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Arsenic (As) is a groundwater contaminant of global concern. The degradation of dissolved organic matter (DOM) can provide a reducing environment for As release. However, the interaction of DOM with local microbial communities and how different sources and types of DOM influence the biotransformation of As in aquifers is uncertain. This study used optical spectroscopy, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), metagenomics, and structural equation modeling (SEM) to demonstrate the how the biotransformation of As in aquifers is promoted. The results indicated that the DOM in high-As groundwater is dominated by highly unsaturated low-oxygen(O) compounds that are quite humic and stable. Metagenomics analysis indicated Acinetobacter, Pseudoxanthomonas, and Pseudomonas predominate in high-As environments; these genera all contain As detoxification genes and are members of the same phylum (Proteobacteria). SEM analyses indicated the presence of Proteobacteria is positively related to highly unsaturated low-O compounds in the groundwater and conditions that promote arsenite release. The results illustrate how the biogeochemical transformation of As in groundwater systems is affected by DOM from different sources and with different characteristics., Competing Interests: Declaration of Competing Interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Xianjun xie reports financial support was provided by China University of Geosciences. Xianjun xie reports a relationship with China University of Geosciences that includes: employment. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier B.V. All rights reserved.)

Biological nitrogen fixation (BNF), the conversion of N 2 into bioavailable nitrogen (N), is the main process for replenishing N loss in the biosphere. However, BNF in groundwater systems remains poorly understood. In this study, we examined the activity, abundance, and community composition of diazotrophs in groundwater in the Hetao Plain of Inner Mongolia using 15 N tracing methods, reverse transcription qPCR (RT-qPCR), and metagenomic/metatranscriptomic analyses. 15 N 2 tracing incubation of near in situ groundwater (9.5-585.4 nmol N L -1 h -1 ) and N 2 -fixer enrichment and isolates (13.2-1728.4 nmol N g -1 h -1 , as directly verified by single-cell resonance Raman spectroscopy), suggested that BNF is a non-negligible source of N in groundwater in this region. The expression of nifH genes ranged from 3.4 × 10 3 to 1.2 × 10 6 copies L -1 and was tightly correlated with dissolved oxygen (DO), Fe(II), and NH 4 . Diazotrophs in groundwater were chiefly aerobes or facultative anaerobes, dominated by Stutzerimonas, Pseudomonas, Paraburkholderia, Klebsiella, Rhodopseudomonas, Azoarcus, and additional uncultured populations. Active diazotrophs, which prefer reducing conditions, were more metabolically diverse and potentially associated with nitrification, sulfur/arsenic mobilization, Fe(II) transport, and CH + . Diazotrophs in groundwater were chiefly aerobes or facultative anaerobes, dominated by Stutzerimonas, Pseudomonas, Paraburkholderia, Klebsiella, Rhodopseudomonas, Azoarcus, and additional uncultured populations. Active diazotrophs, which prefer reducing conditions, were more metabolically diverse and potentially associated with nitrification, sulfur/arsenic mobilization, Fe(II) transport, and CH 4 oxidation. Our results highlight the importance of diazotrophs in subsurface geochemical cycles., (© 2023. The Author(s), under exclusive licence to International Society for Microbial Ecology.)

Bacterial sulfate reduction plays a crucial role in the mobilization of toxic substances in aquifers. However, the role of bacterial sulfate reduction on iodine mobilization in geogenic high-iodine groundwater systems has been unexplored. In this study, the enrichment of groundwater δ 34 S SO4 (15.56 to 69.31‰) and its significantly positive correlation with iodide and total iodine concentrations in deep groundwater samples of the North China Plain suggested that bacterial sulfate reduction participates in the mobilization of groundwater iodine. Similar significantly positive correlations were further observed between the concentrations of iodide and total iodine and the relative abundance of the dsrB gene by qPCR, as well as the composition and abundance of sulfate-reducing bacteria (SRB) predicted from 16S rRNA gene high-throughput sequencing data. Subsequent batch culture experiments by the SRB Desulfovibrio sp. B304 demonstrated that SRB could facilitate iodine mobilization through the enzyme-driven biotic and sulfide-driven abiotic reduction of iodate to iodide. In addition, the dehalogenation of organoiodine compounds by SRB and the reductive dissolution of iodine-bearing iron minerals by biogenic sulfide could liberate bound or adsorbed iodine into groundwater. The role of bacterial sulfate reduction in iodine mobilization revealed in this study provides new insights into our understanding of iodide enrichment in iodine-rich aquifers worldwide.

