Molecular analysis of arsenate-reducing bacteria within Cambodian sediments following amendment with acetate

Reductive dissolution of As-bearing iron oxides: Mediating mechanism of fulvic acid and dissimilated iron reducing bacteria

Fulvic acid (FA) and iron oxides often play regulating roles in the geochemical behavior and ecological risk of arsenic (As) in terrestrial ecosystems. FA can act as electron shuttles to facilitate the reductive dissolution of As-bearing iron (hydr)oxides. However, the influence of FA from different sources on the sequential conversion of Fe/As in As-bearing iron oxides under biotic and abiotic conditions remains unclear. In this work, we exposed prepared As-bearing iron oxides to FAs derived from lignite (FAL) and plant peat (FAP) under anaerobic conditions, tracked the fate of Fe and As in the aqueous phase, and investigated the reduction transformation of Fe(III)/As(V) with or without the presence of Shewanella oneidensis MR-1. The results showed that the reduction efficiency of Fe(III)/As(V) was increased by MR-1, through its metabolic activity and using FAs as electron shuttles. The reduction of Fe(III)/As(V) was closely associated with goethite being more conducive to Fe/As reduction compared to hematite. It is determined that functional groups such as hydroxy, carboxy, aromatic, aldehyde, ketone and aliphatic groups are the primary electron donors. Their reductive capacities rank in the following sequence: hydroxy> carboxy, aromatic, aldehyde, ketone> aliphatic group. Notably, our findings suggest that in the biotic reduction, Fe significantly reduction precedes As reduction, thereby influencing the latter's reduction process across all incubation systems. This work provides empirical support for understanding iron's role in modulating the geochemical cycling of As and is of significant importance for assessing the release risk of arsenic in natural environments.

Activated carbon as a strong DOM adsorbent mitigates antimony and arsenic release in flooded mining-impacted soils

In Southern China, the co-occurrence of arsenic (As) and antimony (Sb) contamination in soils around Sb mines presents an environmental challenge. During the flooding period of mining-impacted soils, anaerobic reduction of iron (Fe) oxides enhances the mobilization and bioavailability of Sb and As, further elevating the risk of Sb and As entering the food chain. To address this problem, activated carbon (AC) and biochar (BC) were applied to remediate flooded mining-impacted soils. Our results explored that AC can significantly decrease mobilization by 9-97 % for Sb and 9-67 % for As through inhibiting Fe(III) mineral reduction and dissolution in flooded soils. In contrast, there was no significant effect of BC. This was attributed to the strong adsorption of soil dissolved organic matter (DOM) by AC compared to BC, while DOM as electron shuttle is crucial for microbial Fe(III) reduction. Consequently, the DOM sequestration by AC effectively mitigates Sb and As leaching in contaminated mining soils.

Unraveling seasonal shifts in microbial and geochemical mediated arsenic mobilization at the estuarine sediment-water interface under redox changes

The mobilization of arsenic (As) at the sediment-water interface (SWI) is crucial for determining the accumulation of dissolved As to potentially toxic levels. However, the specific impacts of redox processes involving iron (Fe) and sulfur (S), as well as microbial activities occurring in sediments, on As mobilization at the marine SWI remain poorly understood. In this study, we investigated As mobilization at the SWI in the Changjiang Estuary during three different seasons with different benthic redox conditions. The preferential reduction of arsenate (As(V)) to arsenite (As(III)) and subsequent re-adsorption onto newly formed crystalline Fe oxides restricted As release in the As(V) reduction layer. Enhanced Fe(III) reduction in the Fe(III) reduction layer contributed to As release, while the presence of low As-high Fe-high SO42- levels resulted in As removal through adsorption onto pyrite in the sulfate reduction layer. Analysis of functional genes indicated that As(V) in sediments was released into porewater through the reductive dissolution of As(V)-bearing Fe(III) oxides by Geobacter species, followed by microbial reduction of the liberated As(V) to As(III) by microbes carrying the arrA gene. The dominant pathway governing As mobilization at the SWI in the Changjiang Estuary shifted from microbial reduction control during the hypoxic summer to Fe redox control during the aerobic autumn and winter. These findings provide valuable insights into the complex mechanisms driving As mobilization and highlight the importance of considering seasonal variations in understanding As dynamics at the marine SWI.

Soil Humic Acid Stimulates Potentially Active Dissimilatory Arsenate-Reducing Bacteria in Flooded Paddy Soil as Revealed by Metagenomic Stable Isotope Probing

Dissimilatory arsenate reduction contributes a large proportion of arsenic flux from flooded paddy soil, which is closely linked to soil organic carbon input and efflux. Humic acid (HA) represents a natural ingredient in soil and is shown to enhance microbial arsenate respiration to promote arsenic mobility. However, the community and function profiles of metabolically active arsenate-respiring bacteria and their interactions with HA in paddy soil remain unclear. To probe this linkage, we performed a genome-centric comparison of potentially active arsenate-respiring bacteria in anaerobic microcosms amended with 13C-lactate and HA by combining stable-isotope probing with genome-resolved metagenomics. Indeed, HA greatly accelerated the microbial reduction of arsenate to arsenite. Enrichment of bacteria that harbor arsenate-respiring reductase genes (arrA) in HA-enriched 13C-DNA was confirmed by metagenomic binning, which are affiliated with Firmicutes (mainly Desulfitobacterium, Bacillus, Brevibacillus, and Clostridia) and Acidobacteria. Characterization of reference extracellular electron transfer (EET)-related genes in these arrA-harboring bacteria supports the presence of EET-like genes, with partial electron-transport chain genes identified. This suggests that Gram-positive Firmicutes- and Acidobacteria-related members may harbor unspecified EET-associated genes involved in metal reduction. Our findings highlight the link between soil HA and potentially active arsenate-respiring bacteria, which can be considered when using HA for arsenic removal.

Chemical-microbial effects of acetic acid, oxalic acid and citric acid on arsenic transformation and migration in the rhizosphere of paddy soil

Low-molecular-weight organic acids (LMWOAs) are essential components of rice roots exudates and an important source of soil organic carbon. The chemical-microbial pathway by which LMWOA affects arsenic (As) cycling in the rhizosphere of paddy soils is still unclear. In this study, three typical LMWOAs (acetic acid (AA), oxalic acid (OA), and citric acid (CA)) in rice root exudates were added to As-contaminated soil at a concentration of 10 mM, mimicking the rhizosphere environment. The results showed that the addition of AA and OA inhibited the mobilization of As in the rhizosphere soil. After 14 days of incubation, the content of As in the porewater of AA and OA decreased by 40% and 22%, respectively, compared with the control. AA hindered the mobilization of As in soil via promoting the formation of secondary minerals. The addition of OA inhibits the mobilization of As via increasing the proportion of As (V) in porewater and promoting the formation of secondary minerals in soil. In addition, OA addition not only significantly increased the aioA gene abundance but also notably enriched the microorganisms containing As (III) methylation functional genes (arsM). The addition of CA greatly expedited the release of As from the soil solid phase through the solubilization of Fe/Mn minerals via the effects of both soil chemistry and microbial action. Furthermore, linear discriminant analysis effect size (LEfSe) revealed the possibility that bacteria such as Burkholderia, Magnetospirillum, and Mycobacterium were involved in the reduction or methylation of As in the rhizosphere of paddy soil. This study revealed the internal causes of LMWOAs regulating As transformation and mobilization in flooded paddy soil and provided theoretical support for reducing As accumulation in rice by breeding rice varieties with high AA and OA secretions.

Thiosulfate driving bio-reduction mechanisms of scorodite in groundwater environment

Reductive dissolution of scorodite results in the release and migration of arsenic (As) in groundwater. The purpose of this study was to explore the possible abiotic and biotic reduction of scorodite in groundwater environment and the effect of microbial-mediated sulfur cycling on the bio-reduction of scorodite. Microcosm experiments consisting of scorodite with bacterium Citrobacter sp. JH012-1 or free sulfide were carried out to determine the effects of thiosulfate on the mobilization of As/Fe. The results show arsenic release is positively correlated with iron reduction. The arsenate [As(V)] released can agglomerate with Fe(II) on the surface of scorodite to form crystalline parasymplesite, while no parasymplesite was detected in the abiotic reduction of scorodite by sulfide. The reduction of scorodite and As(V) was affected by thiosulfate. When 0.5 mM thiosulfate was added, the Fe(III) reduction rate increased from 32% to 82%, and the As(V) reduction rate rose from 54% to 64%. When the addition of thiosulfate was increased from 0.5 mM to 2 mM and 5 mM, Fe(III) reduction rate added 4% and 8%, and As(V) reduction rate increased 11% and 16%, respectively. In addition, the presence of thiosulfate promoted the scorodite almost completely converting to parasymplesite. Therefore, the effect of microbial-mediated sulfur cycling should be considered in arsenic migration and reduction from scorodite.

