Nitrate Stimulates Anaerobic Microbial Arsenite Oxidation in Paddy Soils

Large-scale distribution and ecological risk assessment of inorganic arsenic in surface sediments of Chinese marginal seas

Chinese marginal seas play a crucial role in sea-land interactions, but there is limited large-scale research on the occurrence and ecological risks of As in these areas. This study employed an extensive investigation on the spatial distribution and fractions of As in surface sediments across all four Chinese marginal seas, encompassing 111 sampling sites. The results revealed that the surface sediments exhibited elevated levels of total As content, surpassing those observed in other marginal seas worldwide. The HCl extractable fraction was found to be dominant in the Bohai Sea, Yellow Sea, and East China Sea, while the residual fraction prevailed in the South China Sea. Both environmental factors, including dissolved oxygen (DO), temperature, salinity, and manganese and iron contents in sediments, as well as biological processes such as nitrate-dependent As(III) oxidation, are likely significant contributors to variations in sedimentary As fractions. The distribution of As between reactive iron phases and reactive sulfur phases exhibited spatial variations across nearshore regions and offshore areas, indicating a shift in the relative roles of iron and sulfur in As retention. Generally, this study demonstrated that the ecological risks associated with As in surface sediments in the Chinese marginal seas ranged from low to moderate levels. The occurrence and distribution of As in these sediments were influenced by a complex interplay of environmental conditions, the geochemical cycling of iron and sulfur, and microorganism-mediated electron transfer processes.

New concerns about arsenic contamination in agricultural fields: an in-depth understanding of the occurrence and regulatory strategies for rice straighthead disease

Rice, a pivotal staple crop globally, faces a significant threat from straighthead disease, a pervasive physiological disorder that can diminish or obliterate yields, thereby jeopardizing food security. Currently, it is established that dimethylarsinic acid (DMA), a form of organic arsenic (As) commonly found in paddy fields, is the causal agent inducing rice straighthead disease. However, a systematic understanding of the mechanisms underlying DMA absorption, transport, toxicity, and the induction of straighthead disease remains lacking, as do effective methods for disease prevention and control. This review introduces the incidence of straighthead disease caused by DMA and delves into the potential physiological mechanisms. It synthesizes the factors influencing the uptake of methylated As and their association with straighthead disease in rice. The efficient translocation of DMA from roots to shoots, coupled with its high oxidative toxicity, leads to the manifestation of straighthead disease. Hydrogen peroxide appears to serve as a critical signaling molecule under DMA stress, elevating cell wall pectin levels and inhibiting cell expansion, leading to deformed panicles in diseased rice. Agricultural strategies aimed at diminishing the accumulation of methylated As in rice, such as intermittent flooding, minimizing organic matter input, applying chemical amendments, selecting varieties with low DMA accumulation, and employing bioremediation techniques, could mitigate straighthead disease. This review aims to heighten awareness of methylated As toxicity and straighthead disease, providing a foundational reference to guide future efforts in disease management within rice cultivation.

Effects of arsenic and trace metals on bacterial denitrification process from estuarine sediments and associated nitrous oxide emission

Coastal ecosystems currently face significant challenges due to nutrient enrichment and trace metal contamination. However, the effects of arsenic (As) and other trace metals (copper, lead, zinc, cadmium, mercury) on denitrification processes and nitrous oxide (N2O) emissions in estuarine sediments remain poorly understood. Here, we examined the influence of As and other trace metals on denitrification and N2O emissions in a single denitrifying strain, Marinobacter sp. MSD-1, isolated from metal-contaminated estuarine sediments based on its As(III)-oxidizing and denitrifying abilities and functional microbial composition. The results showed that As did not significantly affect the denitrification or N2O emission of MSD-1. However, Cd(II) at concentrations of 5-10 mg/L significantly induced the accumulation of N2O, while not significantly affecting the reduction of nitrate (NO3-) and nitrite (NO2-). The presence of As(III) further inhibited the N2O reduction under Cd exposure, but it had no significant effect on the N2O reduction after exposure to other trace metals. A negative correlation was observed between N2O reductase (NO2R) activity and N2O emissions, indicating that Cd(II) inhibits the reduction process of N2O mainly by suppressing the activity of NO2R. This study highlights the detrimental effects of cadmium on microbial denitrification and subsequent emissions of the greenhouse gas N2O, thereby improving our understanding of how estuarine and coastal ecosystems respond and adapt to trace metal pollution.

Silicon-iron modified biochar remediates cadmium and arsenic co-contaminated paddy soil by regulating cadmium and arsenic speciation

Introduction

Silicon-iron-modified biochars (SMBCs) were produced to remediate paddy soil contaminated with both cadmium (Cd) and arsenic (As). This study explored the effects of SMBCs on the transformation of Cd and As species in soil and the associated responses of functional genes to elucidate the remediation mechanisms.

Method

Three silicon-iron modified biochars were utilized. (i) Silicon dioxide magnetic biochar (SMBC1), (ii) Calcium silicate magnetic biochar (SMBC2), and (iii) Sodium silicate magnetic biochar (SMBC3) were applied to paddy soil.

Results and discussion

SMBCs increased the soil pH and the concentration of dissolved organic carbon (DOC) by 0.42-0.54 units and 6.6-16.39%, respectively. SMBC treatments reduced the bioavailable concentrations of Cd and As by 29.09-73.63% and 1.67-8.37%, respectively, transforming As(III) into less toxic As(V) and stabilizing soluble Cd into a more inert residual form. Compared to the control, SMBC significantly increased residual Cd concentrations by 2.94-16.17% (p < 0.05) and As(V) concentrations by 11.42-26.07% (p < 0.05). Adding calcium silicate (CaSiO3) at a mass ratio of 5% to magnetic biochar resulted in a residual Cd concentration of 0.79 mg·kg-1 (an increase of 16.86%) and an As(V) concentration of 37.89 mg·kg-1. SMBCs enhanced soil porosity, microbial aioA genes, and sulfate-reducing bacteria, facilitating the oxidation of As(III). Magnetic biochar amended with 5% (CaSiO3) (SMBC2) demonstrated superior efficacy in addressing the co-contamination of Cd and As. The remediation mechanisms include the following: (i) an increase in soil pH and a decrease in dissolved organic carbon (DOC), (ii) enhanced aioA gene activity, promoting the oxidation of As(III) to As(V), and increased dissimilatory sulfite reductase beta subunit (DsrB) gene activity, facilitating the reduction of sulfate ion (SO4 2-) to sulfide ion (S2-), leading to the formation of cadmium sulfide (CdS) precipitates and additional precipitation involving As and Fe. These results highlight the potential of calcium silicate-modified magnetic biochar as an effective additive for Cd and As co-contaminated soils, providing insights into heavy metals' stabilization and transformation mechanisms.

Missing pieces in the puzzle of ecology of microbial arsenate reduction

Arsenic pollution and its associated health risks have raised widespread concern. Under anaerobic conditions, arsenic mobility and toxicity increase when arsenate [As(V)] is reduced to arsenite [As(III)] by microbes through the cytoplasmic and dissimilatory pathways. However, the relative importance of these two pathways in the environment remains unclear, restricting our ability to effectively predict and regulate the environmental behavior of arsenic. Here, we review reports that declared a major role of the cytoplasmic or dissimilatory pathway in the environment and point out their limitations. We then summarize the key environmental factors influencing microbial As(V) reduction. Based on studies examining the expression of genes involved in the two As(V) reduction pathways, we hypothesize that the cytoplasmic pathway predominates at relatively high environmental As(III) concentrations, while the dissimilatory pathway is more significant at low concentrations. Future research is needed to test this hypothesis, and the expression of As(V)-reducing genes as a function of As(III) concentration can be investigated with various environmental samples and gradients.