Dissolved organic matter (DOM) play critical roles in arsenic (As) biotransformation in groundwater, but its compositional characteristics and interactions with indigenous microbial communities remain unclear. In this study, DOM signatures coupled with taxonomy and functions of microbial community were characterized in As-enriched groundwater by excitation-emission matrix, Fourier transform ion cyclotron resonance mass spectrometry and metagenomic sequencing. Results showed that As concentrations were significantly positively correlated with DOM humification (r = 0.707, p < 0.01) and the most dominant humic acid-like DOM components (r = 0.789, p < 0.01). Molecular characterization further demonstrated high DOM oxidation degree, with the prevalence of unsaturated oxygen-low aromatics, nitrogen (N 1 /N 2 )-containing compounds and unique CHO molecules in high As groundwater. These DOM properties were consistent with microbial composition and functional potentials. Both taxonomy and binning analyses demonstrated the dominance of Pseudomonas stutzeri, Microbacterium and Sphingobium xenophagum in As-enriched groundwater which possessed abundant As-reducing gene, with organic carbon degrading genes capable of labile to recalcitrant compounds degradation and high potentials of organic nitrogen mineralization to generate ammonium. Besides, most assembled bins in high As groundwater presented strong fermentation potentials which could facilitate carbon utilization by heterotrophic microbes. This study provides better insight into the potential role of DOM mineralization for As release in groundwater system., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier B.V. All rights reserved.)

Microorganisms play crucial roles in the global iodine cycling through iodine oxidation, reduction, volatilization, and deiodination. In contrast to iodate formation in radionuclide-contaminated groundwater by the iodine-oxidizing bacteria, microbial contribution to the formation of high level of iodide in geogenic high iodine groundwater is poorly understood. In this study, our results of comparative metagenomic analyses of deep groundwater with typical high iodide concentrations in the North China Plain revealed the existence of putative dissimilatory iodate-reducing idrABP1P2 gene clusters in groundwater. Heterologous expression and characterization of an identified idrABP1P2 gene cluster confirmed its functional role in iodate reduction. Thus, microbial dissimilatory iodate reduction could contribute to iodide formation in geogenic high iodine groundwater. In addition, the identified iron-reducing, sulfur-reducing, sulfur-oxidizing, and dehalogenating bacteria in the groundwater could contribute to the release and production of iodide through the reductive dissolution of iron minerals, abiotic iodate reduction of derived ferrous iron and sulfide, and dehalogenation of organic iodine, respectively. These microbially mediated iodate reduction and organic iodine dehalogenation processes may also result in the transformation among iodine species and iodide enrichment in other geogenic iodine-rich groundwater systems worldwide.