Vanadate reducing bacteria and archaea may use different mechanisms to reduce vanadate in vanadium contaminated riverine ecosystems as revealed by the combination of DNA-SIP and metagenomic-binning

Vanadium (V) is a transitional metal that poses health risks to exposed humans. Microorganisms play an important role in remediating V contamination by reducing more toxic and mobile vanadate (V(V)) to less toxic and mobile V(IV). In this study, DNA-stable isotope probing (SIP) coupled with metagenomic-binning was used to identify microorganisms responsible for V(V) reduction and determine potential metabolic mechanisms in cultures inoculated with a V-contaminated river sediment. Anaeromyxobacter and Geobacter spp. were identified as putative V(V)-reducing bacteria, while Methanosarcina spp. were identified as putative V(V)-reducing archaea. The bacteria may use the two nitrate reductases NarG and NapA for respiratory V(V) reduction, as has been demonstrated previously for other species. It is proposed that Methanosarcina spp. may reduce V(V) via anaerobic methane oxidation pathways (AOM-V) rather than via respiratory V(V) reduction performed by their bacterial counterparts, as indicated by the presence of genes associated with anaerobic methane oxidation coupled with metal reduction in the metagenome assembled genome (MAG) of Methanosarcina. Briefly, methane may be oxidized through the "reverse methanogenesis" pathway to produce electrons, which may be further captured by V(V) to promote V(V) reduction. More specially, V(V) reduction by members of Methanosarcina may be driven by electron transport (CoMS-SCoB heterodisulfide reductase (HdrDE), F₄₂₀H₂ dehydrogenases (Fpo), and multi-heme c-type cytochrome (MHC)). The identification of putative V(V)-reducing bacteria and archaea and the prediction of their different pathways for V(V) reduction expand current knowledge regarding the potential fate of V(V) in contaminated sites.

Keystone taxa and functional analysis in arsenic and antimony co-contaminated rice terraces

Both arsenic (As) and antimony (Sb) are primary environmental contaminants that often co-exist at contaminated sites. Though the microbial community compositions of As- and Sb-contaminated sites have been previously described, the changes in microbial community interactions and community functions remain elusive. In the current study, several key metabolic processes, such as As/Sb detoxification and carbon fixation, were enriched under heavily contaminated conditions. Furthermore, the identified keystone taxa, which are associated with the families Nitrosomonadaceae, Pedosphaeraceae, Halieaceae, and Latescibacterota, demonstrated positive correlations with As and Sb concentrations, indicating that they may be resistant to As and Sb toxicities. Accordingly, arsenic resistance-related functions, along with several functions such as carbon fixation, were found to be enriched in heavily contaminated sites. The current study elucidated the key microbial populations in As- and Sb-contaminated rice terraces and may provide useful information for remediation purposes.

Culture-dependent study of arsenic-reducing bacteria in deep aquatic sediments of Bengal Delta

Biogeochemical release of soil-bound arsenic (As) governs mobilization of the toxic metalloid into the groundwater. The present study has examined As^V-reduction ability of bacteria from anoxic aquatic sediments that might contribute to arsenic mobilization in the Bengal Delta. Arsenic-reducing bacteria from deep layers of pond sediment were enriched and isolated in anaerobic environments and As^V reduction was assessed in culture medium. The pond sediment enrichments harboured As^V-reducing bacteria belonging to the phyla Firmicutes and Proteobacteria with dominance of Paraclostridium benzoelyticum and P. bifermentans. Among total 17 isolates, the respiratory reductase genes were not detected by the most common primers and only 3 strains had arsenic reductase ArsC gene suggesting involvement of resistance and some unknown mechanisms in As^V reduction. Presence of high levels of organic matter, As, and As-reducing bacteria might make deep aquatic sediments a hot spot of As mobilization and aquifer contamination.

Variable response of arsenic contaminated groundwater microbial community to electron acceptor regime revealed by microcosm based high-throughput sequencing approach

Arsenic (As) mobilization in alluvial aquifers is facilitated by microbially catalyzed redox transformations that depend on the availability of electron acceptors (EAs). In this study, the response of an As-contaminated groundwater microbial community from West Bengal, India towards varied EAs was elucidated through microcosm based 16S rRNA gene amplicon sequencing. Acinetobacter, Deinococcus, Nocardioides, etc., and several unclassified bacteria (Ignavibacteria) and archaea (Bathyarchaeia, Micrarchaeia) previously not reported from As-contaminated groundwater of West Bengal, characterized the groundwater community. Distinct shifts in community composition were observed in response to various EAs. Enrichment of operational taxonomic units (OTUs) affiliated to Denitratisoma (NO₃-), Spirochaetaceae (Mn⁴⁺), Deinococcus (As⁵⁺), Ruminiclostridium (Fe³⁺), Macellibacteroides (SO₄²-), Holophagae-Subgroup 7 (HCO₃-), Dechloromonas and Geobacter (EA mixture) was noted. Alternatively, As³⁺ amendment as electron donor allowed predominance of Rhizobium. Taxonomy based functional profiling highlighted the role of chemoorganoheterotrophs capable of concurrent reduction of NO₃-, Fe³⁺, SO₄²-, and As biotransformation in As-contaminated groundwater of West Bengal. Our analysis revealed two major aspects of the community, (a) taxa selective toward responding to the EAs, and (b) multifaceted nature of taxa appearing in abundance in response to multiple substrates. Thus, the results emphasized the potential of microbial community members to influence the biogeochemical cycling of As and other dominant anions/cations.

Geochemical, metagenomic, and physiological characterization of the multifaceted interaction between microbiome of an arsenic contaminated groundwater and aquifer sediment

Geomicrobiological details of the interactions between groundwater microbiome (GWM) and arsenic (As)-rich aquifer sediment of Bengal basin was investigated through microcosm incubations. Role of key microorganisms and their specific interactions with As-bearing minerals was demarcated under organic carbon- amended and -unamended conditions. Acinetobacter (50.8 %), Brevundimonas (7.9 %), Sideroxydans (3.4 %), Alkanindiges (3.0 %) dominated the GWM. The microbiome catalysed considerable alterations in As-bearing mineral [Fe-(hydr)oxide and aluminosilicate] phases resulting in substantial changes in overall geochemistry and release of As (65 μg/L) and Fe (118 μg/L). Synergistic roles of autotrophic, NH₄⁺-oxidizing Archaea (Thaumarchaeota) and chemoheterotrophic bacteria (Stenotrophomonas, Pseudomonas, Geobacter) of diverse metabolic abilities (NH₄⁺-oxidizing, NO₃^-, As/Fe-reducing) were noted for observed changes. Organic carbon supported enhanced microbial growth and As mobilization (upto 403.2 μg As/L) from multiple mineral phases (hematite, magnetite, maghemite, biotite, etc.). In presence of high organic carbon, concerted actions of anaerobic, hydrocarbon-utilizing, As-, Fe-reducing Rhizobium, fermentative Escherichia, anaerobic Bacillales, metal-reducing and organic acid-utilizing Pseudomonas and Achromobacter were implicated in altering sediment mineralogy and biogeochemistry. Increase in abundance of arrA, arsC, bssA genes, and dissolution of Fe, Ca, Mg, Mn confirmed that dissimilatory-, cytosolic-As reduction, and mineral weathering fuelled by anaerobic (hydro)carbon metabolism are the predominant mechanisms of As release in aquifers of Bengal basin.

Effects of different dissolved organic matter on microbial communities and arsenic mobilization in aquifers

Dissolved organic matter (DOM) play key roles in the biotransformation of arsenic in groundwater systems. However, the effects of different types of DOM on arsenic biogeochemistry remain poorly understood. In this study, four typical DOM compounds (acetate, lactate, AQS and humic acid) were amended to high As aquifer sediments to investigate their effects on arsenic/iron biotransformation and microbial community response. Results demonstrated that different DOM drove different microbial community shifts and then enhanced microbially-mediated arsenic release and iron reduction. With labile DOM (acetate and lactate) amendment, the abundance of putative dissimilatory iron and sulfate reducers Desulfomicrobium and Clostridium sensu stricto increased within the first week, and subsequently the anaerobic fermentative bacterial genus Acetobacterium and arsenate/sulfate-reducing bacterial genus Fusibacter became predominant. In contrast, recalcitrant DOM (AQS and humic acid) mainly stimulated the abundances of sulfur compounds respiratory genus Desulfomicrobium and fermentative bacterial genus Alkalibacter in the whole incubation. Accompanied with the microbial community structure and function shifts, dissolved organic carbon concentration and oxidation-reduction potential changed and the arsenic/iron reduction increased, which resulted in the enhanced arsenic mobilization. Collectively, the present study linked DOM type to microbial community structure and explored the potential roles of different DOM on arsenic biotransformation in aquifers.

Development of a whole-cell biosensor based on an ArsR-P ars regulatory circuit from Geobacter sulfurreducens

In this study, an Escherichia coli (E. coli) whole-cell biosensor for the specific detection of bioavailable arsenic was developed by placing a green fluorescent protein (GFP) reporter gene under the control of the ArsR1 (GSU2952) regulatory circuit from Geobacter sulfurreducens. E. coli cells only emitted green fluorescence in the presence of arsenite and were more sensitive to arsenite when they were grown in M9 supplemented medium compared to LB medium. Under optimal test conditions, the Geobacter arsR1 promoter had a detection limit of 0.01 μM arsenite and the GFP expression was linear within a range of 0.03-0.1 μM (2.25-7.5 μg/l). These values were well below World Health Organization's drinking water quality standard, which is 10 μg/l. The feasibility of using this whole-cell biosensor to detect arsenic in water samples, such as arsenic polluted tap water and landfill leachate was verified. The biosensor was determined to be just as sensitive as atomic fluorescence spectrometry. This study examines the potential applications of biosensors constructed with Geobacter ArsR-P _ars regulatory circuits and provides a rapid and cost-effective tool that can be used for arsenic detection in water samples.

