Arsenate Reduction Research Articles

Unravelling the mechanism of arsenic resistance and bioremediation in Stenotrophomonas maltophilia: A molecular approach

The mechanism of arsenic resistance in bacteria is under studied and still lacks a clear understanding despite of wide research work. The advanced technologies can help in analysing the arsenic bioremediating bacteria at a molecular level. With this line of idea, highly efficient arsenic bioremediating S. maltophilia was subjected to extensive analysis to understand the mechanism of arsenic resistance and bioremediation. The cell surface analysis revealed that S. maltophilia induces only slight changes in cell surface in the presence of arsenic. Whereas, TEM analysis has indicated the bioaccumulation of arsenic in S. maltophilia. Also, arsenic was found to generate ROS in a concentration dependant manner, and in response, S. maltophilia activated SOD, catalase, thioredoxin reductase etc. to manage oxidative stress which is very much crucial in managing arsenic toxicity. S. maltophilia was found to possess genes such as arsC, aoxB, aoxC and aioA. These genes are involved in arsenic reduction and oxidation. Transcriptomics and proteomics analysis have shown that S. maltophilia detoxifies arsenic by upregulating ars operon, arsH, BetB etc. which are responsible for arsenic reduction, efflux methylation, oxidation etc. A detailed molecular mechanism of arsenic bioremediation in S. maltophilia was put forth.

Midseason draining of paddy water suppresses microbial arsenic methylation in soil and alleviates rice straighthead disease

Arsenic (As) methylation is an important component of As biogeochemical cycle. Microbial As methylation is enhanced under anoxic conditions in paddy soil, producing dimethylarsenate (DMA) which can cause physiological straighthead disease in rice. We conducted field experiments at two sites to test the effect of midseason draining of paddy water on microbial As methylation and the incidence of straighthead disease. Compared to continuous flooding, midseason draining increased soil Eh, decreased the abundances of microbial genes for arsenate reduction (arsC and arrA) and arsenite methylation (arsM), and lowered the concentrations of both inorganic As and DMA in soil porewater. Draining shifted microbial composition, resulting in decreases in the relative abundance of 17–132 amplicon sequence variants. Draining decreased the accumulation of DMA in rice husk and of inorganic As and DMA in rice grain, decreased the incidence of straighthead disease, and increased grain yield by 20–45 %. Further experiments were conducted at eight field sites to assess the effect of midseason draining in a split field design. Draining decreased husk DMA concentration by 40–65 % and increased grain yield by 25–209 %. This study demonstrates that midseason draining can effectively suppress microbial As methylation and alleviate rice straighthead disease, benefiting both grain yield and safety.

Hybrid-genome sequence analysis of Enterobacter cloacae FACU and morphological characterization: insights into a highly arsenic-resistant strain.

Many organisms have adapted to survive in environments with high levels of arsenic (As), a naturally occurring metalloid with various oxidation states and a common element in human activities. These organisms employ diverse mechanisms to resist the harmful effects of arsenic compounds. Ten arsenic-resistant bacteria were isolated from contaminated wastewater in this study. The most efficient bacterial isolate able to resist 15,000ppm Na2HAsO4·7H2O was identified using the 16S rRNA gene and whole genome analysis as Enterobacter cloacae FACU. The arsenic E. cloacae FACU biosorption capability was analyzed. To further unravel the genetic determinants of As stress resistance, the whole genome sequence of E. cloacae FACU was performed. The FACU complete genome sequence consists of one chromosome (5.7Mb) and two plasmids, pENCL 1 and pENCL 2 (755,058 and 1155666bp, respectively). 7152 CDSs were identified in the E. cloacae FACU genome. The genome consists of 130 genes for tRNA and 21 for rRNAs. The average G + C content was found to be 54%. Sequencing analysis annotated 58 genes related to resistance to many heavy metals, including 16 genes involved in arsenic efflux transporter and arsenic reduction (five arsRDABC genes) and 42 genes related to lead, zinc, mercury, nickel, silver, copper, cadmium and chromium in FACU. Scanning electron microscopy (SEM) confirmed the difference between the morphological responses of the As-treated FACU compared to the control strain. The study highlights the genes involved in the mechanism of As stress resistance, metabolic pathways, and potential activity of E. cloacae FACU at the genetic level.