The clay layer is a source to facilitate arsenic (As) enrichment in the aquifer. However, little is known about microbial processes in the clay layer and their roles in As mobilization in the underlying aquifer. In this study, high-throughput sequencing of full-length 16S rRNA gene and metagenomics were used to characterize the microbial composition and functional potential in a sediment borehole across the clay and sand layers in Hetao Basin, Inner Mongolia, China. Results showed the significant differences between the clay layer and underlying sand layer in the geochemistry, microbial composition and functional potential. Fermentation, Fe(III) reduction, As(V) reduction, sulfate reduction, thiosulfate disproportionation, reductive tricarboxylic acid and Wood-Ljungdahl pathway identified in sediments from the clay layer were positively correlated to the observed high levels of TOC, soluble ammonium, H 3 PO 4 -extractable As(III) and As(V) and HCl-extractable Fe(II). Although the microbial compositions of the clay and sand layers were different, the microbial functional potential at the interface between the clay and sand layers was similar with the characteristics of fermentation, ammonification and As(V) reduction. The similarity of microbial functional potential at the interface may be attributable to the interaction between the sand and adjacent clay layer with the fluctuation of groundwater level. These metabolic products from the microbial processes in the clay layers and interface would migrate into the underlying groundwater during groundwater overpumping, which facilitates As enrichment in groundwater., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier B.V. All rights reserved.)

Arsenic (As) mobilization in groundwater involves biogeochemical cycles of carbon, iron, and sulfur. However, few studies have focused on the role of nitrogen-metabolizing bacteria in As mobilization, as well as in the transformation between inorganic and organic As in groundwater. In this study, the nitrogen and As metabolisms of Citrobacter sp. G-C1 and Paraclostridium sp. G-11, isolated from high As groundwater in Hetao Plain, China, were characterized by culture experiments and genome sequencing. The results showed Citrobacter sp. G-C1 was a dissimilatory nitrate-reducing bacterium. The dissimilatory nitrate reduction to ammonia (DNRA) and As-detoxifying pathways identified in the genome enabled Citrobacter sp. G-C1 to simultaneously reduce As(V) during DNRA. Paraclostridium sp. G-11 was a nitrogen-fixing bacterium and its nitrogen-fixing activity was constrained by As. Nitrogen fixation and the As-detoxifying pathways identified in its genome conferred the capability of As(V) reduction during nitrogen fixation. Under anaerobic conditions, Citrobacter sp. G-C1 was able to demethylate organic As and Paraclostridium sp. G-11 performed As(III) methylation with the arsM gene. Collectively, these results not only evidenced that ammonium-generating bacteria with the ars operon were able to transform As(V) to more mobile As(III) during nitrogen-metabolizing processes, but also involved the transformation between inorganic and organic As in groundwater.

Dissimilatory arsenate reduction from arsenic (As)-bearing minerals into highly mobile arsenite is one of the key mechanisms of As release into groundwater. To detect the microbial diversity and As-metabolizing gene clusters of indigenous arsenate-reducing bacteria in high As groundwater in the Hetao Plain of Inner Mongolia, China, three anaerobic arsenate-reducing bacteria were isolated and arrA and arsC gene-based clone libraries of four in situ groundwater samples were constructed. The strains IMARCUG-11(G-11), IMARCUG-C1(G-C1) and IMARCUG-12(G-12) were phylogenetically belonged to genera Paraclostridium, Citrobacter and Klebsiella, respectively. They could reduce >99% of 1 mM arsenate under anoxic conditions with lactate as a carbon source in 60 h, 72 h and 84 h, respectively. As far as we know, this was the first report of arsenate reduction by genus Paraclostridium. Compared with strain G-11 (arsC) and G-C1 (arsRBC), strain G-12 contained two incomplete ars operons (operon1: arsABC, operon2: arsBC), indicating that these strains might present different strategies to resist As toxicity. Phylogenetic analysis illuminating by the arrA genes showed that in situ arsenate-reducing bacterial communities were diverse and mainly composed of Desulfobacterales (53%, dominated by Geobacter), Betaproteobacteria (12%), and unidentified groups (35%). Based on the arsC gene analysis, the indigenous arsenate-reducing bacterial communities were mainly affiliated with Omnitrophica (88%) and Deltaproteobacteria (11%, dominated by Geobacter and Syntrophobacterales). Results of this study expanded our understanding of indigenous arsenic-reducing bacteria in high As groundwater aquifers., (© 2020. Springer Science+Business Media, LLC, part of Springer Nature.)