Microbes involved in arsenic mobilization and respiration: a review on isolation, identification, isolates and implications

Microorganisms play an important role in arsenic (As) cycling in the environment. Microbes mobilize As directly or indirectly, and natural/geochemical processes such as sulphate and iron reduction, oxidative sulphide mineral dissolution, arsenite (AsO₃^3-) oxidation and arsenate (AsO₄^3-) respiration further aid in As cycle in the environment. Arsenate serves as an electron donor for the microbes during anaerobic conditions in the sediment. The present work reviews the recent development in As contamination, various As-metabolizing microbes and their phylogenetic diversity, to understand the role of microbial communities in As respiration and mobilization. It also summarizes the contemporary understanding of the intricate biochemistry and molecular biology of natural As metabolisms. Some successful examples of engineered microbes by harnessing these natural mechanisms for effective remediation are also discussed. The study indicates that there is an exigent need to have a clear understanding of environmental aspects of As mobilization and subsequent oxidation-reduction by a suitable microbial consortium.

Behaviors of heavy metal(loid)s in a cocontaminated alkaline paddy soil throughout the growth period of rice

A pot experiment was conducted to investigate uptake of cadmium (Cd), arsenic (As) and antimony (Sb) by rice from a lime-treated paddy soil contaminated with the three pollutants. The results showed that the content of Cd in the total rice plants decreased as the plant grew, whereas the As and Sb contents increased steadily. The concentration of As in the pore water showed steady increase throughout the growth period, likely due to the reductive dissolution of iron (Fe)-bearing minerals and the reduction of As(V). In contrast, the concentrations of Cd and Sb in the pore water increased initially, likely attributable to the reductive dissolution of Fe-bearing minerals, and then decreased likely due to their adsorptions onto carbonate and Fe sulfides, the reduction of Sb(V), and the formation of CdS. A random forest model was used to quantitatively evaluate the relative contributions of environmental factors to the accumulation of Cd, As, and Sb in the rice plants. The results suggest that sulfides produced through sulfate reduction and the formation of Cd forms associated with sulfur (S) might significantly affected the Cd content in the rice plants. In addition, the dissolved Fe species, the oxidation-reduction potential, and the abundance of the As(V)-respiring gene were major contributors to the As content in the rice plants, suggesting the important role of the reduction of Fe-bearing minerals and As(V). The results also showed that the Sb content in the rice plants was correlated with Fe species, Sb(V) reduction, and acid volatile S. The environmental behaviors of Cd, As, and Sb in the cocontaminated paddy soil exhibited significant differences. Such differences should be considered in remedy of soils contaminated with multiple heavy metals and metalloids.

Role of in Situ Natural Organic Matter in Mobilizing As during Microbial Reduction of FeIII-Mineral-Bearing Aquifer Sediments from Hanoi (Vietnam)

Natural organic matter (NOM) can contribute to arsenic (As) mobilization as an electron donor for microbially-mediated reductive dissolution of As-bearing Fe(III) (oxyhydr)oxides. However, to investigate this process, instead of using NOM, most laboratory studies used simple fatty acids or sugars, often at relatively high concentrations. To investigate the role of relevant C sources, we therefore extracted in situ NOM from the upper aquitard (clayey silt) and lower sandy aquifer sediments in Van Phuc (Hanoi area, Vietnam), characterized its composition, and used 100-day microcosm experiments to determine the effect of in situ OM on Fe(III) mineral reduction, As mobilization, and microbial community composition. We found that OM extracted from the clayey silt (OMC) aquitard resembles young, not fully degraded plant-related material, while OM from the sandy sediments (OMS) is more bioavailable and related to microbial biomass. Although all microcosms were amended with the same amount of C (12 mg C/L), the extent of Fe(III) reduction after 100 days was the highest with acetate/lactate (43 ± 3.5% of total Fe present in the sediments) followed by OMS (28 ± 0.3%) and OMC (19 ± 0.8%). Initial Fe(III) reduction rates were also higher with acetate/lactate (0.53 mg Fe(II) in 6 days) than with OMS and OMC (0.18 and 0.08 mg Fe(II) in 6 days, respectively). Although initially more dissolved As was detected in the acetate/lactate setups, after 100 days, higher concentrations of As (8.3 ± 0.3 and 8.8 ± 0.8 μg As/L) were reached in OMC and OMS, respectively, compared to acetate/lactate-amended setups (6.3 ± 0.7 μg As/L). 16S rRNA amplicon sequence analyses revealed that acetate/lactate mainly enriched Geobacter, while in situ OM supported growth and activity of a more diverse microbial community. Our results suggest that although the in situ NOM is less efficient in stimulating microbial Fe(III) reduction than highly bioavailable acetate/lactate, it ultimately has the potential to mobilize the same amount or even more As.

Abundant and diverse arsenic-metabolizing microorganisms in peatlands treating arsenic-contaminated mining wastewaters

Mining operations produce large quantities of wastewater. At a mine site in Northern Finland, two natural peatlands are used for the treatment of mining‐influenced waters with high concentrations of sulphate and potentially toxic arsenic (As). In the present study, As removal and the involved microbial processes in those treatment peatlands (TPs) were assessed. Arsenic‐metabolizing microorganisms were abundant in peat soil from both TPs (up to 10⁸ cells g_dw⁻¹), with arsenate respirers being about 100 times more abundant than arsenite oxidizers. In uninhibited microcosm incubations, supplemented arsenite was oxidized under oxic conditions and supplemented arsenate was reduced under anoxic conditions, while little to no oxidation/reduction was observed in NaN₃‐inhibited microcosms, indicating high As‐turnover potential of peat microbes. Formation of thioarsenates was observed in anoxic microcosms. Sequencing of the functional genemarkers aioA (arsenite oxidizers), arrA (arsenate respirers) and arsC (detoxifying arsenate reducers) demonstrated high diversity of the As‐metabolizing microbial community. The microbial community composition differed between the two TPs, which may have affected As removal efficiencies. In the present situation, arsenate reduction is likely the dominant net process and contributes substantially to As removal. Changes in TP usage (e.g. mine closure) with lowered water tables and heightened oxygen availability in peat might lead to re‐oxidation and re‐mobilization of bound arsenite.

Unique diversity and functions of the arsenic-methylating microorganisms from the tailings of Shimen Realgar Mine

Microbial arsenic (As) methylation plays important roles in the As biogeochemical cycle. However, little is known about the diversity and functions of As-methylating microorganisms from the tailings of a Realgar Mine, which is characterized as containing extremely high concentrations of As. To address this issue, we collected five samples (T1-T5) from the tailings of Shimen Realgar Mine. Microcosm assays without addition of exogenous As and carbon indicated that all the five samples possess significant As-methylating activities, producing 0.8-5.7 μg/L DMAs^V, and 1.1-10.7 μg/L MMAs^V with an exception of T3, from which MMAs^V was not detectable after 14.0 days of incubation. In comparison, addition of 20.0 mM lactate to the microcosms significantly enhanced the activities of these samples; the produced DMAs^V and MMAs^V are 8.0-39.7 μg/L and 5.8-38.3 μg/L, respectively. The biogenic DMAs^V shows significant positive correlations with the Fe concentrations and negative correlations with the total nitrogen concentrations in the environment. A total of 63 different arsM genes were identified from the five samples, which code for new or new-type ArsM proteins, suggesting that a unique diversity of As-methylating microbes are present in the environment. The microbial community structures of the samples were significantly shaped by the environmental total organic carbon, total As contents and NO₃^- contents. These data help to better understand the microorganisms-catalyzed As methylation occurred in the environment with extremely high contents of As.

Identification of a Chlorodibenzo-p-dioxin Dechlorinating Dehalococcoides mccartyi by Stable Isotope Probing

Polychlorinated dibenzo-p-dioxins (PCDDs) are released into the environment from a variety of both anthropogenic and natural sources. While highly chlorinated dibenzo-p-dioxins are persistent under oxic conditions, in anoxic environments, these organohalogens can be reductively dechlorinated to less chlorinated compounds that are then more amenable to subsequent aerobic degradation. Identifying the microorganisms responsible for dechlorination is an important step in developing bioremediation approaches. In this study, we demonstrated the use of a DNA-stable isotope probing (SIP) approach to identify the bacteria active in dechlorination of PCDDs in river sediments, with 1,2,3,4-tetrachlorodibenzo-p-dioxin (1,2,3,4-TeCDD) as a model. In addition, pyrosequencing of reverse transcribed 16S rRNA of TeCDD dechlorinating enrichment cultures was used to reveal active members of the bacterial community. A set of operational taxonomic units (OTUs) responded positively to the addition of 1,2,3,4-TeCDD in SIP microcosms assimilating ¹³C-acetate as the carbon source. Analysis of bacterial community profiles of the ¹³C labeled heavy DNA fraction revealed that an OTU corresponding to Dehalococcoides mccartyi accounted for a significantly greater abundance in cultures amended with 1,2,3,4-TeCDD than in cultures without 1,2,3,4-TeCDD. This implies the involvement of this Dehalococcoides mccartyi strain in the reductive dechlorination of 1,2,3,4-TeCDD and suggests the applicability of SIP for a robust assessment of the bioremediation potential of organohalogen contaminated sites.