Draft genome sequence of Achromobacter aegrifaciens BAW48 isolated from arsenic-contaminated tubewell water in Bangladesh.

We report the draft genome of Achromobacter aegrifaciens strain BAW48, a bacterium with a genome size of 6,877,653 bp. This genome comprises gene clusters for arsenic conversion, such as arsenic resistance (arsHCsO), arsenite oxidation (aioBA), and arsenate reduction (arsRCDAB), along with genes for heavy metal and antibiotic resistance.

Arsenic Reduces Methane Emissions from Paddy Soils: Insights from Continental Investigation and Laboratory Incubations.

Arsenic (As) contamination and methane (CH4) emissions co-occur in rice paddies. However, how As impacts CH4 production, oxidation, and emission dynamics is unknown. Here, we investigated the abundances and activities of CH4-cycling microbes from 132 paddy soils with different As concentrations across continental China using metagenomics and the reverse transcription polymerase chain reaction. Our results revealed that As was a crucial factor affecting the abundance and distribution patterns of the mcrA gene, which is responsible for CH4 production and anaerobic CH4 oxidation. Laboratory incubation experiments showed that adding 30 mg kg-1 arsenate increased 13CO2 production by 10-fold, ultimately decreasing CH4 emissions by 68.5%. The inhibition of CH4 emissions by As was induced through three aspects: (1) the toxicity of As decreased the abundance and activity of the methanogens; (2) the adaptability and response of methanotrophs to As is beneficial for CH4 oxidation under As stress; and (3) the more robust arsenate reduction would anaerobically consume more CH4 in paddies. Additionally, significant positive correlations were observed between arsC and pmoA gene abundance in both the observational study and incubation experiment. These findings enhance our understanding of the mechanisms underlying the interactions between As and CH4 cycling in soils.

Promoting effects of ferric ions on Microcystis aeruginosa growth and arsenate accumulation and reduction at different phosphorus environments

The effects of different dissolved organic phosphorus (DOP) associated with distinct iron conditions (iron deficient (dFe), ferric ions (Fe3+), and colloidal iron (CFe)) on algal growth and arsenate (As(V)) metabolism were systematically evaluated and compared in Microcystis aeruginosa. Two chemical forms of DOP (D-glucose-6-phosphate (GP) and phytic acid (PA)), as well as dissolved inorganic phosphorus (DIP), were employed as distinct phosphorus environments. The results revealed that As(V) metabolism of M. aeruginosa was more influenced by different phosphorus forms than by different iron conditions. Conversely, the release of microcystins in the media was found to be significantly more affected by the different phosphorus forms than by the iron conditions. Moreover, DOP was observed to promote arsenic (As) biotransformation, particularly the efflux of methylated As from a single algal cell, whereas DIP was found to primarily facilitate As(V) accumulation in algae. The total As metabolism amount per algal cell under PA was observed to be five times that observed under DIP and GP. The influence of iron conditions on the synthesis of algal metabolites was notable, as evidenced by the metabolites identified in algae of aliphatic (δ 1.28–1.68), humic acid-like and aromatic protein-like substances through 1H-NMR spectra and three-dimensional excitation-emission matrix fluorescence spectroscopy analysis. This impact was particularly notable at Fe3+ conditions, due to the role of Fe3+ as a micronutrient with highly bioavailable forms, which enhanced the synthesis of organic compounds in algae and promoted algal growth. Consequently, Fe3+ could inhibit As accumulation under DIP but promote it under DOP. The obtained results facilitate a more comprehensive understanding of the combined role of different phosphorus forms and iron conditions in algal bloom outbreaks and As(V) metabolism.

