, and mobility of heavy metals. Therefore, studies on the mechanism for the microbial transformation of heavy metals are of great importance and can provide guidance for heavy metal bioremediation. Current research studies on the microbial transformation of heavy metals mainly focus on the single oxidation, reduction and methylation pathways. However, complex microbial transformation processes and corresponding bioremediation strategies have never been clarified, which may involve the inherent physicochemical properties of heavy metals. To uncover the underlying mechanism, we reclassified heavy metals into three categories based on their biological transformation pathways, namely, metals that can be chelated, reduced or oxidized, and methylated. Firstly, we comprehensively characterized the difference in transmembrane pathways between heavy metal cations and anions. Further, biotransformation based on chelation by low-molecular-weight organic complexes is thoroughly discussed. Moreover, the progress and knowledge gaps in the microbial redox and (de)methylation mechanisms are discussed to establish a connection linking theoretical advancements with solutions to the heavy metal contamination problem. Finally, several efficient bioremediation strategies for heavy metals and the limitations of bioremediation are proposed. This review presents a solid contribution to the design of efficient microbial remediation strategies applied in the real environment.

Arsenic behavior in soil-plant system under the manure application with the combination of antibiotic and roxarsone

Limited attention has been given to the interaction between antibiotics and arsenic in the soil-plant system. In this investigation, Medicago sativa seedlings were grown in soil treated with cow manure containing oxytetracycline (OTC) or sulfadiazine (SD), as well as arsenic (introduced through roxarsone, referred to as ROX treatment). The study revealed a notable increase in As(III) and dimethylarsinic acid (DMA(V)) levels in rhizosphere soils and plant root tissues as arsenic contamination intensified in the presence of antibiotics, while concentrations of As(V) and monomethylarsonic acid (MMA(V)) decreased. Conversely, elevated antibiotic presence resulted in higher levels of As(V) but reduced DMA concentrations in both rhizosphere soils and plant root tissues in the presence of arsenic. The arsenic biotransformation gene aioA was inhibited by arsenic contamination when antibiotics were present, and suppressed by antibiotic contamination in the presence of arsenic, especially in SD treatments, resulting in reduced expression levels at higher SD concentrations. Conversely, the arsM gene exhibited consistent upregulation under all conditions. However, its expression was found to increase with higher concentrations of ROX in the presence of antibiotics, decrease with increasing SD concentrations, and initially rise before declining with higher levels of OTC in the presence of arsenic. Bacterial genera within the Proteobacteria phylum, such as Geobacter, Lusitaniella, Mesorhizobium, and Methylovirgula, showed significant co-occurrence with both aioA and arsM genes. Correlation analysis demonstrated associations between the four arsenic species and the two arsenic biotransformation genes, emphasizing pH as a critical factor influencing the transformation and uptake of different arsenic species in the soil-plant system. The combined stress of antibiotics and arsenic has the potential to modify arsenic behavior and associated risks in soil-plant systems, highlighting the necessity of considering this interaction in future research endeavors.

Improving sunflower growth and arsenic bioremediation in polluted environments: Insights from ecotoxicology and sustainable mitigation approaches

The issue of arsenic (As) contamination in the environment has become a critical concern, impacting both human health and ecological equilibrium. Addressing this challenge requires a comprehensive strategy encompassing water treatment technologies, regulatory measures for industrial effluents, and the implementation of sustainable agricultural practices. In this study, diverse strategies were explored to enhance As accumulation in the presence of Acinetobacter bouvetii while safeguarding the host from the toxic effects of arsenate exposure. The sunflower seedlings associated with A. bouvetii demonstrated a favorable relative growth rate (RGR) and net assimilation rate (NAR) even less than 100 ppm of As stress. Remarkably, the NAR and RGR of A. bouvetii-associated seedlings outperformed those of control seedlings cultivated without A. bouvetii in As-free conditions. Additionally, a markedly greater buildup of bio-transformed As was observed in A. bouvetii-associated seedlings (P = 0.05). An intriguing observation was the normal levels of reactive oxygen species (ROS) in A. bouvetii-associated seedlings, along with elevated activities of key enzymatic antioxidants like catalases (CAT), ascorbate peroxidase (APX), superoxide dismutase (SOD), and peroxidases (POD), along with non-enzymatic antioxidants (phenols and flavonoids). This coordinated antioxidant defense system likely contributed to the improved survival and growth of the host plant species amidst As stress. A. bouvetii not only augmented the growth of the host plants but also facilitated the uptake of bio-transformed As in the contaminated medium. The rhizobacterium's modulation of various biochemical and physiological parameters indicates its role in ensuring the better survival and progression of the host plants under As stress.

Staphylococcus arlettae mediated defense mechanisms and metabolite modulation against arsenic stress in Helianthus annuus

Introduction

Arsenate, a metalloid, acting as an analog to phosphate, has a tendency to accumulate more readily in plant species, leading to adverse effects.

Methods

In the current study, sunflower seedlings were exposed to 25, 50 and 100 ppm of the arsenic.

Results

Likewise, a notable reduction (p<0.05) was observed in the relative growth rate (RGR) by 4-folds and net assimilation rate (NAR) by 75% of Helianthus annuus when subjected to arsenic (As) stress. Nevertheless, the presence of Staphylococcus arlettae, a plant growth-promoting rhizobacterium with As tolerance, yielded an escalation in the growth of H. annuus within As-contaminated media. S. arlettae facilitated the conversion of As into a form accessible to plants, thereby, increasing its uptake and subsequent accumulation in plant tissues. S. arlettae encouraged the enzymatic antioxidant systems (Superoxide dismutase (SOD), peroxidase (POD), ascorbate peroxidase (APX) and catalase (CAT)) and non-enzymatic antioxidants (flavonoids, phenolics, and glutathione) in H. annuus seedlings following substantial As accumulation. The strain also induced the host plant to produce osmolytes like proline and sugars, mitigating water loss and maintaining cellular osmotic balance under As-induced stress. S. arlettae rectified imbalances in lignin content, reduced high malonaldehyde (MDA) levels, and minimized electrolyte leakage, thus counteracting the toxic impacts of the metal.