Arsenic mobilization in a high arsenic groundwater revealed by metagenomic and Geochip analyses

Microbial metabolisms of arsenic, iron, sulfur, nitrogen and organic matter play important roles in arsenic mobilization in aquifer. In this study, microbial community composition and functional potentials in a high arsenic groundwater were investigated using integrated techniques of RNA- and DNA-based 16S rRNA gene sequencing, metagenomic sequencing and functional gene arrays. 16S rRNA gene sequencing showed the sample was dominated by members of Proteobacteria (62.3-75.2%), such as genera of Simplicispira (5.7-6.7%), Pseudomonas (3.3-5.7%), Ferribacterium (1.6-4.4%), Solimonas (1.8-3.2%), Geobacter (0.8-2.2%) and Sediminibacterium (0.6-2.4%). Functional potential analyses indicated that organics degradation, assimilatory sulfate reduction, As-resistant pathway, iron reduction, ammonification, nitrogen fixation, denitrification and dissimilatory nitrate reduction to ammonia were prevalent. The composition and function of microbial community and reconstructed genome bins suggest that high level of arsenite in the groundwater may be attributed to arsenate release from iron oxides reductive dissolution by the iron-reducing bacteria, and subsequent arsenate reduction by ammonia-producing bacteria featuring ars operon. This study highlights the relationship between biogeochemical cycling of arsenic and nitrogen in groundwater, which potentially occur in other aquifers with high levels of ammonia and arsenic.

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

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

Expression of Genes and Proteins Involved in Arsenic Respiration and Resistance in Dissimilatory Arsenate-Reducing Geobacter sp. Strain OR-1

The reduction of arsenate [As(V)] to arsenite [As(III)] by dissimilatory As(V)-reducing bacteria, such as Geobacter spp., may play a significant role in arsenic release from anaerobic sediments into groundwater. The biochemical and molecular mechanisms by which these bacteria cope with this toxic element remain unclear. In this study, the expression of several genes involved in arsenic respiration (arr) and resistance (ars) was determined using Geobacter sp. strain OR-1, the only cultured Geobacter strain capable of As(V) respiration. In addition, proteins expressed differentially under As(V)-respiring conditions were identified by semiquantitative proteomic analysis. Dissimilatory As(V) reductase (Arr) of strain OR-1 was localized predominantly in the periplasmic space, and the transcription of its gene (arrA) was upregulated under As(V)-respiring conditions. The transcription of the detoxifying As(V) reductase gene (arsC) was also upregulated, but its induction required 500 times higher concentration of As(III) (500 μM) than did the arrA gene. Comparative proteomic analysis revealed that in addition to the Arr and Ars proteins, proteins involved in the following processes were upregulated under As(V)-respiring conditions: (i) protein folding and assembly for rescue of proteins with oxidative damage, (ii) DNA replication and repair for restoration of DNA breaks, (iii) anaplerosis and gluconeogenesis for sustainable energy production and biomass formation, and (iv) protein and nucleotide synthesis for the replacement of damaged proteins and nucleotides. These results suggest that strain OR-1 copes with arsenic stress by orchestrating pleiotropic processes that enable this bacterium to resist and actively metabolize arsenic.IMPORTANCE Dissimilatory As(V)-reducing bacteria, such as Geobacter spp., play significant roles in arsenic release and contamination in groundwater and threaten the health of people worldwide. However, the biochemical and molecular mechanisms by which these bacteria cope with arsenic toxicity remain unclear. In this study, it was found that both respiratory and detoxifying As(V) reductases of a dissimilatory As(V)-reducing bacterium, Geobacter sp. strain OR-1, were upregulated under As(V)-respiring conditions. In addition, various proteins expressed specifically or more abundantly in strain OR-1 under arsenic stress were identified. Strain OR-1 actively metabolizes arsenic while orchestrating various metabolic processes that repair oxidative damage caused by arsenic. Such information is useful in assessing and identifying possible countermeasures for the prevention of microbial arsenic release in nature.

Humic Substances Facilitate Arsenic Reduction and Release in Flooded Paddy Soil

Organic matter is important for controlling arsenic reduction and release under anoxic conditions. Humic substances (HS) represent an important fraction of natural organic matter, yet the manner in which HS affect arsenic transformation in flooded paddy soil has not been thoroughly elucidated. In this study, anaerobic microcosms were established with arsenic-contaminated paddy soil and amended with three extracted humic fractions (fulvic acid, FA; humic acid, HA; and humin, HM). The HS substantially enhanced the extent of arsenic reduction and release in the order FA > HA > HM. It was confirmed that microbially reduced HS acted as an electron shuttle to promote arsenate reduction. HS, particularly FA, provided labile carbon to stimulate microbial activity and increase the relative abundances of Azoarcus, Anaeromyxobacter, and Pseudomonas, all of which may be involved in the reduction of HS, Fe(III), and arsenate. HS also increased the abundance of transcripts for an arsenate-respiring gene ( arrA) and overall transcription in arsenate-respiring Geobacter spp. The increase in both abundances lagged behind the increases in dissolved arsenate levels. These results help to elucidate the pathways of arsenic reduction and release in the presence of HS in flooded paddy soil.

As release under the microbial sulfate reduction during redox oscillations in the upper Mekong delta aquifers, Vietnam: A mechanistic study

The impact of seasonal fluctuations linked to monsoon and irrigation generates redox oscillations in the subsurface, influencing the release of arsenic (As) in aquifers. Here, the biogeochemical control on As mobility was investigated in batch experiments using redox cycling bioreactors and As- and SO₄^2--amended sediment. Redox potential (E_h) oscillations between anoxic (-300-0 mV) and oxic condition (0-500 mV) were implemented by automatically modulating an admixture of N₂/CO₂ or compressed air. A carbon source (cellobiose, a monomer of cellulose) was added at the beginning of each reducing cycle to stimulate the metabolism of the native microbial community. Results show that successive redox cycles can decrease arsenic mobility by up to 92% during reducing conditions. Anoxic conditions drive mainly the conversion of soluble As(V) to As(III) in contrast to oxic conditions. Phylogenetic analyses of 16S rRNA amplified from the sediments revealed the presence of sulfate and iron - reducing bacteria, confirming that sulfate and iron reduction are key factors for As immobilization from the aqueous phase. As and S K-edge X-ray absorption spectroscopy suggested the association of Fe-(oxyhydr)oxides and the importance of pyrite (FeS_2(s)), rather than poorly ordered mackinawite (FeS_(s)), for As sequestration under oxidizing and reducing conditions, respectively. Finally, these findings suggest a role for elemental sulfur in mediating aqueous thioarsenates formation in As-contaminated groundwater of the Mekong delta.

Isolation and characterization of aerobic, culturable, arsenic-tolerant bacteria from lead-zinc mine tailing in southern China

Bioremediation of arsenic (As) pollution is an important environmental issue. The present investigation was carried out to isolate As-resistant novel bacteria and characterize their As transformation and tolerance ability. A total of 170 As-resistant bacteria were isolated from As-contaminated soils at the Kangjiawan lead-zinc tailing mine, located in Hunan Province, southern China. Thirteen As-resistant isolates were screened by exposure to 260 mM Na₂HAsO₄·7H₂O, most of which showed a very high level of resistance to As⁵⁺ (MIC ≥ 600 mM) and As³⁺ (MIC ≥ 10 mM). Sequence analysis of 16S rRNA genes indicated that the 13 isolates tested belong to the phyla Firmicutes, Proteobacteria and Actinobacteria, and these isolates were assigned to eight genera, Bacillus, Williamsia, Citricoccus, Rhodococcus, Arthrobacter, Ochrobactrum, Pseudomonas and Sphingomonas. Genes involved in As resistance were present in 11 of the isolates. All 13 strains transformed As (1 mM); the oxidation and reduction rates were 5-30% and 10-51.2% within 72 h, respectively. The rates of oxidation by Bacillus sp. Tw1 and Pseudomonas spp. Tw224 peaked at 42.48 and 34.94% at 120 h, respectively. For Pseudomonas spp. Tw224 and Bacillus sp. Tw133, the highest reduction rates were 52.01% at 48 h and 48.66% at 144 h, respectively. Our findings will facilitate further research into As metabolism and bioremediation of As pollution by genome sequencing and genes modification.