Evaluation of interaction among arsenic and Brevibacterium sp. strain CS2 and its proteins profiling

In this study, Brevibacterium sp. strain CS2 was used to evaluate the mechanisms of arsenic interaction with the bacterium and its enzymatic and protein profiling under arsenic stress. The bacterium was capable to resist the arsenate 280 mM and arsenite 40 mM as per MIC. The whole genome, available on NCBI, was analyzed for genes associated with arsenic, which confirmed the genes for both arsenic oxidation (aioB) and arsenic reduction arsR, arsC, ACR3, and arsB. The sharpening and shifting of FTIR spectra in the ranges of 3278–2851 cm−1 are due to hydroxyl and amide stretching. SEM analysis showed no significant changes in morphology in arsenic stress while EDX analysis proved the arsenite interaction by showing arsenic peaks in the graph. Both glutathione and non-protein thiol showed different responses in the absence and presence of arsenic stress. Protein bands such as 25, 30, 32, 37, 42, 48, and 100 kDa were expressed more in arsenic-treated samples as compared to the control one. The presence of arsenic oxidizing genes, the ability to resist arsenic, and the varied response of enzymes and proteins in arsenic stress make the bacterium a suitable agent for arsenic eradication from contaminated sites.

Impact of arsenic and PAHs compound contamination on microorganisms in coking sites: From a community to individual perspective

Arsenic (As) and polycyclic aromatic hydrocarbons (PAHs) are highly toxic, carcinogenic and teratogenic, and are commonly found in soils of industrial sites such as coking plants. They exert environmental stresses on soil microorganisms, but their compounding effects have not been systematically studied. Exploring the effects of compound contamination on microbial communities, species and genes is important for revealing the ecological damage caused by compound contamination and offering scientific insights into soil remediation strategies. In this study, we selected soil samples from 0 to 100 cm depth of a coking site with As, PAHs and compound contamination. We investigated the compound effects of As and PAHs on microbial communities by combining high-throughput sequencing, metagenomic sequencing and genome assembly. Compared with single contamination, compound contamination reduced the microbial community diversity by 10.68%–12.07% and reduced the community richness by 8.39%–18.61%. The compound contamination decreased 32.41%–46.02% of microbial PAHs metabolic gene abundance, 11.36%–19.25% of cell membrane transport gene abundance and 12.62%–57.77% of cell motility gene abundance. Xanthobacteraceae, the biomarker for compound contaminated soils, harbors arsenic reduction genes and PAHs degradation pathways of naphthalene, benzo [a]pyrene, fluorene, anthracene, and phenanthrene. Its broad metabolic capabilities, encompassing sulfur metabolism and quorum sensing, facilitate the acquisition of energy and nutrients, thereby conferring ecological niche advantages in compound contaminated environments. This study underscores the significant impacts of As and PAHs on the composition and function of microbial communities, thereby enriching our understanding of their combined effects and providing insights for the remediation of compound contaminated sites.

Loss of microbial diversity increases methane emissions and arsenic release in paddy soils

Microorganisms are vital to the emission of greenhouse gases and transforming pollutants in paddy soils. However, the impact of microbial diversity loss on anaerobic methane (CH4) oxidation and arsenic (As) reduction under flooded conditions remains unclear. In this study, we inoculated microbial suspensions into natural As-contaminated paddy soils using a dilution approach (untreated, 10−2, 10−4, 10−6, 10−8 dilutions) to manipulate microbial diversity levels. The results revealed that the 10−4 and 10−6 dilutions resulted in the highest CH4 emissions (97.0 μmol and 102.3 μmol) compared to untreated groups (27.6 μmol). However, anaerobic CH4 oxidation was not observed in 10−4 dilution groups and higher dilutions, suggesting the loss of diversity inhibited the natural reduction of CH4. Moreover, the porewater As concentration in the dilution groups was 1.8–8.2 times greater than in the untreated groups. The loss of microbial diversity promoted the reductive dissolution of iron (Fe) minerals bearing As, leading to increased concentrations of Fe(II) and dissolved organic carbon (DOC), which further enhanced As release (Fe(II), R = 0.9, p < 0.001) (DOC, R = 0.8, p < 0.001) from soil to porewater. However, CH4-dependent As(V) reduction was almost entirely inhibited under diversity loss. The decline in microbial diversity increased the relative abundances of methanogens (e.g., Methanobacterium and Methanomassiliicoccus), Fe(III)/As(V)-reducing bacteria (e.g., Bacillus, Clostridium_sensu_stricto_10, and Geobacter), and the related functional genes (i.e., mcrA and Geo). These findings suggest that microbial diversity is critical for specialized soil processes, highlighting the detrimental effects of biodiversity loss on CH4 emissions and As release in As-contaminated paddies.