Conclusion

The strain exhibited the capability to concurrently encourage plant growth and remediate Ascontaminated growth media through 2-folds rate of biotransformation and bio-mobilization.

Phosphate Uptake and Its Relation to Arsenic Toxicity in Lactobacilli

The use of probiotic lactobacilli has been proposed as a strategy to mitigate damage associated with exposure to toxic metals. Their protective effect against cationic metal ions, such as those of mercury or lead, is believed to stem from their chelating and accumulating potential. However, their retention of anionic toxic metalloids, such as inorganic arsenic, is generally low. Through the construction of mutants in phosphate transporter genes (pst) in Lactiplantibacillus plantarum and Lacticaseibacillus paracasei strains, coupled with arsenate [As(V)] uptake and toxicity assays, we determined that the incorporation of As(V), which structurally resembles phosphate, is likely facilitated by phosphate transporters. Surprisingly, inactivation in Lc. paracasei of PhoP, the transcriptional regulator of the two-component system PhoPR, a signal transducer involved in phosphate sensing, led to an increased resistance to arsenite [As(III)]. In comparison to the wild type, the phoP strain exhibited no differences in the ability to retain As(III), and there were no observed changes in the oxidation of As(III) to the less toxic As(V). These results reinforce the idea that specific transport, and not unspecific cell retention, plays a role in As(V) biosorption by lactobacilli, while they reveal an unexpected phenotype for the lack of the pleiotropic regulator PhoP.

Microalgae for freshwater arsenic bioremediation: examining cellular toxicity, bioconcentration factor and eluding an alternative arsenic detoxification pathway

Microalgae are photoautotrophic organisms in freshwater systems known to uptake and bioremediate arsenic, a heavy metal. In this study, we compared the growth and arsenic uptake of two microalgae strains, Nostoc and Chlorella, to determine their suitability for arsenic bioremediation. As compared to the control, our results showed that treatment with As (III) enhanced the Nostoc growth by approximately 15% when grown in the absence of phosphate. The highest bioconcentration factor of Nostoc at this treatment was 1463.6, whereas 0.10 mg L-1 As (V) treatment improved the Chlorella growth by 25%, in the presence of phosphate. However, arsenic uptake reduced from 175.7 to 32.3 throughout the cultivation period for Chlorella. This suggests that Nostoc has an upper advantage in the bioremediation of arsenic as compared to the Chlorella strain. To gain insights into the potential of Nostoc in arsenic bioremediation, we further conducted SEM analysis on the vegetative cell surface. The SEM results showed that As (III) disrupted the Nostoc vegetative cell surface and structure. Further to this, pathway analysis and polymerase chain reaction (PCR) were conducted to identify the potential arsenic pathway regulated by Nostoc. The primary As (III)-related pathways elucidated include the arsA transporter and arsD complex that require ATP and As (III) methylation to S-adenosylmethionine. The phosphate deficiency condition resulting in the inability to generate ATP caused As (III) could not be excreted from the Nostoc cells, potentially contributing to the high arsenic concentration accumulated under phosphate-depleted conditions. These insights contribute to understanding the efficacy of microalgae strains in freshwater arsenic bioremediation.

Construction of Tetrahymena strains with highly active arsenic methyltransferase genes for arsenic detoxification in aquatic environments

Biomethylation is an effective means of arsenic detoxification by organisms living in aquatic environments. Ciliated protozoa (including Tetrahymena species) play an important role in the biochemical cycles of aquatic ecosystems and have a potential application in arsenic biotransformation. This study compared arsenic tolerance, accumulation, methylation, and efflux in 11 Tetrahymena species. Nineteen arsenite (As(III)) S-adenosylmethionine (SAM) methyltransferase (arsM) genes, of which 12 are new discoveries, were identified, and protein sequences were studied. We then constructed recombinant cell lines based on the Tetrahymena thermophila (T. thermophila) wild-type SB210 strain and expressed each of the 19 arsM genes under the control of the metal-responsive the MTT1 promoter. In the presence of Cd2+ and As(V), expression of the arsM genes in the recombinant cell lines was much higher than in the donor species. Evaluation of the recombinant cell line identified one with ultra-high arsenic methylation enzyme activity, significantly higher arsenic methylation capacity and much faster methylation rate than other reported arsenic methylated organisms, which methylated 89% of arsenic within 6.5 h. It also had an excellent capacity for the arsenic detoxification of lake water containing As(V), 56% of arsenic was methylated at 250 μg/L As(V) in 48 h. This study has made a significant contribution to our knowledge on arsenic metabolism in protozoa and demonstrates the great potential to use Tetrahymena species in the arsenic biotransformation of aquatic environments.

