Microbiology of inorganic arsenic: From metabolism to bioremediation

Isolation of a potentially arsenic-resistant Halomonas elongata strain (ml10562) from hypersaline systems in the Peruvian Andes, Cusco

Halomonas elongata strain ml10562, was isolated from hypersaline that was collected from Acos Peru. Average Nucleotide Identity (ANI) and dDDH (digital DNA-DNA Hybridization) values between strain ml10562 and type strains of Halomonas elongata species were 71.0-78.4% and 18.8-21.5%, respectively. The draft genome, spanning 4,075,440 base pairs, has a GC content of 64.2% and contains 3,912 genes. Functional characterization revealed the strain's ability to tolerate and resist increasing concentrations of sodium arsenate, with a minimum inhibitory concentration of 25 mM. Bioinformatic analysis revealed the presence of two operons, arsR-arsH-arsB and arsJ-gapdh-arsC, in the genome of strain ml10562, which could play a crucial role in arsenic resistance through transporter-mediated mechanisms. Overall, these results emphasize the potential adaptability of H. elongata ml10562 to arsenic-containing environments and extend our understanding of bacterial arsenic resistance mechanisms, allowing promising applications in bioremediation.

Advances and insights into modeling extracellular electron transfer in anaerobic bioprocesses

Extracellular electron transfer (EET) plays an important role in maintaining redox balance in both natural and engineered anaerobic microbial systems, driving key biochemical processes such as energy generation, bioremediation, and waste degradation. While EET has been characterized in a limited number of microbes and applied in anaerobic digestion and bioelectrochemical systems, further research is needed to explore its mechanism across a broader range of microbial species and anaerobic processes. This review highlights advanced modeling frameworks that provide deeper insights into EET mechanisms and dynamics, aiming to optimize research efforts and minimize time and resource expenditure. Mechanistic models, encompassing thermodynamics and kinetics, are discussed for their utility in calculating conduction rates of electroactive microbes and assessing the energetics of medium chain carboxylic acids production. Genome-scale metabolic models are highlighted for elucidating the roles of cytochromes and conductive pili in the EET pathway. Machine learning is presented as a tool to improve model accuracy and predict EET mechanisms. Furthermore, the integration of quantum mechanics/molecular mechanics methods offers molecular-level insights into electron transfer, while quantum computing addresses limitations of classical computers by simulating complex electron transfer processes in multi-heme cytochromes. Developing advanced modeling techniques will complement experimental techniques, enabling precise predictions and optimization strategies for developing innovative and sustainable anaerobic biotechnologies.

Reviewing arsenic biomineralization: An upcoming strategy for mining wastewater treatment

Human activities are the main cause of arsenic contamination in the environment and water resources, being the mining industry an important source of arsenic contamination because this element is released into the environment in solid, liquid, and gaseous wastes. Currently, several physical and chemical processes could be used for the removal of arsenic in water, but these alternatives depend on the concentration of arsenic. At low concentrations (nanograms or micrograms per liter) arsenic can be removed by membrane technologies. When arsenic is at high concentrations (milligrams or grams per liter), treatment options are reduced to inefficient processes of high economic cost and poor chemical stability of the precipitate, returning consequently arsenic into the environment. Biomineralization is a biological process where microorganisms induce the formation of minerals. This bioprocess has gained interest in recent years for the removal of contaminants from liquid effluents. This review details the harmful effects of arsenic on the health and exposes the relevance of arsenic contamination related to mining activity, whose effluents contain high concentration of arsenic. It also describes and analyzes advances in arsenic treatment strategies through biomineralization using microorganisms, such as sulfate-reducing bacteria, iron- and manganese-oxidizing microorganisms, and ureolytic microorganisms, detailing aspects of effectiveness, applicability, chemical stability of biominerals and future perspectives in their industrial application. To our knowledge, there are no previous reports compiling, analyzing, and explaining in detail the biomineralization of arsenic as a single element. The importance of this review is to deliver in a summarized and systematized way the main aspects and perspectives on the application of microorganisms to remove toxic elements, such as arsenic, from effluents.

Migration, transformation of arsenic, and pollution controlling strategies in paddy soil-rice system: A comprehensive review

Arsenic pollution in paddy fields has become a public concern by seriously threatening rice growth, food security and human health. In this review, we delve into the biogeochemical behaviors of arsenic in paddy soil-rice system, systemically revealing the complexity of its migration and transformation processes, including the release of arsenic from soil to porewater, uptake and translocation of arsenic by rice plants, as well as transformation of arsenic species mediated by microorganism. Especially, microbial processes like reduction, oxidation and methylation of arsenic, and the coupling of arsenic with carbon, iron, sulfur, nitrogen cycling through microbes and related mechanisms were highlighted. Environmental factors like pH, redox potential, organic matter, minerals, nutrient elements, microorganisms and periphyton significantly influence these processes through different pathways, which are discussed in this review. Furthermore, the current progress in remediation strategies, including agricultural interventions, passivation, phytoremediation and microbial remediation is explored, and their potential and limitations are analyzed to address the gaps. This review offers comprehensive perspectives on the complicated behaviors of arsenic and influence factors in paddy soil-rice system, and provides a scientific basis for developing effective arsenic pollution control strategies.

Various microbial taxa couple arsenic transformation to nitrogen and carbon cycling in paddy soils

BACKGROUND

Arsenic (As) metabolism pathways and their coupling to nitrogen (N) and carbon (C) cycling contribute to elemental biogeochemical cycling. However, how whole-microbial communities respond to As stress and which taxa are the predominant As-transforming bacteria or archaea in situ remains unclear. Hence, by constructing and applying ROCker profiles to precisely detect and quantify As oxidation (aioA, arxA) and reduction (arrA, arsC1, arsC2) genes in short-read metagenomic and metatranscriptomic datasets, we investigated the dominant microbial communities involved in arsenite (As(III)) oxidation and arsenate (As(V)) reduction and revealed their potential pathways for coupling As with N and C in situ in rice paddies.