Dissolved organic matter (DOM) play key roles in the biotransformation of arsenic in groundwater systems. However, the effects of different types of DOM on arsenic biogeochemistry remain poorly understood. In this study, four typical DOM compounds (acetate, lactate, AQS and humic acid) were amended to high As aquifer sediments to investigate their effects on arsenic/iron biotransformation and microbial community response. Results demonstrated that different DOM drove different microbial community shifts and then enhanced microbially-mediated arsenic release and iron reduction. With labile DOM (acetate and lactate) amendment, the abundance of putative dissimilatory iron and sulfate reducers Desulfomicrobium and Clostridium sensu stricto increased within the first week, and subsequently the anaerobic fermentative bacterial genus Acetobacterium and arsenate/sulfate-reducing bacterial genus Fusibacter became predominant. In contrast, recalcitrant DOM (AQS and humic acid) mainly stimulated the abundances of sulfur compounds respiratory genus Desulfomicrobium and fermentative bacterial genus Alkalibacter in the whole incubation. Accompanied with the microbial community structure and function shifts, dissolved organic carbon concentration and oxidation-reduction potential changed and the arsenic/iron reduction increased, which resulted in the enhanced arsenic mobilization. Collectively, the present study linked DOM type to microbial community structure and explored the potential roles of different DOM on arsenic biotransformation in aquifers., (Copyright © 2021 Elsevier B.V. All rights reserved.)

The factors that control arsenic (As) mobilization by dissimilatory iron reduction (DIR) are complicated. The association between As mobilization and extracellular polymeric substance (EPS) of dissimilatory iron reducing bacteria (DIRB) remained unclear. In this study, three DIRB were isolated from high arsenic groundwater to understand the effects of EPS on As mobilization. In the laboratory settings, strain Klebsiella oxytoca IR-ZA released As into aqueous phase from As-bearing ferrihydrite, while strain Shewanella putrefaciens IAR-S1 and S. xiamenensis IR-S2 re-sequestrated As by forming secondary minerals during ferrihydrite reduction. Characterization of EPS contents with Fourier Transform Infrared Spectroscopy and high-performance liquid chromatography suggested that mannan and succinic acid were the main different EPS contents of the DIRB. The biomineralization processes were tightly regulated by EPS compositions. Mannan secreted by IAR-S1 and IR-S2 promoted while succinic acid secreted by IR-ZA suppressed the biomineralization and As immobilization. Energy-dispersive X-ray Spectroscopy mapping indicated that As in the secondary minerals was wrapped with EPS. X-ray diffraction and room temperature Mössbauer spectroscopy showed these secondary minerals were vivianite and magnetite, respectively. The amount of As mobilized into aqueous phase was strongly affected by available anions (H 2 PO 4 - and HCO 3 - ). Our results indicated that the EPS of DIRB significantly influenced As mobilization., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier B.V. All rights reserved.)

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

Arsenic methylation is regarded as an effective way of arsenic detoxification. Current knowledge about arsenic biomethylation in high arsenic groundwater remains limited. In the present study, 16 high arsenic groundwater samples from deep wells of the Hetao Plain were investigated using clone library and quantitative polymerase chain reaction (qPCR) analyses of arsM genes as well as geochemical analysis. The concentrations of methylated arsenic (including monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA)) varied from 2.40 to 16.85 μg/L. Both bacterial and archaeal arsenic methylating populations were detected in the high arsenic aquifer. They were dominated by Proteobacteria, Firmicutes, Gemmatimonadetes, Nitrospirae, Methanomicrobia and a large unidentified group. The abundances of predominant populations were correlated positively to either total organic carbon or total arsenic and arsenite concentrations. The arsM gene abundances in high arsenic groundwater ranged from below detection to 5.71 × 10 6 copies/L and accounted for 0-3.32‰ of total bacterial and archaeal 16S rRNA genes. The arsM gene copies in high arsenic groundwater showed closely positive correlations with methylated arsenic concentrations. The overall results implied that arsenic methylating microorganisms were abundant and diverse in high arsenic groundwater. This was the first study of arsenic methylating microbial communities in high arsenic groundwater aquifers and might provide useful information for arsenic bioremediation in groundwater systems.