New insights into the typical nitrogen-containing heterocyclic compound-quinoline degradation and detoxification by microbial consortium: Integrated pathways, meta-transcriptomic analysis and toxicological evaluation

As the primary source of COD in industrial wastewater, quinoline has aroused increasing attention because of its potential teratogenic, carcinogenic, and mutagenic effects in the environment. The activated sludge isolate quinoline-degrading microbial consortium (QDMC) efficiently metabolizes quinoline. However, the molecular underpinnings of the degradation mechanism of quinoline by QDMC have not been elucidated. High-throughput sequencing revealed that the dominant genera included Diaphorobacter, Bacteroidia, Moheibacter and Comamonas. Furthermore, a positive strong correlation was observed between the key bacterial communities (Diaphorobact and Bacteroidia) and quinoline degradation. According to metatranscriptomics, genes associated with quorum sensing, ABC transporters, component systems, carbohydrate, aromatic compound degradation, energy metabolism and amino metabolism showed high expression, thus improving adaptability of microbial community to quinoline stress. In addition, the mechanism of QDMC in adapting and resisting to extreme environmental conditions in line with the corresponding internal functional properties and promoting biogegradation efficiency was illustrated. Based on the identified products, QDMC effectively mineralized quinoline into low-toxicity metabolites through three major metabolic pathways, including hydroxyquinoline, 1,2,3,4-H-quinoline, 5,6,7,8-tetrahydroquinoline and 1-oxoquinoline pathways. Finally, toxicological, genotoxicity and phytotoxicity studies supported the detoxification of quinoline by the QDMC. This study provided a promising approach for the stable, environmental-friendly and efficient bioremediation applications for quinoline-containing wastewater.

Endophytic Enterobacter sp. YG-14 mediated arsenic mobilization through siderophore and its role in enhancing phytostabilization

Soil arsenic (As) phytoremediation has long faced the challenge of efficiently absorbing As by plant accumulators while maintaining their health and fast growth. Even at low doses, arsenic is highly toxic to plants. Therefore, plant growth-promoting microorganisms that can mediate As accumulation in plants are of great interest. In this study, the endophyte Enterobacter sp. YG-14 (YG-14) was found to have soil mobilization activity. By constructing a siderophore synthesis gene deletion mutant (ΔentD) of YG-14, the endophyte was confirmed to effectively mobilize Fe-As complexes in mining soil by secreting enterobactin, releasing bioavailable Fe and As to the rhizosphere. YG-14 also enhances As accumulation in host plants via extracellular polymer adsorption and specific phosphatase transfer protein (PitA) absorption. The root accumulation of As was positively correlated with YG-14 root colonization. In addition, YG-14 promoted plant growth and alleviated oxidative damage in R. pseudoacacia L. under arsenic stress. This is the first study, from phenotype, physiology, and molecular perspectives, to determine the role of endophyte in promoting As phytostabilization and maintaining the growth of the host plant. This demonstrated the feasibility of using endophytes with high siderophore production to assist host plants in As phytoremediation.

Arsenic biogeochemical cycling association with basin-scale dynamics of microbial functionality and organic matter molecular composition

Geogenic arsenic (As)-contaminated groundwater is a sustaining global health concern that is tightly constrained by multiple interrelated biogeochemical processes. However, a complete spectrum of the biogeochemical network of high-As groundwater remains to be established, concurrently neglecting systematic zonation of groundwater biogeochemistry on the regional scale. We uncovered the geomicrobial interaction network governing As biogeochemical pathways by merging in-field hydrogeochemical monitoring, metagenomic analyses, and ultrahigh resolution mass spectrometry (FT-ICR MS) characterization of dissolved organic matter. In oxidizing to weakly reducing environments, the nitrate-reduction and sulfate-reduction encoding genes (narGHI, sat) inhibited the dissolution of As-bearing iron minerals, leading to lower As levels in groundwater. In settings from weakly to moderately reducing, high abundances of sulfate-reduction and iron-transport encoding genes boosted iron mineral dissolution and consequent As release. As it evolved to strongly reducing stage, elevated abundance of methane cycle-related genes (fae, fwd, fmd) further enhanced As mobilization in part by triggering the formation of gaseous methylarsenic. During redox cycling of N, S, Fe, C and As in groundwater, As migration to groundwater and immobilization in mineral particles are geochemically constrained by basin-scale dynamics of microbial functionality and DOM molecular composition. The study constructs a theoretical model to summarize new perspectives on the biogeochemical network of As cycling.

Fundamentals and application in phytoremediation of an efficient arsenate reducing bacterium Pseudomonas putida ARS1

Arsenic is a ubiquitous environmental pollutant. Microbe-mediated arsenic bio-transformations significantly influence arsenic mobility and toxicity. Arsenic transformations by soil and aquatic organisms have been well documented, while little is known regarding effects due to endophytic bacteria. An endophyte Pseudomonas putida ARS1 was isolated from rice grown in arsenic contaminated soil. P. putida ARS1 shows high tolerance to arsenite (As(III)) and arsenate (As(V)), and exhibits efficient As(V) reduction and As(III) efflux activities. When exposed to 0.6 mg/L As(V), As(V) in the medium was completely converted to As(III) by P. putida ARS1 within 4 hr. Genome sequencing showed that P. putida ARS1 has two chromosomal arsenic resistance gene clusters (arsRCBH) that contribute to efficient As(V) reduction and As(III) efflux, and result in high resistance to arsenicals. Wolffia globosa is a strong arsenic accumulator with high potential for arsenic phytoremediation, which takes up As(III) more efficiently than As(V). Co-culture of P. putida ARS1 and W. globosa enhanced arsenic accumulation in W. globosa by 69%, and resulted in 91% removal of arsenic (at initial concentration of 0.6 mg/L As(V)) from water within 3 days. This study provides a promising strategy for in situ arsenic phytoremediation through the cooperation of plant and endophytic bacterium.

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.