RESULTS

Five ROCker models were constructed to quantify the abundance and transcriptional activity of short-read sequences encoding As oxidation (aioA and arxA) and reduction (arrA, arsC1, arsC2) genes in paddy soils. Our results revealed that the sub-communities carrying the aioA and arsC2 genes were predominantly responsible for As(III) oxidation and As(V) reduction, respectively. Moreover, a newly identified As(III) oxidation gene, arxA, was detected in genomes assigned to various phyla and showed significantly increased transcriptional activity with increasing soil pH, indicating its important role in As(III) oxidation in alkaline soils. The significant correlation of the transcriptional activities of aioA with the narG and nirK denitrification genes, of arxA with the napA and nirS denitrification genes and of arrA/arsC2 with the pmoA and mcrA genes implied the coupling of As(III) oxidation with denitrification and As(V) reduction with methane oxidation. Various microbial taxa including Burkholderiales, Desulfatiglandales, and Hyphomicrobiales (formerly Rhizobiales) are involved in the coupling of As with N and C metabolism processes. Moreover, these correlated As and N/C genes often co-occur in the same genome and exhibit greater transcriptional activity in paddy soils with As contamination than in those without contamination.

CONCLUSIONS

Our results revealed the comprehensive detection and typing of short-read sequences associated with As oxidation and reduction genes via custom-built ROCker models, and shed light on the various microbial taxa involved in the coupling of As and N and C metabolism in situ in paddy soils. The contribution of the arxA sub-communities to the coupling of As(III) oxidation with nitrate reduction and the arsC sub-communities to the coupling of As(V) reduction with methane oxidation expands our knowledge of the interrelationships among As, N, and C cycling in paddy soils. Video Abstract.

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.

Detoxification of ars genotypes by arsenite-oxidizing bacteria through arsenic biotransformation

The detoxification process of transforming arsenite (As(III)) to arsenate (As(V)) through bacterial oxidation presents a potent approach for bioremediation of arsenic-polluted soils in abandoned mines. In this study, twelve indigenous arsenic-oxidizing bacteria (AOB) were isolated from arsenic-contaminated soils. Among these, Paenibacillus xylanexedens EBC-SK As2 (MF928871) and Ochrobactrum anthropi EBC-SK As11 (MF928880) were identified as the most effective arsenic-oxidizing isolates. Evaluations for bacterial arsenic resistance demonstrated that P. xylanexedens EBC-SK As2 (MF928871) could resist As(III) up to 40 mM, while O. anthropi EBC-SK As11 (MF928880) could resist As(III) up to 25 mM. From these bacterial strains, genotypes of arsenic resistance system (ars) were detected, encompassing ars leader genes (arsR and arsD), membrane genes (arsB and arsJ), and aox genes known to be crucial for arsenic detoxification. These ars genotypes in the isolated AOBs might play an instrumental role in arsenic-contaminated soils with potential to reduce arsenic contamination.

Leveraging the One Health concept for arsenic sustainability

Arsenic (As) is a naturally occurring chemical element widely distributed in the Earth's crust. Human activities have significantly altered As presence in the environment, posing significant threats to the biota as well as human health. The environmental fates and adverse outcomes of As of various species have been extensively studied in the past few decades. It is imperative to summarize these advances as a whole to provide more profound insights into the As cycle for sustainable development. Embracing the One Health concept, we systematically reviewed previous studies in this work and explored the following three fundamental questions, i.e., what the trends and associated changes are in As contamination, how living organisms interact and cope with As contamination, and most importantly what to do to achieve a sustainable future with As. By focusing on one critical question in each section, this review aims to provide a full picture of the complexity of environmental As. To tackle the significant research challenges and gaps in As pollution and mitigation, we further proposed a One Health framework with potential coping strategies, guiding a coordinated agenda on dealing with legacy As in the environment and ensuring a sustainable As future.

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.

Metagenomic and culture-dependent approaches unveil active microbial community and novel functional genes involved in arsenic mobilization and detoxification in groundwater

BACKGROUND

Arsenic (As) and its species are major pollutants in ecological bodied including groundwater in Bangladesh rendering serious public health concern. Bacteria with arsenotrophic genes have been found in the aquifer, converting toxic arsenite [As (III)] to less toxic arsenate [As (V)] that is easily removed using chemical and biological trappers. In this study, genomic and metagenomic approaches parallel to culture-based assay (Graphical abstract) have made it possible to decipher phylogenetic diversity of groundwater arsenotrophic microbiomes along with elucidation of their genetic determinants.

RESULTS

Seventy-two isolates were retrieved from six As-contaminated (average As concentration of 0.23 mg/L) groundwater samples from Munshiganj and Chandpur districts of Bangladesh. Twenty-three isolates harbored arsenite efflux pump (arsB) gene with high abundance, and ten isolates possessing arsenite oxidase (aioA) gene, with a wide range of minimum inhibitory concentration, MICAs (2 to 32 mM), confirming their role in arsenite metabolism. There was considerable heterogeneity in species richness and microbial community structure. Microbial taxa from Proteobacteria, Firmicutes and Acidobacteria dominated these diversities. Through these combinatorial approaches, we have identified potential candidates such as, Pseudomonas, Acinetobacter, Stenotrophomonas, Achromobacter, Paraburkholderia, Comamonas and Klebsiella and associated functional genes (arsB, acr3, arsD, arsH, arsR) that could significantly contribute to arsenite detoxification, accumulation, and immobilization.

CONCLUSIONS

Culture-dependent and -independent shotgun metagenomic investigation elucidated arsenotrophic microbiomes and their functions in As biogeochemical transformation. These findings laid a foundation for further large-scale researches on the arsenotrophic microbiomes and their concurrent functions in As biogeochemical transformation in As-contaminated areas of Bangladesh and beyond.

Microbial biochemical pathways of arsenic biotransformation and their application for bioremediation

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

Arsenic release dynamics of paddy field soil during groundwater irrigation and natural flooding

Irrigation water in rice cultivation significantly affects the arsenic (As) mobilization in the paddy field soil. This research assessed the effect of rainwater (RW) and groundwater (GW) on the dissolution dynamics of arsenic (As) in paddy field soil. Up-flow column flooding experiments were conducted continuously for 80 d with simulated RW and GW to evaluate As dissolution phenomena in actual field conditions. Arsenic dissolution from the soil was lower in GW (309 μg/kg) irrigation than in RW flooding conditions (1086 μg/kg). The redox potential (Eh) of the soil pore water decreased, and pH increased over-irrigation time in both flooding conditions. The dissolution of arsenic (As) and iron (Fe) in the soil pore increased, while the dissolution of manganese (Mn) decreased over flooding time. The release of As in the soil pore water was attributed to the dissolution of Fe-As and Mn-As minerals and microbial reduction of As. Fe-As dissolution ratios in the soil pore water were relatively low and estimated as 0.68 mol/mol and 4.9 mol/mol for RW and GW, respectively. The dissolution of As and Mn dominated in the initial phase (0-40 d) of flooding, while the dissolution of As and Fe dominated in the second phase (40-80 d). The release of As was much lower in GW flooding than in RW flooding conditions. The Presence of Ca, Mg, and Mn in the GW facilitated the reduction of As dissolution by precipitating Ca-As and Mg-As and the oxidizing dissolved Mn in the soil pore water. The findings of this study provide valuable insights into the mechanisms of As release during monsoon flooding and groundwater flooding to assess the potential risks of As contamination in rice grown in paddy field soils.