Nitrogen, sulfur, iron, and microbial communities co-shape the seasonal biogeochemical behaviors of As and Sb in coastal tidal flat wetlands associated with rivers

Arsenic (As) and antimony (Sb) are affected by complex biogeochemical processes in coastal ecosystems. However, the influence of N, S, Fe, and microbial communities on the biogeochemistry of As and Sb in coastal tidal flat wetlands remain uncertain, particularly when rivers flow into these areas. This study combined diffusive gradients in the thin-film technique with high-throughput sequencing to investigate the release and vertical distribution of As and Sb in river and coastal tidal flat wetland sediments. The results indicated a distinct stratification phenomenon in the As release at depths ranging from 20 mm to -150 mm. At river sites, the release of As occurred in the upper layer (above -40 mm), with peak values of 4.3 and 9.3 μg/L at HS and SY sites in summer, respectively, likely due to anaerobic ammonium oxidation. In the lower layer (below -40 mm), both As and Sb were released, and this was possibly due to Fe reduction. However, at the coastal tidal flat sites, the release of As and Sb may have been driven by anaerobic ammonium oxidation, dissimilatory nitrate reduction to ammonium, sulfate reduction, and Fe reduction. At the river sites, As exhibited increased activity during the summer, and the residual forms were converted more easily into mobile forms. Sb remained relatively stable during both winter and summer. Conversely, both As and Sb primarily existed in residual forms and exhibited higher stability during summer in the coastal tidal flat sites. The microbial phyla Nitrospirota (3.6-7.0 %) and Acidobacteriota (9.5-10.2 %) were more prevalent at the river sites, whereas Desulfobacterota (8.8-12.0 %) and Firmicutes (0.13-27.9 %) were more prevalent at the coastal tidal flat sites. The bacterial genera involved in the N, S, and Fe transformation processes differed between the two sites, and they were primarily Thiobacillus, Limnobacter, and Sulfurovum at the river sites and Sva1033, Anaeromyxobacter, and Sva0485 at the coastal tidal flat sites. In this study, the microorganisms that mediated N, S, and Fe complex processes at various depths in the sediment-water interface were decoupled to elucidate the effect of these processes on the biogeochemical behavior of As and Sb as they move from rivers to coastal tidal flat wetlands.

Nitrous oxide is the main product during nitrate reduction by a novel lithoautotrophic iron(II)-oxidizing culture from an organic-rich paddy soil

Microbial nitrate reduction coupled to iron(II) oxidation (NRFeOx) occurs in paddy soils due to high levels of dissolved iron(II) and regular application of nitrogen fertilizer. However, to date, there is no lithoautotrophic NRFeOx isolate or enrichment culture available from this soil environment. Thus, resulting impacts on greenhouse gas emissions during nitrate reduction (i.e., nitrous oxide [N2O]) and on toxic metalloid (i.e., arsenic) mobility can hardly be investigated. We enriched a lithoautotrophic NRFeOx culture, culture HP (Huilongpu paddy, named after its origin), from a paddy soil (Huilongpu Town, China), which was dominated by Gallionella (71%). The culture reduced 0.45 to 0.63 mM nitrate and oxidized 1.76 to 2.31 mM iron(II) within 4 days leading to N2O as the main N-product (62%-88% N2O-N of total reduced NO3--N). Nitrite was present as an intermediate at a maximum of 0.16 ± 0.1 mM. Cells were associated with, but mostly not encrusted by, poorly crystalline iron(III) minerals (ferrihydrite). Culture HP performed best below an iron(II) threshold of 2.5-3.5 mM and in a pH range of 6.50-7.05. In the presence of 100 µM arsenite, only 0%-18% of iron(II) was oxidized. Due to low iron(II) oxidation, arsenite was not immobilized. However, the proportion of N2O-N of total reduced NO3--N decreased from 77% to 30%. Our results indicate that lithoautotrophic NRFeOx occurs even in organic-rich paddy soils, resulting in denitrification and subsequent N2O emissions. The obtained novel enrichment culture allows us to study the impact of lithoautotrophic NRFeOx on arsenic mobility and N2O emissions in paddy soils.IMPORTANCEPaddy soils are naturally rich in iron(II) and regularly experience nitrogen inputs due to fertilization. Nitrogen fertilization increases nitrous oxide emissions as it is an intermediate product during nitrate reduction. Microorganisms can live using nitrate and iron(II) as electron acceptor and donor, respectively, but mostly require an organic co-substrate. By contrast, microorganisms that only rely on nitrate, iron(II), and CO2 could inhabit carbon-limited ecological niches. So far, no isolate or consortium of lithoautotrophic iron(II)-oxidizing, nitrate-reducing microorganisms has been obtained from paddy soil. Here, we describe a lithoautotrophic enrichment culture, dominated by a typical iron(II)-oxidizer (Gallionella), that oxidized iron(II) and reduced nitrate to nitrous oxide, negatively impacting greenhouse gas dynamics. High arsenic concentrations were toxic to the culture but decreased the proportion of nitrous oxide of the total reduced nitrate. Our results suggest that autotrophic nitrate reduction coupled with iron(II) oxidation is a relevant, previously overlooked process in paddy soils.

Synergistic effect between biochar and nitrate fertilizer facilitated arsenic immobilization in an anaerobic contaminated paddy soil

Nitrate nitrogen fertilizer was usually used to mitigate arsenic (As) release and mobilization in the anaerobic contaminated paddy soil. However, the effect of the interplay between nitrate fertilizer and biochar on As availability as well as the involved mechanism were poorly understood. Herein, the effects and mechanisms of biochar, nitrate fertilizer, and biochar-based nitrate fertilizer on the availability of As in the contaminated paddy soil were investigated via a microcosm incubation experiment. Results indicated that the application of biochar-based nitrate fertilizer significantly lessened the available As concentration in the contaminated paddy soil from 3.01 ± 0.03 (control group) to 2.24 ± 0.08 mg kg-1, which presented an immobilization efficiency of 26.6 % better than those of individual biochar (13.5 %) and nitrate fertilizer (17.6 %), exhibiting a synergistic effect. Moreover, the biochar-based nitrate fertilizer also facilitated the transformation of more toxic arsenite in the contaminated soil to less toxic arsenate. Further, biochar-based nitrate fertilizer increased soil redox potential (Eh), dissolved organic carbon, organic matter, and nitrate yet decreased soil pH and ammonium, which changed the microbial community in the soil, enhancing the relative abundance of Bacillus, Arthrobacter, and Paenibacillus. These functional microorganisms drove the coupled transformation between nitrate denitrification and Fe(II) or As(III) oxidation, favoring As immobilization in the anaerobic paddy soil. Additionally, the co-application of biochar offset the negative effect of single nitrate fertilizer on microbial community diversity. Overall, biochar-based nitrate fertilizer could be a promising candidate for the effective immobilization of As in the anaerobic paddy soil. The current research can provide a valuable reference to the remediation of As-contaminated paddy soil and the production of safe rice.

Interactions between arsenic and nitrogen regulate nitrogen availability and arsenic mobility in flooded paddy soils

In paddy soils, arsenic (As) stress influences nitrogen (N) transformation while application of N fertilizers during rice cropping affects As transformation. However, specific interactive effects between As and N in flooded paddy soils on As mobility and N availability were unclear. Here, we examined N and As dynamics in flooded paddy soils treated with four As levels (0, 30, 80 and 150 mg kg-1) and three urea additions (0, 4 and 8 mmol N kg-1). Arsenic contamination inhibited diazotrophs (nifH) and fungi but promoted AOA and denitrification genes (narG, nirK, nirS), decreasing dissolved organic N, NH4+-N and NO3--N. Besides, urea application stimulated As- and Fe-reducing bacteria (arrA and Geo) coupled with anammox. On Day 28, the addition of 8 mmol N kg-1 increased total As concentrations in solutions of soils treated with 30 and 80 mg As kg-1 by 2.4 and 1.8 times compared with the nil-N control. In contrast, at 150 mg As kg-1, it decreased the total As concentration in soil solution by 63 % through facilitating As(III) oxidation coupled with NO3--N reduction. These results indicate that As contamination decreases N availability, but urea application affects As mobility, depending on As contamination level.