Microbially Mediated Methylation of Arsenic in the Arsenic-Rich Soils and Sediments of Jianghan Plain

Almost nothing is known about the activities and diversities of microbial communities involved in As methylation in arsenic-rich shallow and deep sediments; the correlations between As biomethylation and environmental parameters also remain to be elucidated. To address these issues, we collected 9 arsenic-rich soil/sediment samples from the depths of 1, 30, 65, 95, 114, 135, 175, 200, and 223 m in Jianghan Plain, China. We used microcosm assays to determine the As-methylating activities of the microbial communities in the samples. To exclude false negative results, we amended the microcosms with 0.2 mM As(III) and 20.0 mM lactate. The results indicated that the microbial communities in all of the samples significantly catalyzed arsenic methylation. The arsM genes were detectable from all the samples with the exception of 175 m, and 90 different arsM genes were identified. All of these genes code for new or new-type ArsM proteins, suggesting that new As-methylating microorganisms are widely distributed in the samples from shallow to deep sediments. To determine whether microbial biomethylation of As occurs in the sediments under natural geochemical conditions, we conducted microcosm assays without exogenous As and carbons. After 80.0 days of incubation, approximately 4.5–15.5 μg/L DMAs^V were detected in all of the microcosms with the exception of that from 30 m, and 2.0–9.0 μg/L MMAs^V were detected in the microcosms of 65, 114, 135, 175, 200, and 223 m; moreover, approximately 18.7–151.5 μg/L soluble As(V) were detected from the nine sediment samples. This suggests that approximately 5.3, 0, 8.1, 28.9, 18.0, 8.7, 13.8, 10.2, and 14.9% of total dissolved As were methylated by the microbial communities in the sediment samples from 1, 30, 65, 95, 114, 135, 175, 200, and 223 m, respectively. The concentrations of biogenic DMAs^V show significant positive correlations with the depths of sediments, and negative correlations with the environmental NH₄⁺ and NaCl concentrations, but show no significant correlations with other environmental parameters, such as NO₃^-, SO₄²⁺, TOC, TON, Fe, Sb, Cu, K, Ca, Mg, Mn, and Al. This work helps to better understand the biogeochemical cycles of arsenic in arsenic-rich shallow and deep sediments.

Effect of microbially mediated iron mineral transformation on temporal variation of arsenic in the Pleistocene aquifers of the central Yangtze River basin

Significant seasonal variation of groundwater arsenic (As) concentrations in shallow aquifers of the Jianghan Plain, central Yangtze River Basin has been reported recently, but the underlying mechanisms remain not well understood. To elaborate biogeochemical processes responsible for the observed As concentration variation, 42-day incubation experiments were done using sediment samples collected respectively from the depth of 26, 36 and 60m of the As-affected aquifer which were labeled respectively as JH26, JH36, JH60. Where JH denotes Jianghan Plain, and the number indicates the depth of the sediment sample. The results indicated that As could be mobilized from the sediments of 26m and 36m depth under the stimulation of exogenous organic carbon, with the maximum As release amount of 1.60 and 1.03mgkg^-1, respectively, while the sediments at 60m depth did not show As mobilization. The microbially mediated reductive dissolution of amorphous iron oxides and reduction of As(V) to As(III) could account for the observed As mobilization. The 16S rRNA high-throughput sequencing results indicated that the variation of microbial community correlated with the released As concentration (R=0.7, P<0.05) and the iron-reducing bacteria, including Pseudomonas, Clostridium and Geobacter, were the main drivers for the As mobilization from the sediments at 26m and 36m depth. The increase of arsC gene abundance (up to 1.4×10⁵ copies g^-1) during As release suggested that As reduction was mediated by the resistant reduction mechanism. By contrast, in the 60m sediments where the Fe and As release was absent, the iron-reducing bacteria accounted for a very minor proportion and sulfate-reducing bacteria were predominant in the microbial community. In addition, after 30days of incubation, the released As in the 26m sediments was immobilized via co-precipitation with or adsorption onto the Fe-sulfide mineral newly-formed by the bacterial sulfate reduction. These results are consistent with the results of our previous field monitoring, indicating that the bacterial sulfate reduction could lead to the temporal decrease in groundwater As concentrations. This study provides insights into the mechanism for As mobilization and seasonal As concentration variation in the Pleistocene aquifers from alluvial plains.

Roles of different active metal-reducing bacteria in arsenic release from arsenic-contaminated paddy soil amended with biochar

Although biochar has great potential for heavy metal removal from sediments or soils, its impact on arsenic biogeochemistry in contaminated paddy fields remains poorly characterized. In this study, anaerobic microcosms were established with arsenic-contaminated paddy soil to investigate arsenic transformation as well as the potentially active microbial community and their transcriptional activities in the presence of biochar. The results demonstrated that biochar can simultaneously stimulate microbial reduction of As(V) and Fe(III), releasing high levels of As(III) into the soil solution relative to the control. Total RNAs were extracted to profile the potentially active microbial communities, which suggested that biochar increased the abundance of arsenic- and iron-related bacteria, such as Geobacter, Anaeromyxobacter and Clostridium compared to the control. Reverse transcription, quantitative PCR (RT-qPCR) showed that the abundance of Geobacter transcripts were significantly stimulated by biochar throughout the incubation. Furthermore, significant positive correlations were observed between the abundance of Geobacter transcripts and As(V) concentrations, and between that of Clostridium transcripts and Fe(III) concentrations in biochar-amended microcosms. Our findings suggest that biochar can stimulate the activity of metal-reducing bacteria to promote arsenic mobility. The Geobacter may contribute to As(V) reduction in the presence of biochar, while Clostridium has a role in Fe(III) reduction.

Metal-tolerant thermophiles: metals as electron donors and acceptors, toxicity, tolerance and industrial applications

Metal-tolerant thermophiles are inhabitants of a wide range of extreme habitats like solfatara fields, hot springs, mud holes, hydrothermal vents oozing out from metal-rich ores, hypersaline pools and soil crusts enriched with metals and other elements. The ability to withstand adverse environmental conditions, like high temperature, high metal concentration and sometimes high pH in their niche, makes them an interesting subject for understanding mechanisms behind their ability to deal with multiple duress simultaneously. Metals are essential for biological systems, as they participate in biochemistries that cannot be achieved only by organic molecules. However, the excess concentration of metals can disrupt natural biogeochemical processes and can impose toxicity. Thermophiles counteract metal toxicity via their unique cell wall, metabolic factors and enzymes that carry out metal-based redox transformations, metal sequestration by metallothioneins and metallochaperones as well as metal efflux. Thermophilic metal resistance is heterogeneous at both genetic and physiology levels and may be chromosomally, plasmid or transposon encoded with one or more genes being involved. These effective response mechanisms either individually or synergistically make proliferation of thermophiles in metal-rich habitats possibly. This article presents the state of the art and future perspectives of responses of thermophiles to metals at genetic as well as physiological levels.

Transcriptional Activity of Arsenic-Reducing Bacteria and Genes Regulated by Lactate and Biochar during Arsenic Transformation in Flooded Paddy Soil

Organic substrates and biochar are important in controlling arsenic release from sediments and soils; however, little is known about their impact on arsenic-reducing bacteria and genes during arsenic transformation in flooded paddy soils. In this study, microcosm experiments were established to profile transcriptional activity of As(V)-respiring gene (arrA) and arsenic resistance gene (arsC) as well as the associated bacteria regulated by lactate and/or biochar in anaerobic arsenic-contaminated paddy soils. Chemical analyses revealed that lactate as the organic substrate stimulated microbial reduction of As(V) and Fe(III), which was simultaneously promoted by lactate+biochar, due to biochar's electron shuttle function that facilitates electron transfer from bacteria to As(V)/Fe(III). Sequencing and phylogenetic analyses demonstrated that both arrA closely associated with Geobacter (>60%, number of identical sequences/number of the total sequences) and arsC related to Enterobacteriaceae (>99%) were selected by lactate and lactate+biochar. Compared with the lactate microcosms, transcriptions of the bacterial 16S rRNA gene, Geobacter spp., and Geobacter arrA and arsC genes were increased in the lactate+biochar microcosms, where transcript abundances of Geobacter and Geobacter arrA closely tracked with dissolved As(V) concentrations. Our findings indicated that lactate and biochar in flooded paddy soils can stimulate the active As(V)-respiring bacteria Geobacter species for arsenic reduction and release, which probably increases arsenic bioavailability to rice plants.

Microbial Community Structure and Arsenic Biogeochemistry in Two Arsenic-Impacted Aquifers in Bangladesh

Long-term exposure to trace levels of arsenic (As) in shallow groundwater used for drinking and irrigation puts millions of people at risk of chronic disease. Although microbial processes are implicated in mobilizing arsenic from aquifer sediments into groundwater, the precise mechanism remains ambiguous. The goal of this work was to target, for the first time, a comprehensive suite of state-of-the-art molecular techniques in order to better constrain the relationship between indigenous microbial communities and the iron and arsenic mineral phases present in sediments at two well-characterized arsenic-impacted aquifers in Bangladesh. At both sites, arsenate [As(V)] was the major species of As present in sediments at depths with low aqueous As concentrations, while most sediment As was arsenite [As(III)] at depths with elevated aqueous As concentrations. This is consistent with a role for the microbial As(V) reduction in mobilizing arsenic. 16S rRNA gene analysis indicates that the arsenic-rich sediments were colonized by diverse bacterial communities implicated in both dissimilatory Fe(III) and As(V) reduction, while the correlation analyses involved phylogenetic groups not normally associated with As mobilization. Findings suggest that direct As redox transformations are central to arsenic fate and transport and that there is a residual reactive pool of both As(V) and Fe(III) in deeper sediments that could be released by microbial respiration in response to hydrologic perturbation, such as increased groundwater pumping that introduces reactive organic carbon to depth.IMPORTANCE The consumption of arsenic in waters collected from tube wells threatens the lives of millions worldwide and is particularly acute in the floodplains and deltas of southern Asia. The cause of arsenic mobilization from natural sediments within these aquifers to groundwater is complex, with recent studies suggesting that sediment-dwelling microorganisms may be the cause. In the absence of oxygen at depth, specialist bacteria are thought able to use metals within the sediments to support their metabolism. Via these processes, arsenic-contaminated iron minerals are transformed, resulting in the release of arsenic into the aquifer waters. Focusing on a field site in Bangladesh, a comprehensive, multidisciplinary study using state-of-the-art geological and microbiological techniques has helped better understand the microbes that are present naturally in a high-arsenic aquifer and how they may transform the chemistry of the sediment to potentially lethal effect.