Negative Impacts of Arsenic on Plants and Mitigation Strategies

Arsenic (As) is a metalloid prevalent mainly in soil and water. The presence of As above permissible levels becomes toxic and detrimental to living organisms, therefore, making it a significant global concern. Humans can absorb As through drinking polluted water and consuming As-contaminated food material grown in soil having As problems. Since human beings are mobile organisms, they can use clean uncontaminated water and food found through various channels or switch from an As-contaminated area to a clean area; but plants are sessile and obtain As along with essential minerals and water through roots that make them more susceptible to arsenic poisoning and consequent stress. Arsenic and phosphorus have many similarities in terms of their physical and chemical characteristics, and they commonly compete to cause physiological anomalies in biological systems that contribute to further stress. Initial indicators of arsenic's propensity to induce toxicity in plants are a decrease in yield and a loss in plant biomass. This is accompanied by considerable physiological alterations; including instant oxidative surge; followed by essential biomolecule oxidation. These variables ultimately result in cell permeability and an electrolyte imbalance. In addition, arsenic disturbs the nucleic acids, the transcription process, and the essential enzymes engaged with the plant system's primary metabolic pathways. To lessen As absorption by plants, a variety of mitigation strategies have been proposed which include agronomic practices, plant breeding, genetic manipulation, computer-aided modeling, biochemical techniques, and the altering of human approaches regarding consumption and pollution, and in these ways, increased awareness may be generated. These mitigation strategies will further help in ensuring good health, food security, and environmental sustainability. This article summarises the nature of the impact of arsenic on plants, the physio-biochemical mechanisms evolved to cope with As stress, and the mitigation measures that can be employed to eliminate the negative effects of As.

Moving towards the enhancement of extracellular electron transfer in electrogens

Electrogens are very common in nature and becoming a contemporary theme for research as they can be exploited for extracellular electron transfer. Extracellular electron transfer is the key mechanism behind bioelectricity generation and bioremediation of pollutants via microbes. Extracellular electron transfer mechanisms for electrogens other than Shewanella and Geobacter are less explored. An efficient extracellular electron transfer system is crucial for the sustainable future of bioelectrochemical systems. At present, the poor extracellular electron transfer efficiency remains a decisive factor in limiting the development of efficient bioelectrochemical systems. In this review article, the EET mechanisms in different electrogens (bacteria and yeast) have been focused. Apart from the well-known electron transfer mechanisms of Shewanella oneidensis and Geobacter metallireducens, a brief introduction of the EET pathway in Rhodopseudomonas palustris TIE-1, Sideroxydans lithotrophicus ES-1, Thermincola potens JR, Lysinibacillus varians GY32, Carboxydothermus ferrireducens, Enterococcus faecalis and Saccharomyces cerevisiae have been included. In addition to this, the article discusses the several approaches to anode modification and genetic engineering that may be used in order to increase the rate of extracellular electron transfer. In the side lines, this review includes the engagement of the electrogens for different applications followed by the future perspective of efficient extracellular electron transfer.

Comprehensive characterization of aerobic groundwater biotreatment media

Aerobic biotreatment systems can treat multiple reduced inorganic contaminants in groundwater, including ammonia (NH₃), arsenic (As), iron (Fe), and manganese (Mn). While individual systems treating multiple contaminants simultaneously have been characterized and several systems treating one contaminant have been compared, a comparison of systems treating co-occurring contaminants is lacking. This study assessed the treatment performance and microbial communities within 7 pilot- and full-scale groundwater biotreatment systems in the United States that treated waters with pH 5.6-7.8, 0.1-2.0 mg/L dissolved oxygen, 75-376 mg CaCO₃/L alkalinity, < 0.03-3.79 mg NH₃-N/L, < 4-31 µg As/L, < 0.01-9.37 mg Fe/L, 2-1220 µg Mn/L, and 0.1-5.6 mg/L total organic carbon (TOC). Different reactor configurations and media types were represented, allowing for a broad assessment of linkages between water quality and microbial communities via microscopy, biofilm quantification, and molecular methods. Influent NH₃, TOC, and pH contributed to differences in the microbial communities. Mn oxidase gene copy numbers were slightly negatively correlated with the influent Mn concentration, but no significant relationships between gene copy number and influent concentration were observed for the other contaminants. Extracellular enzyme activities, community composition, and carbon transformation pathways suggested heterotrophic bacteria may be important in nitrifying biofilters. Aerobic groundwater biofilters are complex, and improved understanding could lead to engineering enhancements.

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 Pollution and Anaerobic Arsenic Metabolizing Bacteria in Lake Van, the World's Largest Soda Lake

Arsenic is responsible for water pollution in many places around the world and presents a serious health risk for people. Lake Van is the world's largest soda lake, and there are no studies on seasonal arsenic pollution and arsenic-resistant bacteria. We aimed to determine the amount of arsenic in the lake water and sediment, to isolate arsenic-metabolizing anaerobic bacteria and their identification, and determination of arsenic metabolism. Sampling was done from 7.5 m to represent the four seasons. Metal contents were determined by using ICP-MS. Pure cultures were obtained using the Hungate technique. Growth characteristics of the strains were determined at different conditions as well as at arsenate and arsenite concentrations. Molecular studies were also carried out for various resistance genes. Our results showed that Lake Van's total arsenic amount changes seasonally. As a result of 16S rRNA sequencing, it was determined that the isolates were members of 8 genera with arsC resistance genes. In conclusion, to sustain water resources, it is necessary to prevent chemical and microorganism-based pollution. It is thought that the arsenic-resistant bacteria obtained as a result of this study will contribute to the solution of environmental arsenic pollution problems, as they are the first data and provide the necessary basic data for the bioremediation studies of arsenic from contaminated environmental habitats. At the same time, the first data that will contribute to the creation of the seasonal arsenic map of Lake Van are obtained.