Sulfate-mediated Fe(III) mineral reduction accelerates arsenic mobilization by a Desulfovibrio strain isolated from paddy soil

The biogeochemical cycling of arsenic (As) is often intertwined with iron (Fe) and sulfur (S) cycles, wherein Fe(III)- and sulfate-reducing bacteria (SRB) play a crucial role. Here, we isolated strain DS-1, a strictly anaerobic Fe(III)- and sulfate-reducing bacterium, from As-contaminated paddy soil. Using 16S rRNA gene sequence analysis, strain DS-1 was identified as a member of the genus Desulfovibrio. Strain DS-1 utilized energy derived from ferrihydrite reduction to support its cellular growth. Under anoxic sulfate-reducing conditions, the presence of strain DS-1 significantly increased As mobilization compared to sulfate-free conditions. Mechanistically, SRB-produced sulfide reacts with Fe(III) to form FeS, which disrupts Fe(III) minerals, thereby enhancing As release. These findings highlight the critical role of redox disequilibrium in As mobilization and suggest that SRB-produced sulfide may permeate to the rice rhizosphere, increasing As mobilization through Fe(III) reduction.

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

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

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.

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

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

Bifunctional ArsI Dioxygenase from Acidovorax sp. ST3 with Both Methylarsenite [MAs(III)] Demethylation and MAs(III) Oxidation Activities

Methylated arsenicals, including highly toxic species, such as methylarsenite [MAs(III)], are pervasive in the environment. Certain microorganisms possess the ability to detoxify MAs(III) by ArsI-catalyzed demethylation. Here, we characterize a bifunctional enzyme encoded by the arsI gene from Acidovorax sp. ST3, which can detoxify MAs(III) through both the demethylation and oxidation pathways. Deletion of the 22 C-terminal amino acids of ArsI increased its demethylation activity while reducing the oxidation activity. Further deletion of 44 C-terminal residues enhanced the MAs(III) demethylation activity. ArsI has four vicinal cysteine pairs, with the first pair being necessary for MAs(III) demethylation, while at least one of the other three pairs contributes to MAs(III) oxidation. Molecular modeling and site-directed mutagenesis indicated that one of the C-terminal vicinal cysteine pairs is involved in modulating the switch between oxidase and demethylase activity. These findings underscore the critical role of the C-terminal region in modulating the enzymatic activities of ArsI, particularly in MAs(III) demethylation. This research reveals the structure-function relationship of the ArsI enzyme and advances our understanding of the MAs(III) metabolism in bacteria.

Remediation of arsenic- and nitrate-contaminated groundwater through iron-dependent autotrophic denitrifying culture

Groundwater contamination with arsenic and nitrate poses a pressing concern for the safety of local communities. Bioremediation, utilizing Fe(II)-oxidizing nitrate reducing bacteria, shows promise as a solution to this problem. However, the relatively weak environmental adaptability of a single bacterium hampers practical application. Therefore, this study explored the feasibility and characteristics of a mixed iron-dependent autotrophic denitrifying (IDAD) culture for effectively removing arsenic and nitrate from synthetic groundwater. The IDAD biosystem exhibited stable performace and arsenic resistance, even at a high As(III) concentration of 800 μg/L. Although the nitrogen removal efficiency of the IDAD biosystem decreased from 71.4% to 64.7% in this case, the arsenic concentration in the effluent remained below the standard (10 μg/L) set by WHO. The crystallinity of the lepidocrocite produced by the IDAD culture decreased with increasing arsenic concentration, but the relative abundance of the key iron-oxidizing bacteria norank_f_Gallionellaceae in the culture showed an opposite trend. Metagenomic analysis revealed that the IDAD culture possess arsenic detoxification pathways, including redox, methylation, and efflux of arsenic, which enable it to mitigate the adverse impact of arsenic stress. This study provides theoretical understanding and technical support for the remediation of arsenic and nitrate-contaminated groundwater using the IDAD culture.

Low-molecular-weight organic acids inhibit the methane-dependent arsenate reduction process in paddy soils

Anaerobic methane oxidation (AOM) can drive soil arsenate reduction, a process known as methane-dependent arsenate reduction (M-AsR), which is a critical driver of arsenic (As) release in soil. Low molecular weight organic acids (LMWOAs), an important component of rice root exudates, have an unclear influence and mechanism on the M-AsR process. To narrow this knowledge gap, three typical LMWOAs-citric acid, oxalic acid, and acetic acid-were selected and added to As-contaminated paddy soils, followed by the injection of 13CH4 and incubation under anaerobic conditions. The results showed that LMWOAs inhibited the M-AsR process and reduced the As(III) concentration in soil porewater by 35.1-65.7 % after 14 days of incubation. Among the LMWOAs, acetic acid exhibited the strongest inhibition, followed by oxalic and citric acid. Moreover, LMWOAs significantly altered the concentrations of ferrous iron and dissolved organic carbon in the soil porewater, consequently impacting the release of As in the soil. The results of qPCR and sequencing analysis indicated that LMWOAs inhibited the M-AsR process by simultaneously suppressing microbes associated with ANME-2d and arrA. Our findings provide a theoretical basis for modulating the M-AsR process and enhance our understanding of the biogeochemical cycling of As in paddy soils under rhizosphere conditions.

Prediction of As and Cd dissolution in various soils under flooding condition

Although the mobility of arsenic (As) and cadmium (Cd) in soils during the flooding-drainage process has been intensively studied, predicting their dissolution among various soils still remains a challenge. After comprehensively monitoring multiple parameters related to As and Cd dissolution in 8 soils for a 60-day anaerobic incubation, the redundancy analysis (RDA) and structural equation model (SEM) were employed to identify the key factors and influencing pathways controlling the dynamic release of As and Cd. Results showed that pH alone explained 90.5 % Cd dissolution, while the dissolved-Fe(II) and 5 M-HCl extractable Fe(II) jointly only explained 50.6 % As dissolution. After data normalization, the ratio of Fe(II) to 5 M-HCl extracted total Fe (i.e. FetotII/Fetot) significantly improved the correlation to R2 = 0.824 (p < 0.001) with a fixed slope of 0.393 among the 8 soils. Our results highlight the crucial role played by the reduction degree of total iron contents in determining both the reduction and dissolution of As during flooding. In contrast, dissolved-Fe(II) was too vulnerable to soil properties to be a stable indicator of As dissolution. Therefore, we propose to replace the dissolved-Fe(II) with this novel ratio as the key index to quantitatively assess the kinetic change of As solubility potential across various soils under flooding conditions.

Effects of denitrification on speciation and redistribution of arsenic in estuarine sediments

Microbially-mediated redox processes involving arsenic (As) and its host minerals significantly contribute to the mobilization of As in estuarine sediments. Despite its significance, the coupling between As dynamics and denitrification processes in these sediments is not well understood. This study employed sequential sediment extractions and simultaneous monitoring of dissolved iron (Fe), nitrogen (N), and sulfur (S) to investigate the impact of nitrate (NO3-) on the speciation and redistribution of As, alongside changes in microbial community composition. Our results indicated that NO3- additions significantly enhance anaerobic arsenite (As(III)) oxidation, facilitating its immobilization by increased adsorption onto sediment matrices in As-contaminated estuarine settings. Furthermore, NO3- promoted the conversion of As bound to troilite (FeS) and pyrite (FeS2) into forms associated with Fe oxides, challenging the previously assumed stability of FeS/FeS2-bound As in such environments. Continuous NO3- additions ensured As and Fe oxidation, thereby preventing their reductive dissolution and stabilizing the process that reduces As mobility. Changes in the abundance of bacterial communities and correlation analyses revealed that uncultured Anaerolineaceae and Thioalkalispira may be the main genus involved in these transformations. This study underscores the critical role of NO3- availability in modulating the biogeochemical cycle of As in estuarine sediments, offering profound insights for enhancing As immobilization techniques and informing environmental management and remediation strategies in As-contaminated coastal regions.