A survey was carried out on the microbial community of 20 groundwater samples (4 low and 16 high arsenic groundwater) and 19 sediments from three boreholes (two high arsenic and one low arsenic boreholes) in a high arsenic groundwater system located in Hetao Basin, Inner Mongolia, using the 454 pyrosequencing approach. A total of 233,704 sequence reads were obtained and classified into 12-267 operational taxonomic units (OTUs). Groundwater and sediment samples were divided into low and high arsenic groups based on measured geochemical parameters and microbial communities, by hierarchical clustering and principal coordinates analysis. Richness and diversity of the microbial communities in high arsenic sediments are higher than those in high arsenic groundwater. Microbial community structure was significantly different either between low and high arsenic samples or between groundwater and sediments. Acinetobacter, Pseudomonas, Psychrobacter and Alishewanella were the top four genera in high arsenic groundwater, while Thiobacillus, Pseudomonas, Hydrogenophaga, Enterobacteriaceae, Sulfuricurvum and Arthrobacter dominated high arsenic sediments. Archaeal sequences in high arsenic groundwater were mostly related to methanogens. Biota-environment matching and co-inertia analyses showed that arsenic, total organic carbon, SO4(2-), SO4(2-)/total sulfur ratio, and Fe(2+) were important environmental factors shaping the observed microbial communities. The results of this study expand our current understanding of microbial ecology in high arsenic groundwater aquifers and emphasize the potential importance of microbes in arsenic transformation in the Hetao Basin, Inner Mongolia.

A survey of bacterial and archaeal community structure was carried out in 10 shallow tube wells in a high arsenic groundwater system located in Hetao Basin, Inner Mongolia by 16S rRNA gene based two-step nested PCR-DGGE, clone libraries and 454 pyrosequencing. 12 bacterial and 18 archaeal DGGE bands and 26-136 species-level OTUs were detected for all the samples. 299 bacterial and 283 archaeal 16S rRNA gene clones for two typical samples were identified by phylogenetic analysis. Most of the results from these different methods were consistent with the dominant bacterial populations. But the proportions of the microbial populations were mostly different and the bacterial communities in most of these samples from pyrosequencing were both more abundant and more diverse than those from the traditional methods. Even after quality filtering, pyrosequencing revealed some populations including Alishewanella, Sulfuricurvum, Arthrobacter, Sporosarcina and Algoriphagus which were not detected with traditional techniques. The most dominant bacterial populations in these samples identified as some arsenic, iron, nitrogen and sulfur reducing and oxidizing related populations including Acinetobacter, Pseudomonas, Flavobacterium, Brevundimonas, Massilia, Planococcus, and Aquabacterium and archaeal communities Nitrosophaera and Methanosaeta. Acinetobacter and Pseudomonas were distinctly abundant in most of these samples. Methanogens were found as the dominant archeal population with three methods. From the results of traditional methods, the dominant archaeal populations apparently changed from phylum Thaumarchaeota to Euryarchaeota with the arsenic concentrations increasing. But this structure dynamic change was not revealed with pyrosequencing. Our results imply that an integrated approach combining the traditional methods and next generation sequencing approaches to characterize the microbial communities in high arsenic groundwater is recommended.