Meta-omics analysis reveals the marine arsenic cycle driven by bacteria

Arsenic is a toxic element widely distributed in the Earth's crust and ranked as a class I human carcinogen. Microbial metabolism makes significant contributions to arsenic detoxification, migration and transformation. Nowadays, research on arsenic is primarily in areas affected by arsenic pollution associated with human health activities. However, the biogeochemical traits of arsenic in the global marine ecosystem remain to be explicated. In this study, we revealed that seawater environments were primarily governed by the process of arsenate reduction to arsenite, while arsenite methylation was predominant in marine sediments which may serve as significant sources of arsenic emission into the atmosphere. Significant disparities existed in the distribution patterns of the arsenic cycle between surface and deep seawaters at middle and low latitudes, whereas these situations tend to be similar in the Arctic and Antarctic oceans. Significant variations were also observed in the taxonomic diversity and core microbial community of arsenic cycling across different marine environments. Specifically, γ-proteobacteria played a pivotal role in the arsenic cycle in the whole marine environment. Temperature, dissolved oxygen and phosphate were the crucial factors that related to these differentiations in seawater environments. Overall, our study contributes to a deeper understanding of the marine arsenic cycle.

Synergistic effects of warming and humic substances on driving arsenic reduction and methanogenesis in flooded paddy soil

Microbially-driven arsenic reduction and methane emissions in anaerobic soils are regulated by widespread humic substances (HS), while how this effect responds to climate change remains unknown. We investigated potential synergistic effects of HS in response to temperature changes in arsenic-contaminated paddy soils treated with humic acid (HA) and fulvic acid (FA) at temperatures ranging from 15 to 45 °C. Our results reveal a significant increase in arsenic reduction (5.6 times) and methane emissions (178 times) driven by HS, which can be exponentially stimulated at 45 °C. Acting as a electron shuttle, HS determines microbial arsenic reduction, further stimulated by warming. The top three sensitive genera are Geobacter, Anaeromyxobacter, and Gaiella which are responsible for enhanced arsenic reduction, as well as for the reduction of iron and HS with their functional genes; arrA and Geobacter spp. The top three sensitive methanogens are Methanosarsina, Methanocella, and Methanoculleus. Our study suggests notable synergistic effects between HS and warming in stimulating arsenic reduction and methanogenesis in paddy soils. Overall, the findings of this work highlight the high sensitivity of HS-mediated microbial arsenic transformation and methanogenesis in response to warming, which add potential value in predicting the biogeochemical cycling of arsenic and methane in soil under the context of climate change.