The arsenic-lowering effect of inulin-type prebiotics in end-stage renal disease: a randomized crossover trial

Background: Circulatory imbalance of trace elements is frequent in end-stage renal disease (ESRD), leading to a deficiency of essential elements and excess of toxic elements. The present study aimed to investigate whether inulin-type fructans (ITFs) could ameliorate the circulatory imbalance by modulating gut microbiota and regulating the absorption and elimination of trace elements. Methods: Peritoneal dialysis patients were enrolled in a randomized crossover trial, undergoing interventions with ITFs (10 g d-1) and maltodextrin (placebo) over a 9-month period (with a 3-month washout). The primary outcomes included essential elements Mn, Fe, Co, Cu, Zn, Se, Sr, and Mo and potential toxic elements V, Cr, Ni, As, Cd, Ba, Tl, Pb, Th, and U in plasma. Secondary outcomes included the gut microbiome, short chain fatty acids (SCFAs), bile acids (BAs), and daily removal of trace elements through urine, dialysate and feces. Results: Among the 44 participants initially randomized, 29 completed the prebiotic, placebo or both interventions. The daily dietary intake of macronutrients and trace elements remained consistent throughout the study. The administration of 10 g d-1 ITFs significantly reduced plasma arsenic (As) by 1.03 μg L-1 (95%CI: -1.74, -0.33) (FDR-adjusted P = 0.045) down from the baseline of 3.54 μg L-1 (IQRs: 2.61-4.40) and increased the As clearance rate by urine and dialysis (P = 0.033). Positive changes in gut microbiota were also observed, including an increase in the Firmicutes/Bacteroidetes ratio (P = 0.050), a trend towards higher fecal SCFAs (P = 0.082), and elevated excretion of primary BAs (P = 0.035). However, there were no significant changes in plasma concentrations of other trace elements or their daily removal by urine, dialysis and feces. Conclusions: The daily administration of 10 g d-1 ITFs proved to be effective in reducing the circulating retention of As but demonstrated to be ineffective for other trace elements in ESRD. These sentences are ok to include but as "The clinical trial registry number is ChiCTR-INR-17013739 (https://www.chictr.org.cn/showproj.aspx?proj=21228)".

Risk assessment of toxic and hazardous metals in paddy agroecosystem by biochar-for bio-membrane applications

Toxic and carcinogenic metal (loid)s, such arsenic (As) and cadmium (Cd), found in contaminated paddy soils pose a serious danger to environmental sustainability. Their geochemical activities are complex, making it difficult to manage their contamination. Rice grown in Cd and As-polluted soils ends up in people's bellies, where it can cause cancer, anemia, and the deadly itai sickness. Solving this issue calls for research into eco-friendly and cost-effective remediation technology to lower rice's As and Cd levels. This research delves deeply into the origins of As and Cd in paddy soils, as well as their mobility, bioavailability, and uptake mechanisms by rice plants. It also examines the current methods and reactors used to lower As and Cd contamination in rice. Iron-modified biochar (Fe-BC) is a promising technology for reducing As and Cd toxicity in rice, improving soil health, and boosting rice's nutritional value. Biochar's physiochemical characteristics are enhanced by the addition of iron, making it a potent adsorbent for As and Cd ions. In conclusion, Fe-BC's biomembrane properties make them an attractive option for remediating As- and Cd-contaminated paddy soils. More efficient mitigation measures, including the use of biomembrane technology, can be developed when sustainable agriculture practices are combined with these technologies.

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.

Arsenate microbial reducing behavior regulated by the temperature fields in landfills

Attention should be paid to the As(V) reducing behavior in landfills under different temperature fields. In this study, microcosm tests were conducted using enrichment culture from a landfill. The results revealed that the reduction rate of As(V) was significantly affected by the temperature field, with the highest reduction rate observed at 50 °C, followed by 35 °C, 25 °C, and 10 °C. Different As cycling pathways were observed under various temperature fields. At room and medium temperatures, As4S4 was detected, indicating that both biomineralization and methylation processes occurred after As(V) reduction. However, only biogenic methylation was observed under high or low temperatures, indicating that the viability and adaptability of microorganisms varied depending on the temperature field and As contents. Pseudomonas was found to be the primary genus and dominant As(V) reduction bacteria (ARB) in all reactors. The study revealed that Pseudomonas accounted for a significant proportion of arsC genes, ranging from 87.29% to 97.59%, while arsCs genes were predominantly found in Bacillales and Closestridiales, with a contribution ranging from 89.17% to 96.59%. Interestingly, Bacillus and Clostridium were found to possess arsA genes in their metagenome-ssembled genome, resulting in a higher As(V) reducing rate under medium and high temperatures. These findings underscore the importance of temperature in modulating As(V) reducing behavior and As cycling, and could have implications for managing As pollution in landfill sites.

Biomineralization behavior and mechanism of microbial-mediated removal of arsenate from water

The microbial-mediated removal of arsenate by biomineralization received much attention, but the molecular mechanism of Arsenic (As) removal by mixed microbial populations remains to be elucidated. In this study, a process for the arsenate treatment using sulfate-reducing bacteria (SRB) containing sludge was constructed, and the performance of As removal was investigated at different molar ratios of AsO₄^3- to SO₄^2-. It was found that biomineralization mediated by SRB could achieve the simultaneous removal of arsenate and sulfate from wastewater but only occurred when microbial metabolic processes were involved. The reducing ability of the microorganisms for the sulfate and arsenate was equivalent, so the precipitates produced at the molar ratio of AsO₄^3- to SO₄^2-of 2:3 were most significant. X-ray absorption fine structure (XAFS) spectroscopy was the first time used to determine the molecular structure of the precipitates which were confirmed to be orpiment (As₂S₃). Combined with the metagenomics analysis, the microbial metabolism mechanism of simultaneous removal of sulfate and arsenate by the mixed microbial population containing SRB was revealed, that is, the sulfate and As(V) were reduced by microbial enzymes to produce S^2- and As(III) to further form As₂S₃ precipitates. This research provided a reference and theoretical foundation for the simultaneous removal of sulfate and arsenic mediated by SRB-containing sludge in wastewater treatment.