The concentration and spatial distribution of polycyclic aromatic hydrocarbons (PAHs) in topsoil, groundwater and groundwater suspended solids (SS) at Guozhuang karst water system of northern China were investigated. The total concentration of PAHs ranged from 622 to 87,880 ng/g dry weight in topsoil, from 4739 to 59,314 ng/g dry weight in SS, and from 2137 to 9037 ng/L in groundwater, with mean values of 17,174 ng/g, 11,990 ng/g and 5020 ng/L, respectively. High concentrations of PAHs were mainly observed in the coal mining industrial area and the discharge area. The composition of PAHs indicated that low molecular weight PAHs were predominant in groundwater samples, the content of medium molecular weight PAHs was elevated in SS, and carcinogenic high molecular weight PAHs were frequently detected in topsoil. The high contents of low-medium molecular weight PAHs in groundwater and SS suggested relatively recent local sources of PAHs that were transported into the aquifer via leakage of contaminated surface water and/or infiltration of PAH-containing precipitation. The results of evaluating sources of PAHs using ratios of specific PAH compounds showed that PAHs mainly originated from coal and wood combustion. Furthermore, five sources were identified by positive matrix factorization (PMF) model, and the contribution to the total loadings of groundwater PAHs were: 2% for unburnt oil, 32% for coal combustion, 22% for vehicle emission, 27% for biomass combustion and 18% for coke production, respectively. Furthermore, strong correlations of total PAHs with total organic carbon (TOC) in topsoil indicated co-emission of PAHs and TOC. Poor correlations of PAHs with dissolved organic carbon (DOC) in groundwater indicated that other factors exert stronger influences. Therefore, PAHs might have posed a major threat to the quality of potable groundwater in Guozhuang karst water system., (Copyright © 2014. Published by Elsevier B.V.)

Microbial functional potential in high arsenic (As) groundwater ecosystems remains largely unknown. In this study, the microbial community functional composition of nineteen groundwater samples was investigated using a functional gene array (GeoChip 5.0). Samples were divided into low and high As groups based on the clustering analysis of geochemical parameters and microbial functional structures. The results showed that As related genes (arsC, arrA), sulfate related genes (dsrA and dsrB), nitrogen cycling related genes (ureC, amoA, and hzo) and methanogen genes (mcrA, hdrB) in groundwater samples were correlated with As, SO42-, NH4+ or CH4 concentrations, respectively. Canonical correspondence analysis (CCA) results indicated that some geochemical parameters including As, total organic content, SO42-, NH4+, oxidation-reduction potential (ORP) and pH were important factors shaping the functional microbial community structures. Alkaline and reducing conditions with relatively low SO42-, ORP, and high NH4+, as well as SO42- and Fe reduction and ammonification involved in microbially-mediated geochemical processes could be associated with As enrichment in groundwater. This study provides an overall picture of functional microbial communities in high As groundwater aquifers, and also provides insights into the critical role of microorganisms in As biogeochemical cycling.

Microbial communities can play important role in arsenic release in groundwater aquifers. To investigate the microbial communities in high arsenic groundwater aquifers in agricultural irrigation area, 17 groundwater samples with different arsenic concentrations were collected along the agricultural drainage channels of Hangjinhouqi County, Inner Mongolia and examined by illumina MiSeq sequencing approach targeting the V4 region of the 16S rRNA genes. Both principal component analysis and hierarchical clustering results indicated that these samples were divided into two groups (high and low arsenic groups) according to the variation of geochemical characteristics. Arsenic concentrations showed strongly positive correlations with [Formula: see text] and total organic carbon (TOC). Sequencing results revealed that a total of 329-2823 operational taxonomic units (OTUs) were observed at the 97% OTU level. Microbial richness and diversity of high arsenic groundwater samples along the drainage channels were lower than those of low arsenic groundwater samples but higher than those of high arsenic groundwaters from strongly reducing areas. The microbial community structure in groundwater along the drainage channels was different from those in strongly reducing arsenic-rich aquifers of Hetao Plain and other high arsenic groundwater aquifers including Bangladesh, West Bengal, and Vietnam. Acinetobacter and Pseudomonas dominated with high percentages in both high and low arsenic groundwaters. Alishewanella, Psychrobacter, Methylotenera, and Crenothrix showed relatively high abundances in high arsenic groundwater, while Rheinheimera and the unidentified OP3 were predominant populations in low arsenic groundwater. Archaeal populations displayed a low occurrence and mainly dominated by methanogens such as Methanocorpusculum and Methanospirillum. Microbial community compositions were different between high and low arsenic groundwater samples based on the results of principal coordinate analysis and co-inertia analysis. Other geochemical variables including TOC, [Formula: see text], oxidation-reduction potential, and Fe might also affect the microbial composition.