Arsenic modifies the effect of folic acid in spina bifida prevention, a large hospital-based case-control study in Bangladesh

BackgroundSpina bifida, a developmental malformation of the spinal cord, is associated with high rates of mortality and disability. Although folic acid-based preventive strategies have been successful in reducing rates of spina bifida, some areas continue to be at higher risk because of chemical exposures. Bangladesh has high arsenic exposures through contaminated drinking water and high rates of spina bifida. This study examines the relationships between mother’s arsenic exposure, folic acid, and spina bifida risk in Bangladesh.MethodsWe conducted a hospital-based case-control study at the National Institute of Neurosciences & Hospital (NINS&H) in Dhaka, Bangladesh, between December 2016 and December 2022. Cases were infants under age one year with spina bifida and further classified by a neurosurgeon and imaging. Controls were drawn from children seen at NINS&H and nearby Dhaka Shishu Hospital. Mothers reported folic acid use during pregnancy, and we assessed folate status with serum assays. Arsenic exposure was estimated in drinking water using graphite furnace atomic absorption spectrophotometry (GF-AAS) and in toenails using inductively coupled plasma mass spectrometry (ICP-MS). We used logistic regression to examine the associations between arsenic and spina bifida. We used stratified models to examine the associations between folic acid and spina bifida at different levels of arsenic exposure.ResultsWe evaluated data from 294 cases of spina bifida and 163 controls. We did not find a main effect of mother’s arsenic exposure on spina bifida risk. However, in stratified analyses, folic acid use was associated with lower odds of spina bifida (adjusted odds ratio [OR]: 0.50, 95% confidence interval [CI]: 0.25-1.00, p = 0.05) among women with toenail arsenic concentrations below the median value of 0.46 µg/g, and no association was seen among mothers with toenail arsenic concentrations higher than 0.46 µg/g (adjusted OR: 1.09, 95% CI: 0.52–2.29, p = 0.82).ConclusionsMother’s arsenic exposure modified the protective association of folic acid with spina bifida. Increased surveillance and additional preventive strategies, such as folic acid fortification and reduction of arsenic, are needed in areas of high arsenic exposure.

Acid-based deep eutectic solvents followed by GFAAS for the speciation of As(III), As(V), total inorganic arsenic and total arsenic in rice samples

In the present study, an efficacious, safe, inexpensive and eco-friendly microextraction was provided by deep eutectic solvents based on dispersive liquid–liquid microextraction (DLLME − DES) followed by GFAAS. A series of DESs were synthesised using l-menthol as hydrogen bond acceptor (HBA) and carboxylic acids with 4, 6, 8 and 10 carbon atoms as hydrogen bond donors (HBD). The synthesised DESs were used as extractants of arsenic ions. Under optimised conditions, good linearity with coefficient of determination (r 2) 0.992 and an acceptable linear range of 0.3–100 µg kg−1 was obtained. The limit of detection was 0.1 µg kg−1 (S/N = 3) for arsenite (As(III)) ions, and a high enrichment factor (EF = 200) was obtained. The enhancement factor and extraction recovery (ER%) of the method were 340 and 60%, respectively. RSDs including inter- and intra-day ranged from 3.2% to 5.8% in three examined concentrations. After a specific digestion, the capability of the synthesised DES in the extraction of As(III) from rice was tested. Total inorganic arsenic was separated similarly after reduction of arsenate (As(V)) to As(III), and As(V) concentration was calculated by difference. Using a second digestion method, total arsenic concentration (sum of organic and inorganic arsenic) in the samples was determined.

Impact of Bacillus cereus SPB-10 on Growth Promotion of Wheat (Triticum aestivum L.) Under Arsenic-Contaminated Soil.