Ecological drivers and potential functions of viral communities in flooded arsenic-contaminated paddy soils

This work revealed the profile of viral communities in paddy soils with different levels of arsenic (As) contamination during the flooded period. The structure of viral communities differed significantly in highly and moderately As-contaminated soils. The diversity of soil viral communities under high As contamination decreased. Siphoviridae, Podoviridae, Myoviridae, and Microviridae were the dominant viral families in all samples, and the relative abundances of five of the top 20 viral genera were significantly different between highly and moderately As-contaminated groups. Seventeen dissimilatory As(V)-reducing bacteria were predicted to host 161 viral operational taxonomic units (vOTUs), mainly affiliated with the genera of Sulfurospirillum, Deferribacter, Bacillus and Fusibacter. Among them, 28 vOTUs were also associated with Fe(III)-reducing bacteria, which belonged to different species of the genus Shewanella. Procrustes analysis showed that the community structure of soil viruses was strongly correlated with both prokaryotic community structure and geochemical properties. Random forest analyses revealed that the Total-Fe, DCB-Fe and oxalate-Fe were the most significant variables on viral community richness, while the total-As concentration was an important factor on the Shannon index. Furthermore, As resistance genes (ArsC, ArsR and ArsD), As methylation genes (arsM) and As transporter genes (Pst and Pit) were identified among the auxiliary metabolic genes (AMGs) of the virome. This work revealed that the viruses might influence microbial adaptation in response to As-induced stress, and provided a perspective on the potential virus-mediated biogeochemical cycling of As.

Bacterial metal(loid) resistance genes (MRGs) and their variation and application in environment: A review

Toxic metal(loid)s are widespread and permanent in the biosphere, and bacteria have evolved a wide variety of metal(loid) resistance genes (MRGs) to resist the stress of excess metal(loid)s. Via active efflux, permeability barriers, extracellular/intracellular sequestration, enzymatic detoxification and reduction in metal(loid)s sensitivity of cellular targets, the key components of bacterial cells are protected from toxic metal(loid)s to maintain their normal physiological functions. Exploiting bacterial metal(loid) resistance mechanisms, MRGs have been applied in many environmental fields. Based on the specific binding ability of MRGs-encoded regulators to metal(loid)s, MRGs-dependent biosensors for monitoring environmental metal(loid)s are developed. MRGs-related biotechnologies have been applied to environmental remediation of metal(loid)s by using the metal(loid) tolerance, biotransformation, and biopassivation abilities of MRGs-carrying microorganisms. In this work, we review the historical evolution, resistance mechanisms, environmental variation, and environmental applications of bacterial MRGs. The potential hazards, unresolved problems, and future research directions are also discussed.

Removal of As(III) using a microorganism sustained secrete laccase-straw oxidation system

While laccase oxidation is a novel and promising method for treating arsenite-containing wastewater, the high cost and unsustainability of commercially available enzymes indicate a need to investigate more cost-effective viable alternatives. Here, a microorganism sustained secrete laccase-straw oxidation system (MLOS) was established and subsequently evaluated for the removal of As(III). MLOS showed efficient biological As(III) oxidation, with an As(III) removal efficiency reaching 99.9% at an initial As(III) concentration of 1.0 mg·L^-1. IC-AFS and XPS analysis showed that As(III) was partially oxidized to As(V), and partially As(III) adsorbed on the surface of rice straw. FTIR analysis revealed that hydroxyl, amine and amide groups were all involved in the As(III) removal process. SEM-EDS demonstrated that the surface structure of rice straw was destroyed following Comamonas testosteroni FJ17 (C. testosteroni FJ17) treatment, and the metal ions binding sites of rice straw were increased resulting in elemental arsenic being detected on the material surface. Molecular docking revealed the interaction between key residues of laccase and As(III). Laccase activity was negatively correlated with Cu(II) concentration in the As(III) oxidation. EEM showed that humic-like acids were also involved in the interaction with As(III). Overall, a MLOS derived from biomass waste has a significant potential to be developed as a green and sustainable technology for the treatment of wastewater containing As(III).

Proteomic Analysis of Arsenic Resistance during Cyanide Assimilation by Pseudomonas pseudoalcaligenes CECT 5344

Wastewater from mining and other industries usually contains arsenic and cyanide, two highly toxic pollutants, thereby creating the need to develop bioremediation strategies. Here, molecular mechanisms triggered by the simultaneous presence of cyanide and arsenite were analyzed by quantitative proteomics, complemented with qRT-PCR analysis and determination of analytes in the cyanide-assimilating bacterium Pseudomonas pseudoalcaligenes CECT 5344. Several proteins encoded by two ars gene clusters and other Ars-related proteins were up-regulated by arsenite, even during cyanide assimilation. Although some proteins encoded by the cio gene cluster responsible for cyanide-insensitive respiration decreased in the presence of arsenite, the nitrilase NitC required for cyanide assimilation was unaffected, thus allowing bacterial growth with cyanide and arsenic. Two complementary As-resistance mechanisms were developed in this bacterium, the extrusion of As(III) and its extracellular sequestration in biofilm, whose synthesis increased in the presence of arsenite, and the formation of organoarsenicals such as arseno-phosphoglycerate and methyl-As. Tetrahydrofolate metabolism was also stimulated by arsenite. In addition, the ArsH2 protein increased in the presence of arsenite or cyanide, suggesting its role in the protection from oxidative stress caused by both toxics. These results could be useful for the development of bioremediation strategies for industrial wastes co-contaminated with cyanide and arsenic.