Groundwater has been a relatively safe water source and associated with a series of global pressing issues. It can, however, be vulnerable to pathogen contamination and exhibit risks to public health. While groundwater has often been considered a relatively closed ecosystem, it is important to understand the transport of pathogens into and within groundwater and groundwater-dependent ecosystems in the context of their physiological characteristics, soil/sediment texture, hydrogeological and climatic conditions. The rapid progress in monitoring and modeling tools in recent years has facilitated the detection of pathogens and the prediction of their migration in groundwater, which aids the surveillance and remediation of health-related risks. This review aims to provide an updated understanding about groundwater pathogens, their impacts on human health, surveillance, risk assessment, and remediation strategies in the context of the global environment and climate changes. The challenges and perspectives in protecting groundwater biosafety are also discussed. [ABSTRACT FROM AUTHOR]

• High iodine groundwater occurs in discharge area of fluvial/lacustrine aquifer systems. • Local transformation of sediment Fe minerals explains the enrichment of pore-water iodine in the deep sediments. • Iodine-rich pore-water in the shallow sediments near the coastal area is related to the marine transgressions. The wide occurrence of high iodine groundwater is posing negative effects on human health. The land subsidence due to groundwater over-exploitation causes the release of iodine-rich pore-water trapped in clayey sediment into groundwater, however, the mechanisms of iodine enrichment in pore-water were still unclear. In this study, we selected the Datong Basin (DB) and North China Plain (NCP) with few and severe land subsidence, respectively, to understand the contributions of complex hydro-biogeochemical processes on the enrichment of iodine in groundwater and pore-water by using the inorganic/organic carbon isotope and batch experiments. The results showed that at the DB, the range of groundwater iodine was 4–2175 µg/L, and the enrichment of groundwater iodine is mainly related to the transformation of iron oxyhydroxides triggered by microbial degradation of organic matter, which was reflected by the more depleted δ13C DIC signatures of groundwater and gradual increasing trend of ratio of HCl-extractable Fe(II)/sediment Fe total and sediment δ13C DOC signatures with depth. In comparison, groundwater iodine at the NCP had the range of 2–1106 µg/L, and pore-water rich in iodine preserved in the clayey sediment serves as the main source. The depth profiles of sediment iron mineral fractions and δ13C DOC signatures suggested that the NCP sediments experienced lower degree of reductive transformation of iron minerals and microbial degradation of natural organic matter. At some specific depths higher than 300 m, the evident higher ratios of HCl-extractable Fe(II)/sediment Fe total indicate that the local transformation of iron minerals can explain the pore-water rich in iodine at deep sediments, which was supported by the results of microbial batch experiments. The pore-water with high iodine concentration preserved in the shallow sediment is more likely related to the marine transgressions. This study helps to improve our understanding of the mechanisms of high iodine groundwater, which may provide some new insights on further groundwater management. [ABSTRACT FROM AUTHOR]