Iron-modified biochar effectively mitigates arsenic-cadmium pollution in paddy fields: A meta-analysis

The escalating problem of compound arsenic (As) and cadmium (Cd) contamination in agricultural soils necessitates the urgency for effective remediation strategies. This is compounded by the opposing geochemical behaviors of As and Cd in soil, and the efficacy of biochar treatment remains unclear. This pioneering study integrated 3780 observation pairs referred from 92 peer-reviewed articles to investigate the impact of iron-modified biochar on As and Cd responses across diverse soil environments. Regarding the treatments, 1) biochar significantly decreased the exchangeable and acid-soluble fraction of As (AsF1, 20.9%) and Cd (CdF1, 24.0%) in paddy fields; 2) iron-modified biochar significantly decreased AsF1 (32.0%) and CdF1 (27.4%); 3) iron-modified biochar in paddy fields contributed to the morphological changes in As and Cd, mainly characterized by a decrease in AsF1 (36.5%) and CdF1 (36.3%) and an increase in the reducible fraction of As (19.7%) and Cd (39.2%); and 4) iron-modified biochar in paddy fields increased As (43.1%) and Cd (53.7%) concentrations in the iron plaque on root surfaces. We conclude that iron-modified biochar treatment of paddy fields is promising in remediating As and Cd contamination by promoting the formation of iron plaque.

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

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

Illuminated fulvic acid stimulates denitrification and As(III) immobilization in flooded paddy soils via an enhanced biophotoelectrochemical pathway

Fulvic acid (FA) is a representative photosensitive dissolved organic matter (DOM) compound that occurs naturally in paddy soils. In this study, the effect of a FA + nitrate treatment (0, 4 and 8 mg/L FA + 20 mmol/L nitrate) on denitrification and As(III) immobilization in flooded paddy soils was assessed under dark and intermittently illuminated conditions (12 h light+12 h dark). The FA input stimulated denitrification in illuminated soils (~100 % of nitrate removal within 6 days) compared to dark conditions (~92 % nitrate removal after 6 days). Meanwhile, As(III) (initial concentration of 0.1 mmol/L) was nearly completely immobilized (~100 %) under illuminated conditions after 4 days for the FA + nitrate treatment compared to 90- 93 % retention in the dark. Denitrification and As immobilization were positively related to the FA dosage in the illuminated assays. The stronger denitrification in illuminated soils was ascribed to denitrifiers harvesting photoelectrons from photosensitive substrates/semiconducting minerals. FA addition also increased the activities of denitrifying enzymes (e.g., NAR, NIR and NOR) and the denitrification electron transport system by nearly 0.6-0.7 and 1.5-1.8 times that of the nitrate-alone treatment under illuminated and dark conditions, thereby fostering stronger denitrification. Upon irradiation, As(III) immobilization was not only stimulated by the interactions with the denitrification pathway whereby As(III) acts as an electron donor for denitrifiers, but was also modulated by Fe(III)/oxidative reactive species-derived photooxidation of As(III). Moreover, the FA + nitrate treatment promoted the enrichment of metal-oxidizing bacteria (e.g., Stenotrophomonas and Acidovorax) that are responsible for nitrate-dependent As(III)/Fe(II) oxidation. The results of this study enhance our understanding of interactions among the biogeochemical cycles of As, Fe, N and C, which are intricately linked by a biophotoelectrochemical pathway in flooded paddy soils.

Nitrate-dependent antimony oxidase in an uncultured Symbiobacteriaceae member

Autotrophic antimony (Sb) oxidation coupled to nitrate reduction plays an important role in the transformation and detoxification of Sb. However, the specific oxidase involved in this process has yet to be identified. Herein, we enriched the microbiota capable of nitrate-dependent Sb(III) oxidation and identified a new Sb(III) oxidase in an uncultured member of Symbiobacteriaceae. Incubation experiments demonstrated that nitrate-dependent Sb(III) oxidation occurred in the microcosm supplemented with Sb(III) and nitrate. Both the 16S rRNA gene and metagenomic analyses indicated that a species within Symbiobacteriaceae played a crucial role in this process. Furthermore, carbon-13 isotope labeling with carbon dioxide-fixing Rhodopseudomonas palustris in combination with nanoscale secondary ion mass spectrometry revealed that a newly characterized oxidase from the dimethylsulfoxide reductase family, designated as NaoABC, was responsible for autotrophic Sb(III) oxidation coupled with nitrate reduction. The NaoABC complex functions in conjunction with the nitrate reductase NarGHI, forming a redox loop that transfers electrons from Sb(III) to nitrate, thereby generating the energy necessary for autotrophic growth. This research offers new insights into the understanding of how microbes link Sb and nitrogen biogeochemical cycles in the environment.

Straw incorporation induces rice straighthead disease in As-contaminated paddy soil

Rice straw incorporation is globally recognized as a viable alternative to incineration. However, it might lead to arsenic (As) methylation in soils, resulting in increased accumulation of methylated As in rice plants, potentially contributing to the emergence of rice straighthead disease. To evaluate the effect of straw incorporation on the As transformation in the paddy field system, we conducted a pot experiment for rice cultivation in two paddy soils with different As background levels and also characterized the response of the soil microbial community to straw incorporation. The results showed that straw incorporation elevated the total and methylated As concentration within the soil solution and rice plants, which in turn reduced rice seed setting rate and yield, and caused straighthead disorder in rice cultivated in soils with high As levels. 16S rRNA-based sequencing demonstrated reduced abundance and diversity of microorganisms upon adding straw. Notably, the dominant phylum, Bacteroidetes, exhibited a significant increase in abundance due to straw integration, while the abundance of Proteobacteria and Acidobacteria decreased. At the family level, the prevalence of Rikenellaceae increased only in soils contaminated with As following straw incorporation. Redundancy analysis showed positive associations between Rikenellaceae and levels of methylated As present in both soil porewater and rice husks, suggesting a potentially pivotal role of Rikenellaceae in the As methylation process after straw integration. These findings collectively emphasize that including straw can reshape the soil's microbial community and amplify As methylation in the soil, thereby promoting the uptake and accumulation of methylated As in rice and inducing straighthead disease in As-contaminated soil.

Interplay of legacy irrigation and nitrogen fertilizer inputs to spatial variability of arsenic and uranium within the deep vadose zone

The vadose zone is a reservoir for geogenic and anthropogenic contaminants. Nitrogen and water infiltration can affect biogeochemical processes in this zone, ultimately affecting groundwater quality. In this large-scale field study, we evaluated the input and occurrence of water and nitrogen species in the vadose zone of a public water supply wellhead protection (WHP) area (defined by a 50-year travel time to groundwater for public supply wells) and potential transport of nitrate, ammonium, arsenic, and uranium. Thirty-two deep cores were collected and grouped by irrigation practices: pivot (n = 20), gravity (n = 4) irrigated using groundwater, and non-irrigated (n = 8) sites. Beneath pivot-irrigated sites, sediment nitrate concentrations were significantly (p < 0.05) lower, while ammonium concentrations were significantly (p < 0.05) higher than under gravity sites. The spatial distribution of sediment arsenic and uranium was evaluated against estimated nitrogen and water loading beneath cropland. Irrigation practices were randomly distributed throughout the WHP area and presented a contrasting pattern of sediment arsenic and uranium occurrence. Sediment arsenic correlated with iron (r = 0.32, p < 0.05), uranium negatively correlated to sediment nitrate (r = -0.23, p < 0.05), and ammonium (r = -0.19 p < 0.05). This study reveals that irrigation water and nitrogen influx influence vadose zone geochemistry and mobilization of geogenic contaminants affecting groundwater quality beneath intensive agricultural systems.