Winogradsky Bioelectrochemical System as a Novel Strategy to Enrich Electrochemically Active Microorganisms from Arsenic-Rich Sediments

Bioelectrochemical systems (BESs) have been extensively studied for treatment and remediation. However, BESs have the potential to be used for the enrichment of microorganisms that could replace their natural electron donor or acceptor for an electrode. In this study, Winogradsky BES columns with As-rich sediments extracted from an Andean watershed were used as a strategy to enrich lithotrophic electrochemically active microorganisms (EAMs) on electrodes (i.e., cathodes). After 15 months, Winogradsky BESs registered power densities up to 650 μWcm^-2. Scanning electron microscopy and linear sweep voltammetry confirmed microbial growth and electrochemical activity on cathodes. Pyrosequencing evidenced differences in bacterial composition between sediments from the field and cathodic biofilms. Six EAMs from genera Herbaspirillum, Ancylobacter, Rhodococcus, Methylobacterium, Sphingomonas, and Pseudomonas were isolated from cathodes using a lithoautotrophic As oxidizers culture medium. These results suggest that the tested Winogradsky BES columns result in an enrichment of electrochemically active As-oxidizing microorganisms. A bioelectrochemical boost of centenarian enrichment approaches, such as the Winogradsky column, represents a promising strategy for prospecting new EAMs linked with the biogeochemical cycles of different metals and metalloids.

Mobilization of As, Fe, and Mn from Contaminated Sediment in Aerobic and Anaerobic Conditions: Chemical or Microbiological Triggers?

We integrated aqueous chemistry, spectroscopy, and microbiology techniques to identify chemical and microbial processes affecting the release of arsenic (As), iron (Fe), and manganese (Mn) from contaminated sediments exposed to aerobic and anaerobic conditions. The sediments were collected from Cheyenne River Sioux Tribal lands in South Dakota, which has dealt with mining legacy for several decades. The range of concentrations of total As measured from contaminated sediments was 96 to 259 mg kg^-1, which co-occurs with Fe (21 000-22 005 mg kg^-1) and Mn (682-703 mg kg^-1). The transition from aerobic to anaerobic redox conditions yielded the highest microbial diversity, and the release of the highest concentrations of As, Fe, and Mn in batch experiments reacted with an exogenous electron donor (glucose). The reduction of As was confirmed by XANES analyses when transitioning from aerobic to anaerobic conditions. In contrast, the releases of As, Fe and Mn after a reaction with phosphate was at least 1 order of magnitude lower compared with experiments amended with glucose. Our results indicate that mine waste sediments amended with an exogenous electron donor trigger microbial reductive dissolution caused by anaerobic respiration. These dissolution processes can affect metal mobilization in systems transitioning from aerobic to anaerobic conditions in redox gradients. Our results are relevant for natural systems, for surface and groundwater exchange, or other systems in which metal cycling is influenced by chemical and biological processes.

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

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

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.

Metabolic and residual characteristic of different arsenic species contained in laver during mouse digestion

Laver is one of the major arsenic contributors to human diets. The study on metabolic and residual characteristic of each arsenic species contained in laver is important to scientifically assess the intake risk of arsenic in the laver. The metabolic and residual characteristic of main arsenic species in laver, namely arsenate [As(V)], dimethylarsinic acid [DMA(V)] and two arsenosugars, was investigated by mouse experiments in this study. The results showed that the intake of higher-dose laver did not lead to a notable increase of As(V) concentration in mouse muscle/organs and feces. In contrast, DMA(V) excretion in feces and DMA(V) residue in muscle/organs showed a close correlation with laver-dose intake. Most DMAsSugarMethoxy was translated into other arsenic species and then was together excreted out via mouse feces; two dominant arsenic species, arsenosugar DMAsSugarMethoxy and DMAsSugarPhosphate, were not detected in mouse muscle/organs after 20-Day or 30-Day feeding whether in lower-dose laver groups containing 1/36 (mass ratio) of the laver in mouse feed or higher-dose laver groups containing 1/6 (mass ratio) of the laver in mouse feed. About 65-77% of total arsenic digested by mouse was excreted out via feces; only 0.12-0.78% of it was accumulated in mouse organs/muscle. The results of this study provided valuable knowledge for comprehending the stability and metabolic characteristics of different arsenic species from Fujian laver in vivo, also for more scientifically assessing the intake risk of arsenic in laver.

New Arsenite Oxidase Gene (aioA) PCR Primers for Assessing Arsenite-Oxidizer Diversity in the Environment Using High-Throughput Sequencing

Gene encoding the large subunit of As(III) oxidase (AioA), an important component of the microbial As(III) oxidation system, is a widely used biomarker to characterize As(III)-oxidizing communities in the environment. However, many studies were restricted to a few sequences generated by clone libraries and Sanger sequencing, which may have underestimated the diversity of As(III)-oxidizers in natural environments. In this study, we designed a primer pair, 1109F (5'-ATC TGG GGB AAY RAC AAY TA-3') and 1548R (5'-TTC ATB GAS GTS AGR TTC AT-3'), targeting gene sequence encoding for the conserved molybdopterin center of the AioA protein, yielding amplicons approximately 450 bp in size that are feasible for highly parallel amplicon sequencing. By utilizing in silico analyses and the experimental construction of clone libraries using Sanger sequencing, the specificity and resolution of 1109F/1548R are approximated with two other previously published and commonly used primers, i.e., M1-2F/M3-2R and deg1F/deg1R. With the use of the 1109F/1548R primer pair, the taxonomic composition of the aioA genes was similar both according to the Sanger and next-generation sequencing (NGS) platforms. Furthermore, high-throughput amplicon sequencing using the primer pair, 1109F/1548R, successfully identified the well-known As(III)-oxidizers in paddy soils and sediments, and they also revealed the differences in the community structure and composition of As(III)-oxidizers in above two biotopes. The random forest analysis showed that the dissolved As(III) had the highest relative influence on the Chao1 index of the aioA genes. These observations demonstrate that the newly designed PCR primers enhanced the ability to detect the diversity of aioA-encoding microorganisms in environments using highly parallel short amplicon sequencing.