Biotechnology Advances in Bioremediation of Arsenic: A Review

Arsenic is a highly toxic metalloid widespread in the Earth's crust, and its contamination due to different anthropogenic activities (application of agrochemicals, mining, waste management) represents an emerging environmental issue. Therefore, different sustainable and effective remediation methods and approaches are needed to prevent and protect humans and other organisms from detrimental arsenic exposure. Among numerous arsenic remediation methods, those supported by using microbes as sorbents (microbial remediation), and/or plants as green factories (phytoremediation) are considered as cost-effective and environmentally-friendly bioremediation. In addition, recent advances in genetic modifications and biotechnology have been used to develop (i) more efficient transgenic microbes and plants that can (hyper)accumulate or detoxify arsenic, and (ii) novel organo-mineral materials for more efficient arsenic remediation. In this review, the most recent insights from arsenic bio-/phytoremediation are presented, and the most relevant physiological and molecular mechanisms involved in arsenic biological routes, which can be useful starting points in the creation of more arsenic-tolerant microbes and plants, as well as their symbiotic associations are discussed.

Arsenic Exposure Causes Global Changes in the Metalloproteome of Escherichia coli

Arsenic is a toxic metalloid with differential biological effects, depending on speciation and concentration. Trivalent arsenic (arsenite, AsIII) is more toxic at lower concentrations than the pentavalent form (arsenate, AsV). In E. coli, the proteins encoded by the arsRBC operon are the major arsenic detoxification mechanism. Our previous transcriptional analyses indicate broad changes in metal uptake and regulation upon arsenic exposure. Currently, it is not known how arsenic exposure impacts the cellular distribution of other metals. This study examines the metalloproteome of E. coli strains with and without the arsRBC operon in response to sublethal doses of AsIII and AsV. Size exclusion chromatography coupled with inductively coupled plasma mass spectrometry (SEC-ICPMS) was used to investigate the distribution of five metals (56Fe, 24Mg, 66Zn, 75As, and 63Cu) in proteins and protein complexes under native conditions. Parallel analysis by SEC-UV-Vis spectroscopy monitored the presence of protein cofactors. Together, these data reveal global changes in the metalloproteome, proteome, protein cofactors, and soluble intracellular metal pools in response to arsenic stress in E. coli. This work brings to light one outcome of metal exposure and suggests that metal toxicity on the cellular level arises from direct and indirect effects.

Biogeochemical behavior and pollution control of arsenic in mining areas: A review

Arsenic (As) is one of the most toxic metalloids that possess many forms. As is constantly migrating from abandoned mining area to the surrounding environment in both oxidation and reducing conditions, threatening human health and ecological safety. The biogeochemical reaction of As included oxidation, reduction, methylation, and demethylation, which is closely associated with microbial metabolisms. The study of the geochemical behavior of arsenic in mining areas and the microbial remediation of arsenic pollution have great potential and are hot spots for the prevention and remediation of arsenic pollution. In this study, we review the distribution and migration of arsenic in the mining area, focus on the geochemical cycle of arsenic under the action of microorganisms, and summarize the factors influencing the biogeochemical cycle of arsenic, and strategies for arsenic pollution in mining areas are also discussed. Finally, the problems of the risk control strategies and the future development direction are prospected.

Arsenic perception and signaling: The yet unexplored world

Arsenic is one of the most potent carcinogens in the biosphere, jeopardizing the health of millions of people due to its entrance into the human food chain through arsenic-contaminated waters and staple crops, particularly rice. Although the mechanisms of arsenic sensing are widely known in yeast and bacteria, scientific evidence concerning arsenic sensors or components of early arsenic signaling in plants is still in its infancy. However, in recent years, we have gained understanding of the mechanisms involved in arsenic uptake and detoxification in different plant species and started to get insights into arsenic perception and signaling, which allows us to glimpse the possibility to design effective strategies to prevent arsenic accumulation in edible crops or to increase plant arsenic extraction for phytoremediation purposes. In this context, it has been recently described a mechanism according to which arsenite, the reduced form of arsenic, regulates the arsenate/phosphate transporter, consistent with the idea that arsenite functions as a selective signal that coordinates arsenate uptake with detoxification mechanisms. Additionally, several transcriptional and post-translational regulators, miRNAs and phytohormones involved in arsenic signaling and tolerance have been identified. On the other hand, studies concerning the developmental programs triggered to adapt root architecture in order to cope with arsenic toxicity are just starting to be disclosed. In this review, we compile and analyze the latest advances toward understanding how plants perceive arsenic and coordinate its acquisition with detoxification mechanisms and root developmental programs.