Contradictory Impacts of Nitrate on the Dissimilatory Arsenate-Respiring Prokaryotes-Induced Reductive Mobilization of Arsenic from Contaminated Sediments: Mechanism Insight from Metagenomic and Functional Analyses

Dissimilatory arsenate-respiring prokaryotes (DARPs) are considered to be a key impetus of the reductive dissolution of solid-phase arsenic. However, little is known about the interaction between nitrate and DARPs so far. In this study, we showed that nitrate either inhibited or promoted the DARP population-catalyzed reductive mobilization of As in sediments. Metagenomic analysis of the microbial communities in the microcosms after seven days of As release assays suggested that microbes mainly consisted of: Type-I DARPs having potential to reduce NO3- into NO2- and Type-II DARPs having potential to reduce NO3- to NH4+. We further isolated two cultivable DARPs, Neobacillus sp. A01 and Paenibacillus sp. A02, which represent Type-I and -II DARPs, respectively. We observed that nitrate suppressed A01-mediated release of As(III) but promoted A02-mediated release of As(III). Furthermore, we demonstrated that this observation was due to the fact that nitrite, the end product of incomplete denitrification by Type-I DARPs, suppressed the arrA gene expression per cell and growth of all DARPs, whereas ammonium, the end product of dissimilatory nitrate reduction to ammonium (DNRA) by Type-II DARPs, enhanced the arrA gene expression per cell and significantly promoted the growth of all DARPs. These findings suggest that the actual effects of nitrate on DARP population-catalyzed reductive mobilization of arsenic, largely depend on the ratio of Type-I to Type-II DARPs in sediments.

Sustainable Immobilization of Arsenic by Man-Made Aerenchymatous Tissues in Paddy Soil

Arsenic (As) is a major environmental pollutant and poses a significant health risk to humans through rice consumption. Elevating the soil redox potential (Eh) has been shown to reduce As bioavailability and decrease As accumulation in rice grains. However, sustainable methods for managing the Eh of rice paddies are lacking. To address this issue, we propose a new approach that uses man-made aerenchymatous tissues (MAT) to increase soil Eh by mimicking O2 release from wet plant roots. Our study demonstrated that the MAT method sustainably increased the soil Eh levels from -119 to -80.7 mV (∼30%), over approximately 100 days and within a radius of around 5 cm from the surface of the MAT. Moreover, it resulted in a significant reduction (-28.5% to -63.3%) in dissolved organic carbon, Fe, Mn, and As concentrations. MAT-induced Fe(III) (oxyhydr)oxide minerals served as additional adsorption sites for dissolved As in soil porewater. Furthermore, MAT promoted the oxidation of arsenite to the less mobile arsenate by significantly enhancing the relative abundance of the aioA gene (130% increase in the 0-5 cm soil zone around MAT). The decrease in As bioavailability significantly reduced As accumulation in rice grains (-30.0%). This work offers a low-cost and sustainable method for mitigating As release in rice paddies by addressing the issue of soil Eh management.

Anoxygenic phototrophic arsenite oxidation by a Rhodobacter strain

Microbially mediated arsenic redox transformations are key for arsenic speciation and mobility in rice paddies. Whereas anaerobic anoxygenic photosynthesis coupled to arsenite (As(III)) oxidation has been widely examined in arsenic-replete ecosystems, it remains unknown whether this light-dependent process exists in paddy soils. Here, we isolated a phototrophic purple bacteria, Rhodobacter strain CZR27, from an arsenic-contaminated paddy soil and demonstrated its capacity to oxidize As(III) to arsenate (As(V)) using malate as a carbon source photosynthetically. Genome sequencing revealed an As(III)-oxidizing gene cluster (aioXSRBA) encoding an As(III) oxidase. Functional analyses showed that As(III) oxidation under anoxic phototrophic conditions correlated with transcription of the large subunit of the As(III) oxidase aioA gene. Furthermore, the non-As(III) oxidizer Rhodobacter capsulatus SB1003 heterologously expressing aioBA from strain CZR27 was able to oxidize As(III), indicating that aioBA was responsible for the observed As(III) oxidation in strain CZR27. Our study provides evidence for the presence of anaerobic photosynthesis-coupled As(III) oxidation in paddy soils, highlighting the importance of light-dependent, microbe-mediated arsenic redox changes in paddy arsenic biogeochemistry.

Arsenic removal in flue gas through anaerobic denitrification and sulfate reduction cocoupled arsenic oxidation

Arsenic in flue gas from municipal solid waste incineration can damage to human health and ecological environment. A sulfate-nitrate-reducing bioreactor (SNRBR) for flue gas arsenic removal was investigated. Arsenic removal efficiency attained 89.4%. An integrated metagenomic and metaproteomic investigation showed that three nitrate reductases (NapA, NapB and NarG), three sulfate reductases (Sat, AprAB and DsrAB), and arsenite oxidase (ArxA) regulated nitrate reduction, sulfate reduction and bacterial As(III)-oxidation, respectively. Citrobacter and Desulfobulbus could synthetically regulate the expression of arsenite-oxidizing gene, nitrate reductases and sulfate reducatases, which involved in As(III) oxidation, nitrate and sulfate reduction. A bacterial consortium containing Citrobacter, UG_Enterobacteriaceas, Desulfobulbus and Desulfovibrio could capable of simultaneously arsenic oxidation, sulfate reduction and denitrification. Anaerobic denitrification and sulfate reduction were cocoupled to arsenic oxidation. The biofilm was characterized by FTIR, XPS, XRD, EEM, and SEM. XRD and XPS spectra verified the formation of aarsenic species (As(V)) from flue gas As(III) conversion. Arsenic speciation in biofilms of SNRBR consisted of 77% residual arsenic, 15.9% organic matter-bound arsenic, and 4.3% strongly absorbed arsenic. Flue gas arsenic was bio-stabilized in the form of Fe-As-S and As-EPS through biodeposition, biosorption and biocomplexation. This provides a new way of flue gas arsenic removal using the sulfate-nitrate-reducing bioreactor.

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.

Intercropping efficiency of Pteris vittata with two legume plants: Impacts of soil arsenic concentrations

Intercropping of hyperaccumulators with crops has emerged as a promising method for remediating arsenic (As)-contaminated soil in agroecosystems. However, the response of intercropping hyperaccumulators with different types of legume plants to diverse gradients of As-contaminated soil remains poorly understood. In this study, we assessed the response of plant growth and accumulation of an As hyperaccumulator (Pteris vittata L.) intercropped with two legume plants to three gradients of As-contaminated soil. Results indicated that soil As concentration had a substantial effect on the As uptake by plants. P. vittata growing in slightly As-contaminated soil (80 mg kg^-1) exhibited higher As accumulation (1.52-5.49 folds) than those in higher As-contaminated soil (117 and 148 mg kg^-1), owing to the lower soil pH in high As-contaminated soil. Intercropping with Sesbania cannabina L. increased As accumulation in P. vittata by 19.3%- 53.9% but decreased in intercropping with Cassia tora L. This finding was attributed to S. cannabina providing more NO₃^--N to P. vittata to support its growth, and higher resistance to As. The decreased rhizosphere pH in the intercropping treatment also resulted in the increased As accumulation in P. vittata. Meanwhile, the As concentrations in the seeds of the two legume plants met the national food standards(<0.5 mg kg^-1). Therefore, the intercropping P. vittata with S. cannabina is a highly effective intercropping system in slightly As-contaminated soil and provides a potent method for As phytoremediation.

Remediation methods of heavy metal contaminated soils from environmental and geotechnical standpoints

Heavy metal contaminated soil (HMCS) threatens world health and sustainable growth, owing to which numerous remediation methods have been devised. Meanwhile, environmental sustainability and geotechnical serviceability of remediated HMCS are important considerations for reusing such soils and achieving sustainable development goals; therefore, these considerations are critically reviewed in this article. For this purpose, different onsite and offsite remediation methods are evaluated from environmental and geotechnical standpoints. It was found that each remediation method has its own merits and limitations in terms of environmental sustainability and geotechnical serviceability; generally, sustainable green remediation (SGR) and cementation are regarded as effective solutions for the problems related to the former and latter, respectively. Overall, the impact of remediation techniques on the environment and geotechnical serviceability is a developing area of study that calls for increased efforts to improve the serviceability, sustainability, reusability and environmental friendliness of the remediated HMCS.