A bacterial strain Acinetobacter sp. WSD with phenanthrene-degrading ability was identified based on biochemical tests and 16S rDNA gene sequence analysis. The strain was isolated from polycyclic aromatic hydrocarbons (PAHs)-contaminated groundwater from a coal-mining area of the Guozhuang karst water system in Shanxi province of northern China. Acinetobacter sp. WSD could utilize fluorine (FLO), phenanthrene (PHE) and pyrene (PYR) as its sole carbon source and was able to degrade other PAHs. Approximately 90 % of FLO, 90 % of PHE and 50 % of PYR were degraded after 6 days' incubation. The logistic model well fitted all the experimental data. The specific degradation rates in glucose were higher than that in HCO, indicating that organic carbon source promoted the growth of Acinetobacter sp. WSD and the degradation of PAHs. In presence of humic acids (HA), the total degradation rate was accelerated, although there was a delay at the beginning. In the case of high HA concentration, co-metabolism between PAHs and HA occurred. The metabolic pathway for the three compounds of PAHs was discussed using GC-MS data. Phthalic acid was found in their metabolites. Phenol, 2,5-bis(1,1-dimethylethyl), a new type of PAHs metabolites that have been reported before was found after 2 days degradation. [ABSTRACT FROM AUTHOR]

The abundance, diversity, and relative distribution of sulfate-reducing bacteria (SRB) in high arsenic (As) groundwater aquifers of Hangjinhouqi County in the Hetao Basin, Inner Mongolia was investigated using denaturing gradient gel electrophoresis (DGGE) and quantitative polymerase chain reaction (qPCR) analysis ofdsrBgenes (encoding dissimilatory sulfite reductase beta-subunit). DGGE results revealed that SRB populations were diverse, but were mainly composed ofDesulfotomaculum,Desulfobulbus,Desulfosarcina, andDesulfobacca. The abundance ofDesulfobulbuswas positively correlated with the ratio of Fe(II)/Fe(III). Although qPCR results showed that thedsrBgene abundance in groundwater samples ranged from below detection to 4.9 × 106copies/L, and the highest percentage ofdsrBgene copies to bacterial 16S rRNA gene copies was 2.1%. Geochemical analyses showed that As(III) content and the ratio of Fe(II) to Fe(III) increased with total As, while sulfate concentrations decreased. Interestingly, thedsrBgene abundance was positively correlated with As concentrations. These results indicate that sulfate reduction occurs simultaneously with As and Fe reduction, and might result in increased As release and mobilization when As is not incorporated into iron sulfides. This study improves our understanding of SRB and As cycling in high As groundwater systems. [ABSTRACT FROM AUTHOR]

The microbial diversity and community structure in twenty-one groundwater samples from high arsenic shallow aquifers of Hetao Basin, Inner Mongolia, China was investigated with an integrated approach including polymerase chain reaction (PCR)-denaturing gradient gel electrophoresis (DGGE) and 16S rRNA gene phylogenetic analyses. A total of 25 bacterial and 32 archaeal DGGE bands were exercised for sequencing. Phylogenetic analyses showed that the bacterial DGGE bands were dominated byProteobacteria, and the archaeal bands were dominated byThaumarchaeotaandEuryarchaeota. Based on arsenic concentrations, three samples (corresponding to low, medium, and high level of arsenic, respectively) were selected for construction of 16S rRNA gene clone libraries. A total of 912 (468 and 444 for bacteria and archaea, respectively) 16S rRNA gene clone sequences were obtained and subjected to phylogenetic analyses. The results showed that bacterial communities of these samples were dominated byAcinetobacter,Pseudomonas,Massilia, Dietzia,Planococcus,Brevundimonas,AquabacteriumandGeobacter, and archaeal communities byNitrosophaera,ThermoproteiandMethanosaeta. The relative abundance of major groups varied as a function of changes in groundwater geochemistry.Acinetobacter,Brevundimonas, Geobacter,Thermoprotei and Methanosaetadominated in high arsenic samples with high concentrations of methane and Fe(II), and low concentrations of SO2−4and NO−3, whilePseudomonasandNitrosophaerawere abundant in low arsenic groundwater. These results imply that microbes play an important role in arsenic mobilization in the shallow aquifers of Hetao Basin, Inner Mongolia. [ABSTRACT FROM AUTHOR]