Lysinibacillus sphaericus mediates stress responses and attenuates arsenic toxicity in Caenorhabditis elegans

Exposure to toxic metals alters host response and that leads to disease development. Studies have revealed the effects of metals on microbial physiology, however, the role of metal resistant bacteria on host response to metals is unclear. The hypothesis that xenobiotic interactions between gut microbes and arsenic influence the host physiology and toxicity was assessed in a Caenorhabditis elegans model. The arsenic-resistant Lysinibacillus sphaericus B1CDA was fed to C. elegans to determine the host responses to arsenic in comparison to Escherichia coli OP50 food. L. sphaericus diet extended C. elegans lifespan compared to E. coli diet, with an increased expression of genes involved in lifespan, stress response and immunity (hif-1, hsp-16.2, mtl-2, abf-2, clec-60), as well as reduced fat accumulation. Arsenic-exposed worms fed L. sphaericus also had a longer lifespan than those fed E. coli and had an increased expression of genes involved in cytoprotection, stress resistance (mtl-1, mtl-2) and oxidative stress response (cyp-35A2, isp-1, ctl-2, sod-1), together with a decreased accumulation of reactive oxygen species (ROS). In comparison with E. coli, L. sphaericus B1CDA diet increased C. elegans fitness while detoxifying arsenic induced ROS and extending lifespan.

Transcriptome analysis of an arsenite-/antimonite-oxidizer, Bosea sp. AS-1 reveals the importance of the type 4 secretion system in antimony resistance

Bosea sp. AS-1 is an arsenite [As(III)] and antimonite [Sb(III)] oxidizer previously isolated by our group from the Xikuangshan Antimony (Sb) Mine area. Our previous study showed that Bosea sp. AS-1 had a preference for oxidizing As(III) or Sb(III) with different carbon sources, which suggested that different metabolic mechanisms may be utilized by the bacteria to survive in As(III)- or Sb(III)-contaminated environments. Here, we conducted whole-genome and transcriptome sequencing to reveal the molecular mechanisms utilized by Bosea sp. AS-1 to resist As(III) or Sb(III). We discovered that AS-1 acquired various As- and Sb-resistant genes in its genome and might resist As(III) or Sb(III) through the regulation of multiple pathways, such as As and Sb metabolism, the bacterial secretion system, oxidative phosphorylation, the TCA cycle and bacterial flagellar motility. Interestingly, we discovered that genes of the type IV secretion system (T4SS) were activated in response to Sb(III), and inhibiting T4SS activity in AS-1 dramatically reduced its oxidation efficiency and tolerance to Sb(III). To our knowledge, this is the first study showing the activation of T4SS genes by Sb and a direct involvement of T4SS in bacterial Sb resistance. Our findings establish the T4SS as an important Sb resistance factor in bacteria and may help us understand the spread of Sb resistance genes in the environment.

Arsenic Release from Soil Induced by Microorganisms and Environmental Factors

In rhizospheric soil, arsenic can be activated by both biological and abiotic reactions with plant exudates or phosphates, but little is known about the relative contributions of these two pathways. The effects of microorganisms, low-molecular-weight organic acid salts (LMWOASs), and phosphates on the migration of As in unrestored and nano zero-valent iron (nZVI)-restored soil were studied in batch experiments. The results show that As released by microbial action accounted for 17.73%, 7.04%, 92.40%, 92.55%, and 96.68% of the total As released in unrestored soil with citrate, phytate, malate, lactate, and acetate, respectively. It was only suppressed in unrestored soil with oxalate. In restored soil, As was still released in the presence of oxalate, citrate, and phytate, but the magnitude of As release was inhibited by microorganisms. The application of excess nZVI can completely inhibited As release processes induced by phosphate in the presence of microorganisms. Microbial iron reduction is a possible mechanism of arsenic release induced by microorganisms. Microorganisms and most environmental factors promoted As release in unrestored soil, but the phenomenon was suppressed in restored soil. This study helps to provide an effective strategy for reducing the secondary release of As from soils due to replanting after restoration.

Molecular insight into arsenic uptake, transport, phytotoxicity, and defense responses in plants: a critical review

A critical investigation into arsenic uptake and transportation, its phytotoxic effects, and defense strategies including complex signaling cascades and regulatory networks in plants. The metalloid arsenic (As) is a leading pollutant of soil and water. It easily finds its way into the food chain through plants, more precisely crops, a common diet source for humans resulting in serious health risks. Prolonged As exposure causes detrimental effects in plants and is diaphanously observed through numerous physiological, biochemical, and molecular attributes. Different inorganic and organic As species enter into the plant system via a variety of transporters e.g., phosphate transporters, aquaporins, etc. Therefore, plants tend to accumulate elevated levels of As which leads to severe phytotoxic damages including anomalies in biomolecules like protein, lipid, and DNA. To combat this, plants employ quite a few mitigation strategies such as efficient As efflux from the cell, iron plaque formation, regulation of As transporters, and intracellular chelation with an array of thiol-rich molecules such as phytochelatin, glutathione, and metallothionein followed by vacuolar compartmentalization of As through various vacuolar transporters. Moreover, the antioxidant machinery is also implicated to nullify the perilous outcomes of the metalloid. The stress ascribed by the metalloid also marks the commencement of multiple signaling cascades. This whole complicated system is indeed controlled by several transcription factors and microRNAs. This review aims to understand, in general, the plant-soil-arsenic interaction, effects of As in plants, As uptake mechanisms and its dynamics, and multifarious As detoxification mechanisms in plants. A major portion of this article is also devoted to understanding and deciphering the nexus between As stress-responsive mechanisms and its underlying complex interconnected regulatory networks.

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.