Insight into universality and characteristics of nitrate reduction coupled with arsenic oxidation in different paddy soils

Nitrate reduction coupled with arsenic (As) oxidation strongly influences the bioavailability and toxicity of As in anaerobic environments. In the present study, five representative paddy soils developed from different parent materials were used to investigate the universality and characteristics of nitrate reduction coupled with As oxidation in paddy soils. Experimental results indicated that 99.8 % of highly toxic aqueous As(III) was transformed to dissolved As(V) and Fe-bound As(V) in the presence of nitrate within 2-8 d, suggesting that As was apt to be reserved in its low-toxic and nonlabile form after nitrate treatment. Furthermore, nitrate additions also significantly induced the higher abundance of 16S rRNA and As(III) oxidase (aioA) genes in the five paddy soils, especially in the soils developed from purple sand-earth rock and quaternary red clay, which increased by 10 and 3-5 times, respectively, after nitrate was added. Moreover, a variety of putative novel nitrate-dependent As(III)-oxidizing bacteria were identified based on metagenomic analysis, mainly including Aromatoleum, Paenibacillus, Microvirga, Herbaspirillum, Bradyrhizobium, Azospirillum. Overall, all these findings indicate that nitrate reduction coupled with As(III) oxidation is an important nitrogen-As coupling process prevalent in paddy environments and emphasize the significance of developing and popularizing nitrate-based biotechnology to control As pollution in paddy soils and reduce the risk of As compromising food security.

Elevated antimony concentration stimulates rare taxa of potential autotrophic bacteria in the Xikuangshan groundwater

Microbial communities composed of few abundant and many rare species are widely involved in the biogeochemical cycles of elements. Yet little is known about the ecological roles of rare taxa in antimony (Sb) contaminated groundwater. Groundwater samples were collected along an Sb concentration gradient in the Xikuangshan antimony mine area and subjected to high through-put sequencing of 16S rRNA genes to investigate the bacterial communities. Results suggested that both abundant and rare sub-communities were dominated by Betaproteobacteria, Gammaproteobacteria, and Alphaproteobacteria, whereas rare sub-communities showed higher alpha-diversities. Multivariate analysis showed that both the abundant and rare taxa were under the stress of Sb, but the impact on rare taxa was greater. Nitrate explained a large part for the variation of the abundant sub-communities, indicating the critical role of nitrate for their activities under anoxic conditions. In contrast, bicarbonate significantly impacted rare sub-communities, suggesting their potential autotrophic characteristics. To further explore the role of rare taxa in the communities and the mechanism of affecting the community composition, a network was constructed to display the co-occurrence pattern of bacterial communities. The rare taxa contributed most of the network nodes and served as keystone species to maintain the stability of community. Abiotic factors (mainly Sb and pH) and bacterial interspecific interactions (interactions between keystone species and other bacterial groups) jointly affect the community dynamics. Functional prediction was performed to further reveal the ecological function of rare taxa in the Sb-disturbed groundwater environment. The results indicated that the rare taxa harbored much more diverse functions than their abundant counterparts. Notably, elevated Sb concentration promoted some potential autotrophic functions in rare taxa such as the oxidation of S-, N-, and Fe(II)-compounds. These results offer new insights into the roles of rare species in elemental cycles in the Sb-impacted groundwater.

Environmental Mn(II) enhances the activity of dissimilatory arsenate-respiring prokaryotes from arsenic-contaminated soils

Many investigations suggest that dissimilatory arsenate-respiring prokaryotes (DARPs) play a key role in stimulating reductive mobilization of As from solid phase into groundwater, but it is not clear how environmental Mn(II) affects the DARPs-mediated reductive mobilization of arsenic. To resolve this issue, we collected soil samples from a realgar tailings-affected area. We found that there were diverse arsenate-respiratory reductase (arr) genes in the soils. The microbial communities had high arsenate-respiring activity, and were able to efficiently stimulate the reductive mobilization of As. Compared to the microcosms without Mn(II), addition of 10 mmol/L Mn(II) to the microcosms led to 23.99%-251.79% increases in the microbial mobilization of As, and led to 133.3%-239.2% increases in the abundances of arr genes. We further isolated a new cultivable DARP, Bacillus sp. F11, from the arsenic-contaminated soils. It completely reduced 1 mmol/L As(V) in 5 days under the optimal reaction conditions. We further found that it was able to efficiently catalyze the reductive mobilization and release of As from the solid phase; the addition of 2 mmol/L Mn(II) led to 98.49%-248.78% increases in the F11 cells-mediated reductive mobilization of As, and 70.6%-104.4% increases in the arr gene abundances. These data suggest that environmental Mn(II) markedly increased the DARPs-mediated reductive mobilization of As in arsenic-contaminated soils. This work provided a new insight into the close association between the biogeochemical cycles of arsenic and manganese.

Metagenome based analysis of groundwater from arsenic contaminated sites of West Bengal revealed community diversity and their metabolic potential

The study of microbial community in groundwater systems is considered to be essential to improve our understanding of arsenic (As) biogeochemical cycling in aquifers, mainly as it relates to the fate and transport of As. The present study was conducted to determine the microbial community composition and its functional potential using As-contaminated groundwater from part of the Bengal Delta Plain (BDP) in West Bengal, India. Geochemical analyses indicated low to moderate dissolved oxygen (0.42-3.02 mg/L), varying As (2.5-311 µg/L) and Fe (0.19-1.2 mg/L) content, while low concentrations of total organic carbon (TOC), total inorganic carbon (TIC), nitrate, and sulfate were detected. Proteobacteria was the most abundant phylum, while the indiscriminate presence of an array of archaeal phyla, Euryarchaeota, Crenarchaeota, Nanoarchaeota, etc., was noteworthy. The core community members were affiliated to Sideroxydans, Acidovorax, Pseudoxanthomonas, Brevundimonas, etc. However, diversity assessed over multiple seasons indicated a shift from Sideroxydans to Pseudomonas or Brevundimonas dominant community, suggestive of microbial response to seasonally fluctuating geochemical stimuli. Taxonomy-based functional potential showed prospects for As biotransformation, methanogenesis, sulfate respiration, denitrification, etc. Thus, this study strengthened existing reports from this region by capturing the less abundant or difficult-to-culture taxa collectively forming a major fraction of the microbial community.

Multiple-functionalized biochar affects rice yield and quality via regulating arsenic and lead redistribution and bacterial community structure in soils under different hydrological conditions

Rice grown in soils contaminated with arsenic (As) and lead (Pb) can cause lower rice yield and quality due to the toxic stress. Herein, we examined the role of functionalized biochars (raw phosphorus (P)-rich (PBC) and iron (Fe)-modified P-rich (FePBC)) coupled with different irrigation regimes (continuously flooded (CF) and intermittently flooded (IF)) in affecting rice yield and accumulation of As and Pb in rice grain. Results showed that FePBC increased the rice yield under both CF (47.4%) and IF (19.6%) conditions, compared to the controls. Grain As concentration was higher under CF (1.94-2.42 mg kg^-1) than IF conditions (1.56-2.31 mg kg^-1), whereas the concentration of grain Pb was higher under IF (0.10-0.76 mg kg^-1) than CF (0.12-0.48 mg kg^-1) conditions. Application of PBC reduced grain Pb by 60.1% under CF conditions, while FePBC reduced grain As by 12.2% under IF conditions, and increased grain Pb by 2.9 and 6.6 times under CF and IF conditions, respectively, compared to the controls. Therefore, application of the multiple-functionalized biochar can be a promising strategy for increasing rice yield and reducing the accumulation of As in rice grain, particularly under IF conditions, whereas it is inapplicable for remediation of paddy soils contaminated with Pb.

Multi-site field trials demonstrate the effectiveness of silicon fertilizer on suppressing dimethylarsenate accumulation and mitigating straighthead disease in rice

Rice accumulates both inorganic arsenic (iAs) and organic As species such as dimethylarsenate (DMA). Although DMA is less toxic to humans, it has been shown in hydroponic studies to induce rice straighthead disease, a physiological disorder prevalent in some rice growing regions causing large yield losses. We investigated the effects of different amendments on As species dynamics in soil porewater, accumulation of As species in rice husks and grains, and the incidence of straighthead disease in five field experiments conducted over 2 years at three sites where straighthead disease was observed in previous seasons. The amendments included silicon (Si) fertilizer, micronized zero valent iron (μZVI), sulfate, nitrate, Si-rich biochar, and a mixture of trace and major elements. Straighthead disease was observed in all five experiments. Rice panicles showing the straighthead disease symptoms contained much higher DMA concentrations in husks and grains than normal panicles. Silicon fertilizer was highly effective at decreasing the disease incidence rate and increasing seed setting rate, resulting in 14.9-58.1% increase in grain yield. Silicon fertilizer increased soil porewater iAs and DMA concentrations, but decreased iAs and DMA accumulation in husks and grains, suggesting that Si suppressed the uptake of iAs and DMA by rice plants. Other amendments alleviated straighthead disease to smaller extents than Si fertilizer, with the effect of biochar and the mixture of trace and major elements likely also being attributed to the addition of Si. Results from this field-based study demonstrate that excessive accumulation of DMA is the main cause of rice straighthead disease and Si fertilizer is highly effective at mitigating this disease by suppressing DMA accumulation.