Recent Advances in Enzymes for the Bioremediation of Pollutants

Nowadays, pollution of the environment is a huge problem for humans and other organisms' health. Conventional methods of pollutant removal like membrane filtration or ion exchange are not efficient enough to lower the number of pollutants to standard levels. Biological methods, because of their higher efficiency and biocompatibility, are preferred for the remediation of pollutants. These cost-effective and environment-friendly methods of reducing pollutants are called bioremediation. In bioremediation methods, enzymes play the most crucial role. Enzymes can remedy different types of organic and inorganic pollutants, including PAHs, azo dyes, polymers, organocyanides, lead, chromium, and mercury. Different enzymes isolated from various species have been used for the bioremediation of pollutants. Discovering new enzymes and new subtypes with specific physicochemical characteristics would be a promising way to find more efficient and cost-effective tools for the remediation of pollutants.

Citrobacter arsenatis sp. nov., an arsenate-reducing bacterium isolated from freshwater sediment

A novel arsenate-reducing bacterium, LY-1^T, was isolated from freshwater sediment in Huangshi, China. Morphological analysis indicated that the cells were shaped like rods and were gram-negative. The major fatty acids (> 10%) were C_16:0, summed feature 3 (C_16:1 ω7c, C_16:1 ω6c) and summed feature 8 (C_18:1 ω7c, C_18:1 ω6c). An assessment of the phylogeny based on 16S rRNA gene sequences indicated that the strain LY-1^T belonged to the genus Citrobacter, while further analysis based on the recN gene indicated that LY-1^T occupies a distinct phylogenetic niche within the Citrobacter genus. Moreover, average nucleotide identity and digital DNA-DNA hybridization between the strain LY-1^T and the type strains of closely related species of the genus Citrobacter (C. europaeus, C. brakii, C. portucalensis, C. freundii, C. werkmanii, C. cronae, C. youngae, C. pasteurii, C. tructae, C. gillenii, and C. murliniae) were 85.8-93.8% and 31.2-56.9%, respectively. In addition, the LY-1^T strain's capacity to metabolize various compounds and its characteristic G + C content of 51.9% were also distinct from other species of the Citrobacter genus. These discriminatory features cumulatively indicate the LY-1^T strain as a new species within the Citrobacter genus. We propose the species name Citrobacter arsenatis for this new species, with LY-1^T (= CCTCC AB 2019169^T = KCTC 72440^T) as the type strain.

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

A novel dissimilatory antimonate [Sb(V)]-reducing bacterium, strain SVR, was isolated from soil of a former antimony (Sb) mine. Strain SVR coupled Sb(V) reduction to acetate oxidation with an apparent reduction rate of 2.4 mM d^-1. The reduction of Sb(V) was followed by the precipitation and accumulation of white microcrystals in the liquid medium. The precipitates were initially small and amorphous, but they eventually developed to the crystal phase with a length > 50 µm. Strain SVR removed 96% of dissolved Sb as the precipitates. An X-ray diffraction analysis indicated that the microcrystals were the orthorhombic Sb trioxide (Sb₂O₃), i.e., valentinite. Phylogenetic and physiological analyses revealed that strain SVR is a member of the genus Geobacter. The cell suspension of strain SVR incubated with acetate and Sb(V) at pH 7.0 was able to form valentinite. Interestingly, at pH 8.0, the cell suspension formed another crystalline Sb₂O₃ with a cubic structure, i.e., senarmontite. Our findings provide direct evidence that Geobacter spp. are involved in Sb(V) reduction in nature. Considering its superior capacity for Sb removal, strain SVR could be used for the recovery of Sb and the individual productions of valentinite and senarmontite from Sb-contaminated wastewater.

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

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

The Effects of Forest Litter and Waterlogging on the Ecotoxicity of Soils Strongly Enriched in Arsenic in a Historical Mining Site

This study examined the effects of waterlogging and forest litter introduced to soil on chemical properties of soil pore water and ecotoxicity of soils highly enriched in As. These effects were examined in a 21-day incubation experiment. Tested soil samples were collected from Złoty Stok, a historical centre of arsenic and gold mining: from a forested part of the Orchid Dump (19,600 mg/kg As) and from a less contaminated site situated in a neighboring forest (2020 mg/kg As). An unpolluted soil was used as control. The concentrations of As, Fe and Mn in soil pore water were measured together with a redox potential Eh. A battery of ecotoxicological tests, including a bioassay with luminescence bacteria Vibrio fischeri (Microtox) and several tests on crustaceans (Rapidtox, Thamnotox and Ostracodtox tests), was used to assess soil ecotoxicity. The bioassays with crustaceans (T. platyurus, H. incongruens) were more sensitive than the bacterial test Microtox. The study confirmed that the input of forest litter into the soil may significantly increase the effects of toxicity. Waterlogged conditions facilitated a release of As into pore water, and the addition of forest litter accelerated this effect thus causing increased toxicity.

Effects of in situ Remediation With Nanoscale Zero Valence Iron on the Physicochemical Conditions and Bacterial Communities of Groundwater Contaminated With Arsenic

Nanoscale Zero-Valent Iron (nZVI) is a cost-effective nanomaterial that is widely used to remove a broad range of metal(loid)s and organic contaminants from soil and groundwater. In some cases, this material alters the taxonomic and functional composition of the bacterial communities present in these matrices; however, there is no conclusive data that can be generalized to all scenarios. Here we studied the effect of nZVI application in situ on groundwater from the site of an abandoned fertilizer factory in Asturias, Spain, mainly polluted with arsenic (As). The geochemical characteristics of the water correspond to a microaerophilic and oligotrophic environment. Physico-chemical and microbiological (cultured and total bacterial diversity) parameters were monitored before and after nZVI application over six months. nZVI treatment led to a marked increase in Fe(II) concentration and a notable fall in the oxidation-reduction potential during the first month of treatment. A substantial decrease in the concentration of As during the first days of treatment was observed, although strong fluctuations were subsequently detected in most of the wells throughout the six-month experiment. The possible toxic effects of nZVI on groundwater bacteria could not be clearly determined from direct observation of those bacteria after staining with viability dyes. The number of cultured bacteria increased during the first two weeks of the treatment, although this was followed by a continuous decrease for the following two weeks, reaching levels moderately below the initial number at the end of sampling, and by changes in their taxonomic composition. Most bacteria were tolerant to high As(V) concentrations and showed the presence of diverse As resistance genes. A more complete study of the structure and diversity of the bacterial community in the groundwater using automated ribosomal intergenic spacer analysis (ARISA) and sequencing of the 16S rRNA amplicons by Illumina confirmed significant alterations in its composition, with a reduction in richness and diversity (the latter evidenced by Illumina data) after treatment with nZVI. The anaerobic conditions stimulated by treatment favored the development of sulfate-reducing bacteria, thereby opening up the possibility to achieve more efficient removal of As.