This study investigates the impact of bacteria on arsenic reduction in wheat plants, highlighting the potential of microbe-based eco-friendly strategies for plant growth. In the present study, bacterial isolate SPB-10 was survived at high concentration against both form of arsenic (As3+ and As5+). SPB-10 produced 5.2g/L and 11.3g/L of exo-polysaccharide at 20ppm of As3+ and As5+, respectively, whereas qualitative examination revealed the highest siderophores ability. Other PGP attributes such as IAA production were recorded 52.12mg/L and 95.82mg/L, phosphate solubilization was 90.23mg/L and 129mg/L at 20ppm of As3+ and As5+, respectively. Significant amount of CAT, APX, and Proline was also observed at 20ppm of As3+ and As5+ in SPB-10. Isolate SPB-10 was molecularly identified as Bacillus cereus through 16S rRNA sequencing. After 42days, wheat plants inoculated with SPB-10 had a 25% increase in shoot length and dry weight, and 26% rise in chlorophyll-a pigment under As5+ supplemented T4 treatment than control. Reducing sugar content was increased by 24% in T6-treated plants compared to control. Additionally, SPB-10 enhanced the content of essential nutrients (NPK), CAT, and APX in plant's-leaf under both As3+ and As5+ stressed conditions after 42days. The study found that arsenic uptake in plant roots and shoots decreased in SPB-10-inoculated plants, with the maximum reduction observed in As5+ treated plants. Bio-concentration factor-BCF was reduced by 90.89% in SPB-10-inoculated treatment T4 after 42days. This suggests that Bacillus cereus-SPB-10 may be beneficial for plant growth in arsenic-contaminated soil.

High sensitivity atomic fluorescence spectroscopy for the detection of AsIII by selective electrolysis of arsenic on nanoflowers-like Fe/NFE

Modified electrosynthetic sample introduction technique is a reliable means of solving the problem of high sensitivity analysis of trace arsenite. This article attempts to achieve selective electroreduction of AsIII through the construction of electrode surfaces with different structures and materials from the perspective of interface reactions. Among the four transition metal modifiers, the iron modified nickel foam electrode with nano-flower structure documented higher efficiency in inducing arsenic reduction and better species selectivity. Systematic electrochemical and spectroscopic tests suggest that strong adsorption effect between Fe and AsIII, appropriate hydrogen evolution potential, and catalytic activity jointly promote efficient electroreduction of AsIII. Optimization based on electrode materials and electrolysis conditions, with high sensitivity, wide linear range (0.1–50 μg L−1), and excellent species selectivity, this paper offers an efficient and economic sample introduction method for trace AsIII/V selective atomic spectroscopy direct determination.

Sodium citrate enhancing electrodeposition of metallic arsenic from toxic trivalent arsenic and the mechanism understanding

Electrochemical conversion of hypertoxic trivalent arsenic to value-added metallic arsenic can not only contribute to pollution abatement, but also resources reutilization, therefore being widely explored. Electrochemical reduction of trivalent arsenic as a promising way is widely explored. However, the high efficiency conversion is retarded by the sluggish reduction kinetics of AsO33− and fierce evolution of side products of both H2 and toxic AsH3. Herein, by using the sodium citrate as the additive, the current efficiency for metal arsenic production is increased greatly from 60% to 91%, with the accompanied evolution of hypertoxic AsH3 being restrained from 0.15 Nm3/tAs to 0.022 Nm3/tAs, promising a high-efficiency and green process. The electrochemical tests and electrode surface characterizations as well as DFT calculations indicate that the added sodium citrate promotes both the diffusion of reactive AsO33− towards the cathode and its subsequent adsorption on the Ti cathode, contributing to smoother reduction for generating metal arsenic, with the evolution of toxic AsH3 being hindered at the same time. The results can provide new insights for the high-efficiency and greener conversion of hypertoxic trivalent arsenic to value-added metallic arsenic.

Biochar-supported zero-valent iron enhanced arsenic immobilization in a paddy soil: the role of soil organic matter

Arsenic (As) detoxification in polluted soils by iron-based materials can be mediated by the endogenous soil organic matter (SOM), nevertheless the mechanisms remain unclear. Herein, endogenous SOM in a paddy soil was substantially removed to understand its roles on As immobilization by biochar-supported zero-valent iron (ZVI/BC). The results demonstrated that ZVI/BC application significantly decreased As bioavailability by 64.2% compared with the control soil under the anaerobic condition. XPS and HR-TEM suggested As immobilization by ZVI/BC mainly invoked the formation of ternary complexes (i.e., As-Fe-SOM). However, SOM depletion compromised the efficacy of ZVI/BC for As immobilization by 289.8%. This is likely because SOM depletion increased the fulvic acid and OH− contents in soils. Besides, ZVI/BC increased the proportion of As(III) in available As fraction, but SOM depletion altered the mechanisms associated with As(V) reduction. That is, As(V) reduction resulted from the reductive capacity of ZVI in the pristine soil, but the As(V)-reducing bacteria contributed greater to As(V) reduction in the SOM-depleted soil. Additionally, SOM depletion boosted the abundances of Fe(III)- and As(V)-reducing bacteria such as Bacillus and Ammoniphilus in soils, which enhanced the dissimilatory arsenate reduction. Thus, this work highlighted the importance of SOM in the remediation of As-contaminated soils by ZVI/BC.Graphical