Distinct arsenic uptake feature in rice reveals the importance of N fertilization strategies

The environmental behavior of arsenic (As) is commonly affected by the biogeochemical processes of iron (Fe) and nitrogen (N). In this study, field experiments were conducted to explore As uptake in rice and As translation and distribution in As-contaminated iron-rich paddy soils after applying different forms of N fertilizers, including urea (CO(NH2)2), ammonium bicarbonate (NH4HCO3), nitrate of potash (KNO3), and ammonium bicarbonate + nitrate of potash (NH4HCO3 + KNO3). The results indicated that applying nitrate N fertilizer inhibited the reduction and dissolution of As-bearing iron minerals and promoted microbial-mediated As(III) oxidation in flooded soil, thus reducing the soil As bioavailability. The concentrations of total As and inorganic As ratio (iAs/TAs) in rice grain decreased by 32.4 % and 15.4 %, respectively. However, the application of ammonium nitrogen promoted the reductive dissolution of As-bearing iron minerals and stimulated microbial As(V) reduction in flooded soil, leading to the release of As from soil to porewater. The total As concentration and inorganic As uptake ratio in rice grain increased by 20.1 % and 6.2 %, respectively, when urea was applied, and by 29.6 % and 10.5 %, respectively, when ammonium bicarbonate was applied. However, the simultaneous application of NH4+ and NO3- had no significant effect on As concentration in rice grain and its transformation in paddy soils. Ammonium nitrogen enhanced the organic As concentration in rice grain because the increased As(III) promoted As methylation in soil. In contrast, nitrate decreased the organic As uptake by rice grain because the decreased As(III) diminished As methylation in soil. The results provide reasonable N fertilization strategies for regulating the As biogeochemical process and reducing the risk of As contamination in rice.

Nitrate Controls on the Extent and Type of Metal Retention in Fine-Grained Sediments of a Simulated Aquifer

Aquifer groundwater quality is largely controlled by sediment composition and physical heterogeneity, which commonly sustains a unique redox gradient pattern. Attenuation of heavy metals within these heterogeneous aquifers is reliant on multiple factors, including redox conditions and redox-active species that can further influence biogeochemical cycling. Here, we simulated an alluvial aquifer system using columns filled with natural coarse-grained sediments and two domains of fine-grained sediment lenses. Our goal was to examine heavy metal (Ni and Zn) attenuation within a complex aquifer network and further explore nitrate-rich groundwater conditions. The fine-grained sediment lenses sustained reducing conditions and served as a sink for Ni sequestration─in the form of Ni-silicates, Ni-organic matter, and a dominant Ni-sulfide phase. The silicate clay and sulfide pools were also important retention mechanisms for Zn; however, Ni was associated more extensively with organic matter compared to Zn, which formed layered double hydroxides. Nitrate-rich conditions promoted denitrification within the lenses that was coupled to the oxidation of Fe(II) and the concomitant precipitation of an Fe(III) phase with higher structural distortion. A decreased metal sulfide pool also resulted, where nitrate-rich conditions generated an average 20% decrease in solid-phase Ni, Zn, and Fe. Ultimately, nitrate plays a significant role in the aquifer's biogeochemical cycling and the capacity to retain heavy metals.

Transformation of arsenic species by diverse endophytic bacteria of rice roots

Rice growing in flooded paddy soil often accumulates considerable levels of inorganic and organic arsenic (As) species, which may cause toxicity to plants and/or pose a risk to human health. The bioavailability and toxicity of As in soil depends on its chemical species, which undergo multiple transformations driven primarily by soil microbes. However, the role of endophytes inside rice roots in As species transformation remains largely unknown. We quantified the abundances of microbial functional genes involved in As transformation in the endosphere and rhizosphere of rice roots growing in three paddy soils in a pot experiment. We also isolated 46 different bacterial endophytes and tested their abilities to transform various As species. The absolute abundances of the arsenate reductase gene arsC and the dissimilatory arsenate reductase gene arrA in the endosphere were comparable to those in the rhizosphere, whereas the absolute abundances of the arsenite methylation gene arsM and arsenite oxidation gene aioA in the endosphere were lower. After normalization based on the bacterial 16S rRNA gene, all four As transformation genes showed higher relative abundances in the endosphere than in the rhizosphere. Consistent with the functional gene data, all of the 30 aerobic endophytic isolates were able to reduce arsenate, but only 3 strains could oxidize arsenite. Among the 16 anaerobic endophytic isolates, 4 strains belonging to Desulfovibrio, Terrisporobacter or Clostridium could methylate arsenite and/or methylarsenite. Six strains of aerobic endophytes could demethylate methylarsenite, among which three strains also could reduce and demethylate methylarsenate. None of the isolates could demethylate dimethylarsenate. These results suggest that diverse endophytes living inside rice roots could participate in As species transformation and affect As accumulation and species distribution in rice plants.

Can As concentration in crop be controlled by Se fertilization? A meta-analysis and outline of As sequestration mechanisms

Arsenic (As) is a pollutant with a strong toxic effect on animals, plants and human beings. Exogenous selenium (Se) has been suggested to reduce the accumulation of As in crops, but contradictory results were found in the published literature. In order to clarify the possible processes, we collected the literature that reports on the effects of Se application on As uptake and accumulation in crops, analyzed the data by meta-analysis, and tested the effects of different factors on As accumulation by meta-regression model and subgroup analysis. The results highlighted a significant dose-dependent reduction of As content in crops after Se addition. Exogenous Se can significantly reduce As concentrations in grains by 18.76%. The reduction was dose-dependent for rice grains under aerobic soil conditions but not for rice grains under anoxic soil conditions. Se-enriched soils (greater than 0.5 mg kg^-1) significantly reduced As concentrations in grains. Selenium significantly decreased the transfer factor of As from root to shoot. Moreover, selenite had a stronger inhibiting effect on the transport of As from root to shoot than selenate. The inhibition of selenium fertilization on As concentrations seems to take place in root and soil, while physiological processes in rice may be involved in restricting uptake and transport from root to shoot. These findings provide new ideas for effectively alleviating the transfer of As to the human body through the food chain.

How different nitrogen fertilizers affect arsenic mobility in paddy soil after straw incorporation?

In straw return fields, nitrogen-fertilizers are added to mitigate microbial competition for nitrogen with plants. However, in arsenic (As)-contaminated paddy fields, the specific effects of different nitrogen fertilizers on As mobility after straw incorporation and the interactions among iron(Fe)/carbon(C)/nitrogen(N)/As are not well understood. In the reported microcosm experiment we monitored As-mobility as a function of different dosages of KNO₃, NH₄Cl and rice straw incorporation. Addition of both KNO₃ and NH₄Cl significantly inhibited the As mobilization induced by straw incorporation. Following the KNO₃ addition, the As concentration in porewater dropped by 51-66% after 2 days of the incubation by restraining Fe reduction and enhancing Fe oxidation. High-dose NH₄Cl addition reduced As in porewater by 22-43% throughout the incubation by decreasing porewater pH. High-throughput sequencing results demonstrated that KNO₃ addition enriches both the denitrifying and Fe-oxidizing bacteria, while diminishing Fe-reducing bacteria; NH₄Cl addition has the opposite effect on Fe-reducing bacteria. Network analysis revealed that As and Fe concentrations in porewater were positively correlated with the abundance of denitrifying and Fe-reducing bacteria. This study broadens our insight into the As biogeochemistry associated with the N/C/Fe balance in soil, which are of great significance for agronomic management and mitigation the risk of As-contaminated paddy fields.