Progress in Molecular Nanoarchitectonics and Materials Nanoarchitectonics

Although various synthetic methodologies including organic synthesis, polymer chemistry, and materials science are the main contributors to the production of functional materials, the importance of regulation of nanoscale structures for better performance has become clear with recent science and technology developments. Therefore, a new research paradigm to produce functional material systems from nanoscale units has to be created as an advancement of nanoscale science. This task is assigned to an emerging concept, nanoarchitectonics, which aims to produce functional materials and functional structures from nanoscale unit components. This can be done through combining nanotechnology with the other research fields such as organic chemistry, supramolecular chemistry, materials science, and bio-related science. In this review article, the basic-level of nanoarchitectonics is first presented with atom/molecular-level structure formations and conversions from molecular units to functional materials. Then, two typical application-oriented nanoarchitectonics efforts in energy-oriented applications and bio-related applications are discussed. Finally, future directions of the molecular and materials nanoarchitectonics concepts for advancement of functional nanomaterials are briefly discussed.

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

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

Arsenic speciation and biotransformation pathways in the aquatic ecosystem: The significance of algae

The contamination of aquatic systems with arsenic (As) is considered to be an internationally-important health and environmental issue, affecting over 115 countries globally. Arsenic contamination of aquatic ecosystems is a global threat as it can enter the food chain from As-rich water and cause harmful impacts on the humans and other living organisms. Although different factors (e.g., pH, redox potential, iron/manganese oxides, and microbes) control As biogeochemical cycling and speciation in water systems, the significance of algal species in biotransformation of As is poorly understood. The overarching attribute of this review is to briefly elaborate various As sources and its distribution in water bodies and factors affecting As biogeochemical behavior in aqueous ecosystems. This review elucidates the intriguing role of algae in biotransformation/volatilization of As in water bodies under environmentally-relevant conditions. Also, we critically delineate As sorption, uptake, oxidation and reduction pathways of As by algae and their possible role in bioremediation of As-contaminated water (e.g., drinking water, wastewater). The current review provides the updated and useful framework for government and water treatment agencies to implement algae in As remediation programs globally.

Microbial Oxidation of Arsenite: Regulation, Chemotaxis, Phosphate Metabolism and Energy Generation

Arsenic (As) is a metalloid that occurs widely in the environment. The biological oxidation of arsenite [As(III)] to arsenate [As(V)] is considered a strategy to reduce arsenic toxicity and provide energy. In recent years, research interests in microbial As(III) oxidation have been growing, and related new achievements have been revealed. This review focuses on the highlighting of the novel regulatory mechanisms of bacterial As(III) oxidation, the physiological relevance of different arsenic sensing systems and functional relationship between microbial As(III) oxidation and those of chemotaxis, phosphate uptake, carbon metabolism and energy generation. The implication to environmental bioremediation applications of As(III)-oxidizing strains, the knowledge gaps and perspectives are also discussed.

Edaphic factors influencing vegetation colonization and encroachment on arsenical gold mine tailings near Sudbury, Ontario

Mine tailings are found worldwide and can have significant impacts on ecosystem and human health. In this study, natural vegetation patterns on arsenical (As) gold (Au) mine tailings located in Sudbury, Ontario were assessed using transects located at the edge of the tailings and on the tailings. Vegetation communities were significantly different between the edge and open tailings areas of the site. Arsenic concentrations in both areas were extremely variable (from 285-17,567 mg/kg) but were not significantly correlated with vegetation diversity at the site. Nutrients (carbon (C), phosphorus (P)) and organic matter concentrations were associated with higher diversity and with the presence of climax vegetation on the tailings, but there were no significant relationships between tailings chemistry and vegetation indices on the edge. Encroachment onto the tailings from the edge occurred in conventional succession patterns, with a clear gradient from grasses (Agrostis gigantea) to trees such as Picea glauca. On the tailings, a nucleation pattern was visible, distinct from conventional succession. Trees and shrubs such as Betula papyrifera and Diervilla lonicera were associated with higher diversity and higher nutrient concentrations in the underlying tailings, whereas grasses such as A. gigantea were not. We concluded that at all areas of the site, vegetation - particularly trees - was facilitating amelioration of the underlying tailings. Despite high concentrations of As, nutrients appeared to have a greater influence than metals on vegetation diversity.

Bioremediation of toxic heavy metals (THMs) contaminated sites: concepts, applications and challenges

Heavy metal contamination is a global issue, where the prevalent contaminants are arsenic (As), cadmium (Cd), chromium (Cr)(VI), mercury (Hg), and lead (Pb). More often, they are collectively known as "most problematic heavy metals" and "toxic heavy metals" (THMs). Their treatment through a variety of biological processes is one of the prime interests in remediation studies, where heavy metal-microbe interaction approaches receive high interest for their cost effective and ecofriendly solutions. In this review, we provide an up to date information on different microbial processes (bioremediation) for the removal of THMs. For the same, emphasis is put on oxidation-reduction, biomineralization, bioprecipitation, bioleaching, biosurfactant technology, biovolatilization, biosorption, bioaccumulation, and microbe-assisted phytoremediation with their selective advantages and disadvantages. Further, the literature briefly discusses about the various setups of cleaning processes of THMs in environment under ex situ and in situ applications. Lately, the study sheds light on the manipulation of microorganisms through genetic engineering and nanotechnology for their advanced treatment approaches.

Effects of Fe-Mn-Ce oxide-modified biochar on As accumulation, morphology, and quality of rice (Oryza sativa L.)