Influence of bacterial strains on Oryza sativa grown under arsenic tainted soil: Accumulation and detoxification response

The present study was conducted to study the role of bacterial inoculants in growth, accumulation and tolerance responses in rice grown in arsenic (As) contaminated soil. Results revealed that out of five isolated bacterial strains, strain BBAU/MMM₁ (Babasaheb Bhimrao Ambedkar University/Mari Matamandir) exhibited resistant to As(III) to the level of 400 μM As(III) in comparison to other strain which showed toxicity. The isolated strain BBAU/MMM_{1 1}was characterized as gram negative, rod shape, showed positive test nitrate, citrate, catalase and phosphate solubilization with high production of IAA and siderophore. Arsenic treated rice seedlings supplemented with bacterial inoculants BBAU/MMM₁ showed increased growth characteristics viz., root length (35.41%), shoot length (32.8%) and fresh weight (30.31%) in comparison to rice treated with As(III) only. In addition, reduced lipid peroxidation and induces cysteine, GSH (reduced glutathione), GSSG (Oxidized glutathione) and antioxidant enzymatic activities were also observed in bacterial supplemented rice seedlings. Further, reduced metal accumulation in root (1351.46-968.67 μg g^-1fw), shoot (488.01-378.02 μg g^-1fw) and reduced translocation factor (0.583-0.390) in As(III) treated rice seedlings inoculated with bacterial strains clearly reflect protective response of bacterial strain against As toxicity. Thus, isolated bacterial strain could be used as bioremediator as well as growth inducer in paddy field contaminated with arsenic.

Novel arsenic-transforming bacteria and the diversity of their arsenic-related genes and enzymes arising from arsenic-polluted freshwater sediment

Bacteria are essential in arsenic cycling. However, few studies have addressed 16S rRNA and arsenic-related functional gene diversity in long-term arsenic-contaminated tropical sediment. Here, using culture-based, metagenomic and computational approaches, we describe the diversity of bacteria, genes and enzymes involved in AsIII and AsV transformation in freshwater sediment and in anaerobic AsIII- and AsV-enrichment cultures (ECs). The taxonomic profile reveals significant differences among the communities. Arcobacter, Dechloromonas, Sedimentibacter and Clostridium thermopalmarium were exclusively found in ECs, whereas Anaerobacillus was restricted to AsV-EC. Novel taxa that are both AsV-reducers and AsIII-oxidizers were identified: Dechloromonas, Acidovorax facilis, A. delafieldii, Aquabacterium, Shewanella, C. thermopalmarium and Macellibacteroides fermentans. Phylogenic discrepancies were revealed among the aioA, arsC and arrA genes and those of other species, indicating horizontal gene transfer. ArsC and AioA have sets of amino acids that can be used to assess their functional and structural integrity and familial subgroups. The positions required for AsV reduction are conserved, suggesting strong selective pressure for maintaining the functionality of ArsC. Altogether, these findings highlight the role of freshwater sediment bacteria in arsenic mobility, and the untapped diversity of dissimilatory arsenate-reducing and arsenate-resistant bacteria, which might contribute to arsenic toxicity in aquatic environments.

Linking Genes to Microbial Biogeochemical Cycling: Lessons from Arsenic

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

Bacterial community and arsenic functional genes diversity in arsenic contaminated soils from different geographic locations

To understand how soil microbial communities and arsenic (As) functional genes respond to soil arsenic (As) contamination, five soils contaminated with As at different levels were collected from diverse geographic locations, incubated for 54 days under flooded conditions, and examined by both MiSeq sequencing of 16S rRNA gene amplicons and functional gene microarray (GeoChip 4.0). The results showed that both bacterial community structure and As functional gene structure differed among geographical locations. The diversity of As functional genes correlated positively with the diversity of 16S rRNA genes (P< 0.05). Higher diversities of As functional genes and 16S rRNA genes were observed in the soils with higher available As. Soil pH, phosphate-extractable As, and amorphous Fe content were the most important factors in shaping the bacterial community structure and As transformation functional genes. Geographic location was also important in controlling both the bacterial community and As transformation functional potential. These findings provide insights into the variation of As transformation functional genes in soils contaminated with different levels of As at different geographic locations, and the impact of environmental As contamination on the soil bacterial community.

New Arsenate Reductase Gene (arrA) PCR Primers for Diversity Assessment and Quantification in Environmental Samples

The extent of arsenic contamination in drinking water and its potential threat to human health have resulted in considerable research interest in the microbial species responsible for arsenic reduction. The arsenate reductase gene (arrA), an important component of the microbial arsenate reduction system, has been widely used as a biomarker to study arsenate-reducing microorganisms. A new primer pair was designed and evaluated for quantitative PCR (qPCR) and high-throughput sequencing of the arrA gene, because currently available PCR primers are not suitable for these applications. The primers were evaluated in silico and empirically tested for amplification of arrA genes in clones and for amplification and high-throughput sequencing of arrA genes from soil and groundwater samples. In silico, this primer pair matched (≥90% DNA identity) 86% of arrA gene sequences from GenBank. Empirical evaluation showed successful amplification of arrA gene clones of diverse phylogenetic groups, as well as amplification and high-throughput sequencing of independent soil and groundwater samples without preenrichment, suggesting that these primers are highly specific and can amplify a broad diversity of arrA genes. The arrA gene diversity from soil and groundwater samples from the Cache Valley Basin (CVB) in Utah was greater than anticipated. We observed a significant correlation between arrA gene abundance, quantified through qPCR, and reduced arsenic (AsIII) concentrations in the groundwater samples. Furthermore, we demonstrated that these primers can be useful for studying the diversity of arsenate-reducing microbial communities and the ways in which their relative abundance in groundwater may be associated with different groundwater quality parameters.

IMPORTANCE

Arsenic is a major drinking water contaminant that threatens the health of millions of people worldwide. The extent of arsenic contamination and its potential threat to human health have resulted in considerable interest in the study of microbial species responsible for the reduction of arsenic, i.e., the conversion of AsV to AsIII In this study, we developed a new primer pair to evaluate the diversity and abundance of arsenate-reducing microorganisms in soil and groundwater samples from the CVB in Utah. We observed significant arrA gene diversity in the CVB soil and groundwater samples, and arrA gene abundance was significantly correlated with the reduced arsenic (AsIII) concentrations in the groundwater samples. We think that these primers are useful for studying the ecology of arsenate-reducing microorganisms in different environments.

Microbial communities involved in arsenic mobilization and release from the deep sediments into groundwater in Jianghan plain, Central China

It was shown that groundwater in Jianghan Plain was severely contaminated by arsenic; however, little is known about the mechanism by which the mineral arsenic was mobilized and released into groundwater from the high-arsenic sediments in this area. Here, we collected sediment samples from the depths of 5-230m in Jianghan Plain. Although all of the samples contain high contents of total arsenic, the soluble arsenic was only detectable in few of the shallow sediments, but was readily detectable in all of the deep sediments at the depths of 190-230m. Analysis of the genes of arsenate-respiring reductases indicated that they were not present in all of the shallow sediments from the depths of 5-185m, but were detectable in all of the deep sediments from the depths of 190-230m; all of the identified reductase genes are new or new-type, and they display unique diversity. Microcosm assay indicated that the microbial communities from the deep sediments were able to reduce As(V) into As(III) using lactate, formate, pyruvate or acetate as an electron donor under anaerobic condition. Arsenic release assay demonstrated that these microbial communalities efficiently catalyzed the mobilization and release of the mineral arsenic into aqueous phase. We also isolated a novel cultivable dissimilatory As(V)-respiring bacterium Aeromonas sp. JH155 from the sediments. It is able to completely reduce 2.0mM As(V) into As(III) in 72h, and efficiently promote the reduction and release of the mineral arsenic into aqueous phase. Analysis of the 16S rRNA genes indicated that the deep sediments contain diversities of microbial communities, which were shaped by the environmental factors, such as As, SO₄^2-, NO₃^-, Fe and pH value. These data suggest that the microorganisms in the deep sediments in Jianghan Plain played key roles in the mobilization and release of insoluble arsenic into the groundwater.