Global genomic analysis of microbial biotransformation of arsenic highlights the importance of arsenic methylation in environmental and human microbiomes

Arsenic is a ubiquitous toxic element, the global cycle of which is highly affected by microbial redox reactions and assimilation into organoarsenic compounds through sequential methylation reactions. While microbial biotransformation of arsenic has been studied for decades, the past years have seen the discovery of multiple new genes related to arsenic metabolism. Still, most studies focus on a small set of key genes or a small set of cultured microorganisms. Here, we leveraged the recently greatly expanded availability of microbial genomes of diverse organisms from lineages lacking cultivated representatives, including those reconstructed from metagenomes, to investigate genetic repertoires of taxonomic and environmental controls on arsenic metabolic capacities. Based on the collection of arsenic-related genes, we identified thirteen distinct metabolic guilds, four of which combine the aio and ars operons. We found that the best studied phyla have very different combinations of capacities than less well-studied phyla, including phyla lacking isolated representatives. We identified a distinct arsenic gene signature in the microbiomes of humans exposed or likely exposed to drinking water contaminated by arsenic and that arsenic methylation is important in soil and in human microbiomes. Thus, the microbiomes of humans exposed to arsenic have the potential to exacerbate arsenic toxicity. Finally, we show that machine learning can predict bacterial arsenic metabolism capacities based on their taxonomy and the environment from which they were sampled.

Microalgal bioremediation of heavy metal pollution in water: Recent advances, challenges, and prospects

With the rapid economy development and population surge, the water resources available for direct use on the earth have been in shortage. Therefore, water pollution remediation inevitably becomes the focus of global attention. Aside from their capacity to fix and effectively control the emission of carbon dioxide thus achieve negative carbon emission, microalgae and its products modified by genetic engineering and other technologies also have a broad prospect in sewage treatment such as efficiently removing all kinds of pollutants in water and producing high-quality biofuels after use. Therefore, research on these organisms has gradually deepened in recent years. This paper summarizes the bioremediation mechanism of heavy metal ions in water by using microalgae and their modified products. The relevant research progresses since 2015 are critically reviewed and discussed. Challenges and prospects are also put forward for their industrial implementation.

Phosphorus-arsenic interaction in the 'soil-plant-microbe' system and its influence on arsenic pollution

Elevated arsenic (As) in soil is of public concern due to the carcinogenicity. Phosphorus (P) strongly influences the adsorption, absorption, transport, and transformation of As in the soil and in organisms due to the similarity of the chemical properties of P and As. In soil, P, particularly inorganic P, can release soil-retained As (mostly arsenate) by competing for adsorption sites. In plant and microbial systems, P usually reduces As (mainly arsenate) uptake and affects As biotransformation by competing for As transporters. The intensity and pattern of PAs interaction are highly dependent on the forms of As and P, and strongly influenced by various biotic and abiotic factors. An understanding of the PAs interaction in 'soil-plant-microbe' systems is of great value to prevent soil As from entering the human food chain. Here, we review PAs interactions and the main influential factors in soil, plant, and microbial subsystems and their effects on the As release, absorption, transformation, and transport in the 'soil-plant-microbe' system. We also analyze the application potential of P fertilization as a control for As pollution and suggest the research directions that need to be followed in the future.

Arsenic biotransformation and mobilization: the role of bacterial strains and other environmental variables

Over several decades, arsenic (As) toxicity in the biosphere has affected different flora, fauna, and other environmental components. The majority of these problems are linked with As mobilization due to bacterial dissolution of As-bearing minerals and its transformation in other reservoirs such as soil, sediments, and ground water. Understanding the process, mechanism, and various bacterial species involved in these processes under the influence of some ecological variables greatly contributes to a better understanding of the fate and implications of As mobilization into the environments. This article summarizes the process, role, and various types of bacterial species involved in the transformation and mobilization of As. Furthermore, insight into how Fe(II) oxidation and resistance mechanisms such as methylation and detoxification against the toxic effect of As(III) was highlighted as a potential immobilization and remediation strategy in As-contaminated sites. Furthermore, the significance and comparative advantages of some useful analytical tools used in the evaluation, speciation, and analysis of As are discussed and how their in situ and ex situ applications support assessing As contamination in both laboratory and field settings. Nevertheless, additional research involving advanced molecular techniques is required to elaborate on the contribution of these bacterial consortia as a potential agronomic tool for reducing As availability, particularly in natural circumstances. Graphical abstract. Courtesy of conceptual model: Aminu Darma.

Isolation, Identification, and Analysis of Potential Functions of Culturable Bacteria Associated with an Invasive Gall Wasp, Leptocybe invasa

Symbioses between invasive insects and bacteria are one of the key drivers of insect invasion success. Gall-inducing insects stimulate host plants to produce galls, which affects the normal growth of plants. Leptocybe invasa Fisher et La Salle, an invasive gall-inducing wasp, mainly damages Eucalyptus plantations in Southern China, but little is known about its associated bacteria. The aim of this study was to assess the diversity of bacterial communities at different developmental stages of L. invasa and to identify possible ecological functions of the associated bacteria. Bacteria associated with L. invasa were isolated using culture-dependent methods and their taxonomic statuses were determined by sequencing the 16S rRNA gene. A total of 88 species belonging to four phyla, 27 families, and 44 genera were identified by phylogenetic analysis. The four phyla were Proteobacteria, Actinobacteria, Firmicutes, and Bacteroidetes, mainly from the genera Pantoea, Enterobacter, Pseudomonas, Bacillus, Acinetobacter, Curtobacterium, Sphingobium, Klebsiella, and Rhizobium. Among them, 72 species were isolated in the insect gall stage and 46 species were isolated from the adult stage. The most abundant bacterial species were γ-Proteobacteria. We found significant differences in total bacterial counts and community compositions at different developmental stages, and identified possible ecological roles of L. invasa-associated bacteria. This study is the first to systematically investigate the associated bacteria of L. invasa using culture-dependent methods, and provides a reference for other gall-inducing insects and associated bacteria.

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.