The fluidity of arsenic (As) in soil used for rice cultivation under flooding conditions is the main reason for its high accumulation in rice, which poses a serious threat to human's health. Biochar can immobilize heavy metal (for example lead) of soil because of the strong binding of heavy metals to the inner biochar particles. We conducted a pot experiment to evaluate the effects of biochar (BC) and Fe-Mn-Ce oxide-modified biochar composites (FMCBCs) on the morphology, As accumulation, and grain quality of rice grown in As-contaminated soils. The biochar and FMCBC treatments significantly increased the dry weight of roots, stems, leaves, and rice grains grown in As-contaminated soil (P < 0.05). The As concentration in different parts of rice was significantly lower with treatment FMCBC_3-2 (BC, Fe, Mn, and Ce weight ratio of 24:2:3:10) than with the BC and control (no BC) treatments. The application of FMCBC_3-2 maximized the yield and quality of rice grains: rice grain yields were 61.45-68.41% higher over control and the proportion of essential amino acids in the rice grains was 31.01-44.62%. The application of FMCBCs also increased the concentration of Fe-Mn plaques, which prevent the uptake of As by rice, thereby mitigating the toxic effects of As-contaminated soil on rice. In summary, Fe-Mn-Ce oxide-modified BC composites fixed As, reducing its fluidity and the As concentration in rice. Our results show that FMCBC₃ could play an important role in reducing As accumulation and increasing the grain yield and quality of rice, thus ensuring food safety in regions contaminated with As.

Microbial driven iron reduction affects arsenic transformation and transportation in soil-rice system

The microbe-driven iron cycle plays an important role in speciation transformation and migration of arsenic (As) in soil-rice systems. In this study, pot experiments were used to investigate the effect of bacterial iron (Fe) reduction processes in soils on As speciation and migration, as well as on As uptake in soil-rice system. During the rice growth period, pH and electrical conductivity (EC) in soil solutions initially increased and then decreased, with the ranges of 7.4-8.8 and 116.3-820 mS cm-1, respectively. The concentrations of Fe, total As and As(III) showed an increasing trend in the rhizosphere and non-rhizosphere soil solutions with the increasing time. Fe concentrations were significantly positively correlated with total As and As(III) concentrations (***p < 0.001) in the soil solutions. The abundances of the arsenate reductase gene (arsC) and the As(III) S-adenosylmethionine methyltransferase gene (arsM) in rhizosphere soils were higher than those in non-rhizosphere soils, while the abundance of the Fe-reducing bacteria (Geo) showed an opposite trend. Moreover, it showed that the Geo abundance was significantly positively correlated with that of the arsC (***p < 0.001) and arsM (**p < 0.01) genes, respectively. The abundances of Geo, arsC and arsM genes were significantly positively correlated with the concentrations of Fe, total As and As(III) in the soil solutions (*p < 0.05). Moreover, the abundances of arsC and arsM genes were significantly negatively correlated with total As and As(III) in rice grains (*P < 0.05). These results showed that the interaction of bacterial Fe reduction process and radial oxygen loss from roots promoted the reduction and methylation of As, and then decreased As uptake by rice, which provided a theoretical basis for alleviating As pollution in paddy soils.

Behavior and Mechanism of Cesium Biosorption from Aqueous Solution by Living Synechococcus PCC7002

Many efforts have focused on the adsorption of metals from contaminated water by microbes. Synechococcus PCC7002, a major marine cyanobacteria, is widely applied to remove metals from the ocean's photic zone. However, its ability to adsorb cesium (Cs) nuclides has received little attention. In this study, the biosorption behavior of Cs(I) from ultrapure distilled water by living Synechococcus PCC7002 was investigated based on kinetic and isotherm studies, and the biosorption mechanism was characterized by Fourier-transform infrared spectroscopy, Raman&nbsp;spectroscopy, scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectrometry, and three-dimensional excitation emission matrix fluorescence spectroscopy. Synechococcus PCC7002 showed extremely high tolerance to Cs ions and its minimal inhibitory concentration was 8.6 g/L. Extracellular polymeric substances (EPS) in Synechococcus PCC7002 played a vital role in this tolerance. The biosorption of Cs by Synechococcus PCC7002 conformed to a Freundlich-type isotherm model and pseudo-second-order kinetics. The binding of Cs(I) was primarily attributed to the extracellular proteins in EPS, with the amino, hydroxyl, and phosphate groups on the cell walls contributing to Cs adsorption. The biosorption of Cs involved two mechanisms: Passive adsorption on the cell surface at low Cs concentrations and active intracellular adsorption at high Cs concentrations. The results demonstrate that the behavior and mechanism of Cs adsorption by Synechococcus PCC7002 differ based on the Cs ions concentration.

Artificial light at night alter the impact of arsenic on microbial decomposers and leaf litter decomposition in streams

Artificial light at night (ALAN, also known as light pollution) has been proved to be a contributor to environmental change and a biodiversity threat worldwide, yet little is known about its potential interaction with different metal pollutants, such as arsenic (As), one of the largest threats to aquatic ecosystems. To narrow this gap, an indoor microcosm study was performed using an ALAN simulation device to examine whether ALAN exposure altered the impact of arsenic on plant litter decomposition and its associated fungi. Results revealed that microbial decomposers involved in the conversion of As(III) to As(V), and ALAN exposure enhanced this effect; ALAN or arsenic only exposure altered fungal community composition and the correlations between fungi species, as well as stimulated or inhibited litter decomposition, respectively. The negative effects of arsenic on the decomposition of Pterocarya stenoptera leaf litter was alleviated by ALAN resulting in the enhanced photodegradation of leaf litter lignin and microbiological oxidation of As(III) to As(V), the increased microbial biomass and CBH activity, as well as the enhanced correlations between CBH and litter decomposition rate. Overall, results expand our understanding of ALAN on environment and highlight the contribution of ALAN to the toxicity of arsenic in aquatic ecosystems.

Isolation and characterization of bacteria from a brazilian gold mining area with a capacity of arsenic bioaccumulation

In Paracatu, a city in Minas Gerais State (Brazil), the gold mineral extraction produces wastes that contribute to environmental contamination by arsenic. This work describes the evaluation of arsenic concentration from soil of a gold mining area in Paracatu and the selection of arsenic resistant bacteria. In the process of culturing enrichment, 38 bacterial strains were isolated and the minimum inhibitory concentration (MIC) was determined in solid medium for each strain. Three bacterial strains named P1C1Ib, P2Ic and P2IIB were resistant to 3000 mg L^-1 of arsenite. Analysis of 16S rDNA gene sequences revealed that these bacteria belong to Bacillus cereus and Lysinibacillus boronitolerans species. After cultivation of the strains P1C1Ib, P2Ic and P2IIIb, 69.38%-71.88% of arsenite and 82.39%-85.72% of arsenate concentrations were reduced from the culture medium, suggesting the potential application of theses strains in bioremediation processes.