Arsenic Detoxification by Geobacter Species

Insight into the mechanisms for arsenic detoxification by Geobacter species is expected to improve the understanding of global cycling of arsenic in iron-rich subsurface sedimentary environments. Analysis of 14 different Geobacter genomes showed that all of these species have genes coding for an arsenic detoxification system (ars operon), and several have genes required for arsenic respiration (arr operon) and methylation (arsM). Genes encoding four arsenic repressor-like proteins were detected in the genome of G. sulfurreducens; however, only one (ArsR1) regulated transcription of the ars operon. Elimination of arsR1 from the G. sulfurreducens chromosome resulted in enhanced transcription of genes coding for the arsenic efflux pump (Acr3) and arsenate reductase (ArsC). When the gene coding for Acr3 was deleted, cells were not able to grow in the presence of either the oxidized or reduced form of arsenic, while arsC deletion mutants could grow in the presence of arsenite but not arsenate. These studies shed light on how Geobacter influences arsenic mobility in anoxic sediments and may help us develop methods to remediate arsenic contamination in the subsurface.

IMPORTANCE

This study examines arsenic transformation mechanisms utilized by Geobacter, a genus of iron-reducing bacteria that are predominant in many anoxic iron-rich subsurface environments. Geobacter species play a major role in microbially mediated arsenic release from metal hydroxides in the subsurface. This release raises arsenic concentrations in drinking water to levels that are high enough to cause major health problems. Therefore, information obtained from studies of Geobacter should shed light on arsenic cycling in iron-rich subsurface sedimentary environments, which may help reduce arsenic-associated illnesses. These studies should also help in the development of biosensors that can be used to detect arsenic contaminants in anoxic subsurface environments. We examined 14 different Geobacter genomes and found that all of these species possess genes coding for an arsenic detoxification system (ars operon), and some also have genes required for arsenic respiration (arr operon) and arsenic methylation (arsM).

The patterns of bacterial community and relationships between sulfate-reducing bacteria and hydrochemistry in sulfate-polluted groundwater of Baogang rare earth tailings

Microorganisms are the primary agents responsible for the modification, degradation, and/or detoxification of pollutants, and thus, they play a major role in their natural attenuation; yet, little is known about the structure and diversity of the subsurface community and relationships between microbial community and groundwater hydrochemistry. In this study, denaturing gradient gel electrophoresis (DGGE) and terminal restriction fragment length polymorphism (T-RFLP) allowed a comparative microbial community analysis of sulfate-contaminated groundwater samples from nine different wells in the region of Baogang rare earth tailings. Using real-time PCR, the abundance of total bacteria and the sulfate-reducing genes of aprA and dsrB were quantified. Statistical analyses showed a clear distinction of the microbial community diversity between the contaminated and uncontaminated samples, with Proteobacteria being the most dominant members of the microbial community. SO₄^2- concentrations exerted a significant effect on the variation of the bacterial community (P < 0.05), with higher concentrations of sulfate reducing the microbial diversity (H' index), indicating that human activity (e.g., mining industries) was a possible factor disturbing the structure of the bacterial community. Quantitative analysis of the functional genes showed that the proportions of dsrB to total bacteria were 0.002-2.85 %, and the sulfate-reducing bacteria (SRB) were predominant within the prokaryotic community in the groundwater. The uncontaminated groundwater with low sulfate concentration harbored higher abundance of SRB than that in the polluted samples, while no significant correlation was observed between sulfate concentrations and SRB abundances in this study, suggesting other environmental factors possibly contributed to different distributions and abundances of SRB in the different sites. The results should facilitate expanded studies to identify robust microbe-environment interactions and provide a strong foundation for qualitative exploration of the bacterial diversity in rare earth tailings groundwater that might ultimately be incorporated into the remediation of environmental contamination.

An arsenate-reducing and alkane-metabolizing novel bacterium, Rhizobium arsenicireducens sp. nov., isolated from arsenic-rich groundwater

A novel arsenic (As)-resistant, arsenate-respiring, alkane-metabolizing bacterium KAs 5-22^T, isolated from As-rich groundwater of West Bengal was characterized by physiological and genomic properties. Cells of strain KAs 5-22^T were Gram-stain-negative, rod-shaped, motile, and facultative anaerobic. Growth occurred at optimum of pH 6.0-7.0, temperature 30 °C. 16S rRNA gene affiliated the strain KAs 5-22^T to the genus Rhizobium showing maximum similarity (98.4 %) with the type strain of Rhizobium naphthalenivorans TSY03b^T followed by (98.0 % similarity) Rhizobium selenitireducens B1^T. The genomic G + C content was 59.4 mol%, and DNA-DNA relatedness with its closest phylogenetic neighbors was 50.2 %. Chemotaxonomy indicated UQ-10 as the major quinone; phosphatidylethanolamine, phosphatidylglycerol, and diphosphatidylglycerol as major polar lipids; C_16:0, C_17:0, 2-OH C_10:0, 3-OH C_16:0, and unresolved C_18:1 ɷ7C/ɷ9C as predominant fatty acids. The cells were found to reduce O₂, As⁵⁺, NO₃^-, SO₄^2- and Fe³⁺ as alternate electron acceptors. The strain's ability to metabolize dodecane or other alkanes as sole carbon source using As⁵⁺ as terminal electron acceptor was supported by the presence of genes encoding benzyl succinate synthase (bssA like) and molybdopterin-binding site (mopB) of As⁵⁺ respiratory reductase (arrA). Differential phenotypic, chemotaxonomic, genotypic as well as physiological properties revealed that the strain KAs 5-22^T is separated from its nearest recognized Rhizobium species. On the basis of the data presented, strain KAs 5-22^T is considered to represent a novel species of the genus Rhizobium, for which the name Rhizobium arsenicireducens sp. nov. is proposed as type strain (=LMG 28795^T=MTCC 12115^T).

Metabolic Network Modeling of Microbial Interactions in Natural and Engineered Environmental Systems

We review approaches to characterize metabolic interactions within microbial communities using Stoichiometric Metabolic Network (SMN) models for applications in environmental and industrial biotechnology. SMN models are computational tools used to evaluate the metabolic engineering potential of various organisms. They have successfully been applied to design and optimize the microbial production of antibiotics, alcohols and amino acids by single strains. To date however, such models have been rarely applied to analyze and control the metabolism of more complex microbial communities. This is largely attributed to the diversity of microbial community functions, metabolisms, and interactions. Here, we firstly review different types of microbial interaction and describe their relevance for natural and engineered environmental processes. Next, we provide a general description of the essential methods of the SMN modeling workflow including the steps of network reconstruction, simulation through Flux Balance Analysis (FBA), experimental data gathering, and model calibration. Then we broadly describe and compare four approaches to model microbial interactions using metabolic networks, i.e., (i) lumped networks, (ii) compartment per guild networks, (iii) bi-level optimization simulations, and (iv) dynamic-SMN methods. These approaches can be used to integrate and analyze diverse microbial physiology, ecology and molecular community data. All of them (except the lumped approach) are suitable for incorporating species abundance data but so far they have been used only to model simple communities of two to eight different species. Interactions based on substrate exchange and competition can be directly modeled using the above approaches. However, interactions based on metabolic feedbacks, such as product inhibition and synthropy require extensions to current models, incorporating gene regulation and compounding accumulation mechanisms. SMN models of microbial interactions can be used to analyze complex “omics” data and to infer and optimize metabolic processes. Thereby, SMN models are suitable to capitalize on advances in high-throughput molecular and metabolic data generation. SMN models are starting to be applied to describe microbial interactions during wastewater treatment, in-situ bioremediation, microalgae blooms methanogenic fermentation, and bioplastic production. Despite their current challenges, we envisage that SMN models have future potential for the design and development of novel growth media, biochemical pathways and synthetic microbial associations.

Profiling microbial community in a watershed heavily contaminated by an active antimony (Sb) mine in Southwest China

Located in Southwest China, the Chahe watershed has been severely contaminated by upstream active antimony (Sb) mines. The extremely high concentrations of Sb make the Chahe watershed an excellent model to elucidate the response of indigenous microbial activities within a severe Sb-contaminated environment. In this study, water and surface sediments from six locations in the Chahe watershed with different levels of Sb contamination were analyzed. Illumina sequencing of 16S rRNA amplicons revealed more than 40 phyla from the domain Bacteria and 2 phyla from the domain Archaea. Sequences assigned to the genera Flavobacterium, Sulfuricurvum, Halomonas, Shewanella, Lactobacillus, Acinetobacter, and Geobacter demonstrated high relative abundances in all sequencing libraries. Spearman's rank correlations indicated that a number of microbial phylotypes were positively correlated with different speciation of Sb, suggesting potential roles of these phylotypes in microbial Sb cycling. Canonical correspondence analysis further demonstrated that geochemical parameters, including water temperature, pH, total Fe, sulfate, aqueous Sb, and Eh, significantly structured the overall microbial community in Chahe watershed samples. Our findings offer a direct and reliable reference to the diversity of microbial communities in the presence of extremely high Sb concentrations, and may have potential implications for in situ bioremediation strategies of Sb contaminated sites.