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.

Exploring Key Soil Parameters Relevant to Arsenic and Cadmium Accumulation in Rice Grain in Southern China

Paddy soils in some areas of southern China are contaminated by arsenic (As) and cadmium (Cd), threatening human health via the consumption of As- and/or Cd-tainted rice. To date, a quantitative understanding of how soil characteristics control As and Cd accumulation in rice grains under field conditions is still deficient. Based on 31 paired soil-grain samples collected in southern China, we statistically explored which soil parameter or parameter combination from various soil analyses best estimates As and Cd in rice. We found that CaCl2 extraction of field-moist soil collected at rice harvest provided the best estimation (R2adj = 0.47–0.60) for grain Cd followed by dry soil CaCl2 extraction (R2adj = 0.38–0.49), where CaCl2 extractable Cd from moist or dry soil was the dominant soil parameter. Compared to soil totals, parameters from neither dry soil ascorbate-citrate extraction nor anoxic soil incubation improved model performance for grain As (R2adj ≤ 0.44), despite their closer relevance to soil redox conditions during plant As uptake. A key role of soil-available sulfur in controlling grain As was suggested by our models. Our approach and results may help develop potential soil amendment strategies for decreasing As and/or Cd accumulation in soils.

Elucidating heterogeneous iron biomineralization patterns in a denitrifying As(iii)-oxidizing bacterium: implications for arsenic immobilization

Anaerobic nitrate-dependent iron(ii) oxidation is a process common to many bacterial species, which promotes the formation of Fe(iii) minerals that can influence the fate of soil and groundwater pollutants, such as arsenic. Herein, we investigated simultaneous nitrate-dependent Fe(ii) and As(iii) oxidation by Acidovorax sp. strain ST3 with the aim of studying the Fe biominerals formed, their As immobilization capabilities and the metabolic effect on cells. X-ray powder diffraction (XRD) and scanning transmission electron microscopy (STEM) nanodiffraction were applied for biomineral characterization in bulk and at the nanoscale, respectively. NanoSIMS (nanoscale secondary ion mass spectrometry) was used to map the intra and extracellular As and Fe distribution at the single-cell level and to trace metabolically active cells, by incorporation of a ¹³C-labeled substrate (acetate). Metabolic heterogeneity among bacterial cells was detected, with periplasmic Fe mineral encrustation deleterious to cell metabolism. Interestingly, Fe and As were not co-localized in all cells, indicating delocalized sites of As(iii) and Fe(ii) oxidation. The Fe(iii) minerals lepidocrocite and goethite were identified in XRD, although only lepidocrocite was identified via STEM nanodiffraction. Extracellular amorphous nanoparticles were formed earlier and retained more As(iii/v) than crystalline "flakes" of lepidocrocite, indicating that longer incubation periods promote the formation of more crystalline minerals with lower As retention capabilities. Thus, the addition of nitrate promotes Fe(ii) oxidation and formation of Fe(iii) biominerals by ST3 cells which retain As(iii/v), and although this process was metabolically detrimental to some cells, it warrants further examination as a viable mechanism for As removal in anoxic environments by biostimulation with nitrate.

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

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

Flexible 2D Cu Metal: Organic Framework@MXene Film Electrode with Excellent Durability for Highly Selective Electrocatalytic NH3 Synthesis

Electrocatalytic nitrate reduction to ammonia (ENRA) is an effective strategy to resolve environmental and energy crisis, but there are still great challenges to achieve high activity and stability synergistically for practical application in a fluid environment. The flexible film electrode may solve the abovementioned problem of practical catalytic application owing to the advantages of low cost, light weight, eco-friendliness, simple and scalable fabrication, extensive structural stability, and electrocatalytic reliability. Herein, 2D hybridization copper 1,4-benzenedi-carboxylate (CuBDC) has been grown on electronegative MXene nanosheets (Ti₃C₂T_x) seamlessly to prepare a 2D flexible CuBDC@Ti₃C₂T_x electrode for ENRA. The flexible electrode simultaneously exhibits high Faradaic efficiency (86.5%) and excellent stability for NH₃ synthesis, which are comparable to previously reported nanomaterials toward ENRA. Especially, the flexible electrode maintains outstanding FE _NH3 toward ENRA after the bending, twisting, folding, and crumpling tests, indicating excellent electroconductibility, high stability, and durability. This work not only provides mild permeation-mediated strategy to fabricate a flexible electrode but also explores the practical applications of the electrode with effectively environmental adaptability in solving global environmental contamination and energy crisis by effective ENRA.

Effects of anode/cathode electroactive microorganisms on arsenic removal with organic/inorganic carbon supplied

This study reports the effects of an external voltage (0 V, 0.4 V and 0.9 V) on soil arsenic (As) release and sequestration when amended with organic carbon (NaAc) and inorganic carbon (NaHCO₃), respectively, in a soil bioelectrochemistry system (BES). The results demonstrated that although an external voltage had no effect on the As removal capacity in an oligotrophic environment fueled with NaHCO_3, 93.6% of As(III) in the supernatant was removed at 0.9 V with an NaAc amendment. Interestingly, the content of As detected on the electrodes was higher than that removed from the supernatant, implying a continuous release of soil As under external voltages and rapid adsorption onto the electrodes, especially the cathode. In addition, the species of As on the cathode were similar to those in the supernatant (the As(III)/As(V) ratio was approximately 3:1), indicating that the removal capacity was independent of preoxidation. From the viewpoint of electroactive microorganisms (EABs), the relative abundances of the arrA gene and Geobacter genus were specifically enriched at the anode, thus signifying stimulation of the reduction and release of soil As in the anode region. By comparison, Bacillus was particularly abundant at the cathode, which could contribute to the oxidation and sequestration of As in the cathode region. Additionally, specific extracellular polymeric substances (EPSs) secreted by EABs could combine with As, which was followed by electrostatic attraction to the cathode under the effect of an electric field. Furthermore, the formation of secondary minerals and coprecipitation in the presence of iron (Fe) may have also contributed to As removal from solution. The insights from this study will enable us to further understand the biogeochemical cycle of soil As and to explore the feasibility of in situ As bioremediation techniques, combining the aspects of microbial and physicochemical processes in soil bioelectrochemical systems.

Demethylation of the Antibiotic Methylarsenite is Coupled to Denitrification in Anoxic Paddy Soil

Arsenic (As) biomethylation is an important component of the As biogeochemical cycle, which produces methylarsenite [MAs(III)] as an intermediate product. Its high toxicity is used by some microbes as an antibiotic to kill off other microbes and gain a competitive advantage. Some aerobic microbes have evolved a detoxification mechanism to demethylate MAs(III) via the dioxygenase C-As lyase ArsI. How MAs(III) is demethylated under anoxic conditions is unclear. We found that nitrate addition to a flooded paddy soil enhanced MAs(III) demethylation. A facultative anaerobe Bacillus sp. CZDM1 isolated from the soil was able to demethylate MAs(III) under anoxic nitrate-reducing conditions. A putative C-As lyase gene (BcarsI) was identified in the genome of strain CZDM1. The expression of BcarsI in the As-sensitive Escherichia coli AW3110 conferred the bacterium the ability to demethylate MAs(III) under anoxic nitrate-reducing condition and enhanced its resistance to MAs(III). Both Bacillus sp. CZDM1 and E. coli AW3110 harboring BcarsI could not demethylate MAs(III) under fermentative conditions. Five conserved amino acid resides of cysteine, histidine, and glutamic acid are essential for MAs(III) demethylation under anoxic nitrate-reducing conditions. Putative arsI genes are widely present in denitrifying bacteria, with 75% of the sequenced genomes containing arsI, also possessing dissimilatory nitrate reductase genes narG or napA. These results reveal a novel mechanism in which MAs(III) is demethylated via ArsI by coupling to denitrification, and such a mechanism is likely to be common in an anoxic environment such as paddy soils and wetlands.