Dissolution and final fate of arsenic associated with gypsum, calcite, and ferrihydrite: Influence of microbial reduction of As(V), sulfate, and Fe(III)

Several studies have demonstrated that gypsum (CaSO₄·2H₂O) and calcite (CaCO₃) can be important hosts of arsenic in contaminated hydrogeological systems. However, the extent to which microbial reducing processes contribute to the dissolution and transformation of carbonate and sulfate minerals and, thereby, to arsenic mobilization is poorly understood. These processes are likely to have a strong impact on arsenic mobility in iron-poor environments and in reducing aquifers where iron oxyhydroxides become unstable. Anoxic batch bioassays with arsenate (As(V)) coprecipitated with calcite, gypsum, or ferrihydrite (Fe(OH)₃) were conducted in the presence of sulfate or molybdate to examine the impact of bioprocesses (i.e. As(V), sulfate, and Fe(III)-reduction) on arsenic dissolution, speciation, and eventual remineralization. Microbial reduction of As(V)-bearing calcite caused an important dissolution of arsenite, As(III), which remained in solution up to the end of the experiment (30 days). The reduction of As(V) from gypsum-As(V) also led to the release of As(III), which was subsequently remineralized, possibly as arsenic sulfides. The presence of sulfate triggered arsenic dissolution in the bioassays with ferrihydrite-As(V). This study showed that although gypsum and calcite have a lower capacity to bind arsenic, compared to iron oxides, they can play a critical role in the biogeochemical cycle of arsenic in natural calcareous and gypsiferous systems depleted of iron since they can be a source of electron acceptors for reducing bioprocesses.

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.

Soil Microbial Communities Involved in Reductive Dissolution of Arsenic from Arsenate-Laden Minerals with Different Carbon Sources

The natural microbial communities involved in arsenic (As) extraction under biostimulated conditions are still unclear. In this study, soil slurry was incubated with arsenate [As(V)]-laden Fe(III) or Al (hydr)oxides with lactate or acetate. After 40 d, dissolved As released from As(V)-laden Fe(III) accounted for 54% of the initial solid-phase As in lactate-amended slurries, while much less As was released from acetate-amended slurries. As was released more rapidly from As(V)-laden Al, but the total release was relatively low (45%). High-throughput Illumina sequencing of 16S rRNA genes revealed that dissimilatory metal(loid) reducers such as Desulfitobacterium became predominant in lactate-amended slurries. Moreover, anaerobic fermenters in the Sporomusaceae family were predominant. Interestingly, a Sporomusaceae bacterial strain isolated from the slurry was capable of releasing As from both As(V)-laden (hydr)oxides in the presence of lactate. The strain first released As as As(V) and subsequently reduced it to As(III) in the aqueous phase. These results suggest that lactate is a suitable carbon source for As extraction by natural microbial communities, and that both dissimilatory metal(loid) reducers and certain anaerobic fermenters play significant roles in As extraction. Microbial reductive dissolution of As may be expected to be a cost-effective restoration technique for As-contaminated soils.

Role of extracellular polymeric substance (EPS) in toxicity response of soil bacteria Bacillus sp. S3 to multiple heavy metals

Heavy metal resistant bacteria are of great interest because of their potential use in bioremediation. Understanding the survival and adaptive strategies of these bacteria under heavy metal stress is important for better utilization of these bacteria in remediation. The objective of this study was to investigate the role of bacterial extracellular polymeric substance (EPS) in detoxifying against different heavy metals in Bacillus sp. S3, a new hyper antimony-oxidizing bacterium previously isolated from contaminated mine soils. The results showed that Bacillus sp. S3 is a multi-metal resistant bacterial strain, especially to Sb(III), Cu(II) and Cr(VI). Toxic Cd(II), Cr(VI) and Cu(II) could stimulate the secretion of EPS in Bacillus sp. S3, significantly enhancing the adsorption and detoxification capacity of heavy metals. Both Fourier transform infrared spectroscopy (FTIR) and three-dimensional excitation-emission matrix (3D-EEM) analysis further confirmed that proteins were the main compounds of EPS for metal binding. In contrast, the EPS production was not induced under Sb(III) stress. Furthermore, the TEM-EDX micrograph showed that Bacillus sp. S3 strain preferentially transported the Sb(III) to the inside of the cell rather than adsorbed it on the extracellular surface, indicating intracellular detoxification rather than extracellular EPS precipitation played an important role in microbial resistance towards Sb(III). Together, our study suggests that the toxicity response of EPS to heavy metals is associated with difference in EPS properties, metal types and corresponding environmental conditions, which is likely to contribute to microbial-mediated remediation.

Capability for arsenic mobilization in groundwater is distributed across broad phylogenetic lineages

Despite the importance of microbial activity in mobilizing arsenic in groundwater aquifers, the phylogenetic distribution of contributing microbial metabolisms is understudied. Groundwater samples from Ohio aquifers were analyzed using metagenomic sequencing to identify functional potential that could drive arsenic cycling, and revealed mechanisms for direct (i.e., Ars system) and indirect (i.e., iron reduction) arsenic mobilization in all samples, despite differing geochemical conditions. Analyses of 194 metagenome-assembled genomes (MAGs) revealed widespread functionality related to arsenic mobilization throughout the bacterial tree of life. While arsB and arsC genes (components of an arsenic resistance system) were found in diverse lineages with no apparent phylogenetic bias, putative aioA genes (aerobic arsenite oxidase) were predominantly identified in Methylocystaceae MAGs. Both previously described and undescribed respiratory arsenate reduction potential via arrA was detected in Betaproteobacteria, Deltaproteobacteria, and Nitrospirae MAGs, whereas sulfate reduction potential was primarily limited to members of the Deltaproteobacteria and Nitrospirae. Lastly, iron reduction potential was detected in the Ignavibacteria, Deltaproteobacteria, and Nitrospirae. These results expand the phylogenetic distribution of taxa that may play roles in arsenic mobilization in subsurface systems. Specifically, the Nitrospirae are a much more functionally diverse group than previously assumed and may play key biogeochemical roles in arsenic-contaminated ecosystems.