Diversity and Distribution of Arsenic-Related Genes Along a Pollution Gradient in a River Affected by Acid Mine Drainage

Some microorganisms have the capacity to interact with arsenic through resistance or metabolic processes. Their activities contribute to the fate of arsenic in contaminated ecosystems. To investigate the genetic potential involved in these interactions in a zone of confluence between a pristine river and an arsenic-rich acid mine drainage, we explored the diversity of marker genes for arsenic resistance (arsB, acr3.1, acr3.2), methylation (arsM), and respiration (arrA) in waters characterized by contrasted concentrations of metallic elements (including arsenic) and pH. While arsB-carrying bacteria were representative of pristine waters, Acr3 proteins may confer to generalist bacteria the capacity to cope with an increase of contamination. arsM showed an unexpected wide distribution, suggesting biomethylation may impact arsenic fate in contaminated aquatic ecosystems. arrA gene survey suggested that only specialist microorganisms (adapted to moderately or extremely contaminated environments) have the capacity to respire arsenate. Their distribution, modulated by water chemistry, attested the specialist nature of the arsenate respirers. This is the first report of the impact of an acid mine drainage on the diversity and distribution of arsenic (As)-related genes in river waters. The fate of arsenic in this ecosystem is probably under the influence of the abundance and activity of specific microbial populations involved in different As biotransformations.

Arsenic(V) Incorporation in Vivianite during Microbial Reduction of Arsenic(V)-Bearing Biogenic Fe(III) (Oxyhydr)oxides

The dissolution of arsenic-bearing iron(III) (oxyhydr)oxides during combined microbial iron(III) and arsenate(V) reduction is thought to be the main mechanism responsible for arsenic mobilization in reducing environments. Besides its mobilization during bioreduction, arsenic is often resequestered by newly forming secondary iron(II)-bearing mineral phases. In phosphate-bearing environments, iron(II) inputs generally lead to vivianite precipitation. In fact, in a previous study we observed that during bioreduction of arsenate(V)-bearing biogenic iron(III) (oxyhydr)oxides in phosphate-containing growth media, arsenate(V) was immobilized by the newly forming secondary iron(II) and iron(II)/iron(III)mineral phases, including vivianite. In the present study, changes in arsenic redox state and binding environment in these experiments were analyzed. We found that arsenate(V) partly replaced phosphate in vivianite, thus forming a vivianite-symplesite solid solution identified as Fe3(PO4)1.7(AsO4)0.3·8H2O. Our data suggests that in order to predict the fate of arsenic during the bioreduction of abiogenic and biogenic iron(III) (oxyhydr)oxides in arsenic-contaminated environments, the formation of symplesite-vivianite minerals needs to be considered. Indeed, such mineral phases could contribute to a delayed and slow release of arsenic in phosphate-bearing surface and groundwater environments.

Indigenous arsenic(V)-reducing microbial communities in redox-fluctuating near-surface sediments of the Mekong Delta

Arsenic (As) cycling within soils and sediments of the Mekong Delta of Cambodia is affected by drastic redox fluctuations caused by seasonal monsoons. Extensive flooding during monsoon seasons creates anoxic soil conditions that favor anaerobic microbial processes, including arsenate [As(V)] respiration-a process contributing to the mobilization of As. Repeated oxidation and reduction in near-surface sediments, which contain 10-40 mg kg(-1) As, lead to the eventual downward movement of As to the underlying aquifer. Amplification of a highly conserved functional gene encoding dissimilatory As(V) reductase, arrA, can be used as a molecular marker to detect the genetic potential for As(V) respiration in environmental samples. However, few studies have successfully amplified arrA from sediments without prior enrichment, which can drastically shift community structure. In the present study, we examine the distribution and diversity of arrA genes amplified from multiple sites within the Cambodian Mekong Delta as a function of near-surface depth (10, 50, 100, 200, and 400 cm), where sediments undergo seasonal redox fluctuations. We report successful amplification of 302 arrA gene sequences (72 OTUs) from near-surface Cambodian soils (without prior enrichment or stimulation with carbon amendments), where a large majority (>70%) formed a well-supported clade that is phylogenetically distinct from previously reported sequences from Cambodia and other South and Southeast Asian sediments, with highest sequence similarity to known Geobacter species capable of As(V) respiration, further supporting the potentially important role of Geobacter sp. in arsenic mobilization in these regions.

Diversity, metabolic properties and arsenic mobilization potential of indigenous bacteria in arsenic contaminated groundwater of West Bengal, India

Arsenic (As) mobilization in alluvial aquifers is caused by a complex interplay of hydro-geo-microbiological activities. Nevertheless, diversity and biogeochemical significance of indigenous bacteria in Bengal Delta Plain are not well documented. We have deciphered bacterial community compositions and metabolic properties in As contaminated groundwater of West Bengal to define their role in As mobilization. Groundwater samples showed characteristic high As, low organic carbon and reducing property. Culture-independent and -dependent analyses revealed presence of diverse, yet near consistent community composition mostly represented by genera Pseudomonas, Flavobacterium, Brevundimonas, Polaromonas, Rhodococcus, Methyloversatilis and Methylotenera. Along with As-resistance and -reductase activities, abilities to metabolize a wide range carbon substrates including long chain and polyaromatic hydrocarbons and HCO3, As3+ as electron donor and As5+/Fe3+ as terminal electron acceptor during anaerobic growth were frequently observed within the cultivable bacteria. Genes encoding cytosolic As5+ reductase (arsC) and As3+ efflux/transporter [arsB and acr3(2)] were found to be more abundant than the dissimilatory As5+ reductase gene arrA. The observed metabolic characteristics showed a good agreement with the same derived from phylogenetic lineages of constituent populations. Selected bacterial strains incubated anaerobically over 300 days using natural orange sand of Pleistocene aquifer showed release of soluble As mostly as As3+ along with several other elements (Al, Fe, Mn, K, etc.). Together with the production of oxalic acid within the biotic microcosms, change in sediment composition and mineralogy indicated dissolution of orange sand coupled with As/Fe reduction. Presence of arsC gene, As5+ reductase activity and oxalic acid production by the bacteria were found to be closely related to their ability to mobilize sediment bound As. Overall observations suggest that indigenous bacteria in oligotrophic groundwater possess adequate catabolic ability to mobilize As by a cascade of reactions, mostly linked to bacterial necessity for essential nutrients and detoxification.

Diverse arsenic- and iron-cycling microbial communities in arsenic-contaminated aquifers used for drinking water in Bangladesh

Subsurface removal of arsenic by injection with oxygenated groundwater has been proposed as a viable technology for obtaining 'safe' drinking water in Bangladesh. While the oxidation of ferrous iron to solid ferric iron minerals, to which arsenic adsorbs, is assumed to be driven by abiotic reactions, metal-cycling microorganisms may potentially affect arsenic removal. A cultivation-independent survey covering 24 drinking water wells in several geographical regions in Bangladesh was conducted to obtain information on microbial community structure and diversity in general, and on specific functional groups capable of the oxidation or reduction of arsenic or iron. Each functional group, targeted by either group-specific 16S rRNA or functional gene amplification, occurred in at least 79% of investigated samples. Putative arsenate reducers and iron-oxidizing Gallionellaceae were present at low diversity, while more variation in potentially arsenite-oxidizing microorganisms and iron-reducing Desulfuromonadales was revealed within and between samples. Relations between community composition on the one hand and hydrochemistry on the other hand were in general not evident, apart from an impact of salinity on iron-cycling microorganisms. Our data suggest widespread potential for a positive contribution of arsenite and iron oxidizers to arsenic removal upon injection with oxygenated water, but also indicate a potential risk for arsenic re-mobilization by anaerobic arsenate and iron reducers once injection is halted.

Integrated phytobial remediation for sustainable management of arsenic in soil and water

Arsenic (As), cited as the most hazardous substance by the U.S. Agency for Toxic Substance and Disease Registry (ATSDR, 2005), is an ubiquitous metalloid which when ingested for prolonged periods cause extensive health effects leading to ultimate untimely death. Plants and microbes can help mitigate soil and groundwater As problem since they have evolved elaborate detoxification machineries against this toxic metalloid as a result of their coexistence with this since the origin of life on earth. Utilization of the phytoremediation and bioremediation potential of the plants and microbes, respectively, is now regarded as two innovative tools that encompass biology, geology, biotechnology and allied sciences with cutting edge applications for sustainable mitigation of As epidemic. Discovery of As hyperaccumulating plants that uptake and concentrate large amounts of this toxic metalloid in their shoots or roots offered new hope to As phytoremediation, solar power based nature's own green remediation. This review focuses on how phytoremediation and bioremediation can be merged together to form an integrated phytobial remediation which could synergistically achieve the goal of large scale removal of As from soil, sediment and groundwater and overcome the drawbacks of the either processes alone. The review also points to the feasibility of the introduction of transgenic plants and microbes that bring new hope for more efficient treatment of As. The review identifies one critical research gap on the importance of remediation of As contaminated groundwater not only for drinking purpose but also for irrigation purpose and stresses that more research should be conducted on the use of constructed wetland, one of the most suitable areas of application of phytobial remediation. Finally the review has narrowed down on different phytoinvestigation and phytodisposal methods, which constitute the most essential and the most difficult part of pilot scale and field scale applications of phytoremediation programs.