Identification of Novel QTL for Mercury Accumulation in Maize Using an Enlarged SNP Panel.

Mercury (Hg) pollution not only poses a threat to the environment but also adversely affects the growth and development of plants, with potential repercussions for animals and humans through bioaccumulation in the food chain. Maize, a crucial source of food, industrial materials, and livestock feed, requires special attention in understanding the genetic factors influencing mercury accumulation. Developing maize varieties with low mercury accumulation is vital for both maize production and human health. In this study, a comprehensive genome-wide association study (GWAS) was conducted using an enlarged SNP panel comprising 1.25 million single nucleotide polymorphisms (SNPs) in 230 maize inbred lines across three environments. The analysis identified 111 significant SNPs within 78 quantitative trait loci (QTL), involving 169 candidate genes under the Q model. Compared to the previous study, the increased marker density and optimized statistical model led to the discovery of 74 additional QTL, demonstrating improved statistical power. Gene ontology (GO) enrichment analysis revealed that most genes participate in arsenate reduction and stress responses. Notably, GRMZM2G440968, which has been reported in previous studies, is associated with the significant SNP chr6.S_155668107 in axis tissue. It encodes a cysteine proteinase inhibitor, implying its potential role in mitigating mercury toxicity by inhibiting cysteine. Haplotype analyses provided further insights, indicating that lines carrying hap3 exhibited the lowest mercury content compared to other haplotypes. In summary, our study significantly enhances the statistical power of GWAS, identifying additional genes related to mercury accumulation and metabolism. These findings offer valuable insights into unraveling the genetic basis of mercury content in maize and contribute to the development of maize varieties with low mercury accumulation.

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.

Microbially mediated sulfur oxidation coupled with arsenate reduction within oligotrophic mining-impacted habitats.

Arsenate [As(V)] reduction is a major cause of arsenic (As) release from soils, which threatens more than 200 million people worldwide. While heterotrophic As(V) reduction has been investigated extensively, the mechanism of chemolithotrophic As(V) reduction is less studied. Since As is frequently found as a sulfidic mineral in the environment, microbial mediated sulfur oxidation coupled to As(V) reduction (SOAsR), a chemolithotrophic process, may be more favorable in sites impacted by oligotrophic mining (e.g. As-contaminated mine tailings). While SOAsR is thermodynamically favorable, knowledge regarding this biogeochemical process is still limited. The current study suggested that SOAsR was a more prevalent process than heterotrophic As(V) reduction in oligotrophic sites, such as mine tailings. The water-soluble reduced sulfur concentration was predicted to be one of the major geochemical parameters that had a substantial impact on SOAsR potentials. A combination of DNA stable isotope probing and metagenome binning revealed members of the genera Sulfuricella, Ramlibacter, and Sulfuritalea as sulfur oxidizing As(V)-reducing bacteria (SOAsRB) in mine tailings. Genome mining further expanded the list of potential SOAsRB to diverse phylogenetic lineages such as members associated with Burkholderiaceae and Rhodocyclaceae. Metagenome analysis using multiple tailing samples across southern China confirmed that the putative SOAsRB were the dominant As(V) reducers in these sites. Together, the current findings expand our knowledge regarding the chemolithotrophic As(V) reduction process, which may be harnessed to facilitate future remediation practices in mine tailings.