Arsenic (As) bioavailability to rice plants is elevated in flooded paddy soils due to reductive mobilization of arsenite [As(III)]. However, some microorganisms are able to mediate anaerobic As(III) oxidation by coupling to nitrate reduction, thus attenuating As mobility. In this study, we investigated the impact of nitrate additions on As species dynamics in the porewater of four As-contaminated paddy soils. The effects of nitrate on microbial community structure and the abundance and diversity of the As(III) oxidase (aioA) genes were quantified using 16S rRNA sequencing, quantitative PCR, and aioA gene clone libraries. Nitrate additions greatly stimulated anaerobic oxidation of As(III) to As(V) and decreased total soluble As in the porewater in flooded paddy soils. Nitrate additions significantly enhanced the abundance of aioA genes and changed the microbial community structure by increasing the relative abundance of the operational taxonomic units (OTUs) from the genera Acidovorax and Azoarcus. The aioA gene sequences from the Acidovorax related OTU were also stimulated by nitrate. A bacterial strain (ST3) belonging to Acidovorax was isolated from nitrate-amended paddy soil. The strain was able to oxidize As(III) and Fe(II) under anoxic conditions using nitrate as the electron acceptor. Abiotic experiments showed that Fe(II), but not As(III), could be oxidized by nitrite. These results show that nitrate additions can stimulate As(III) oxidation in flooded paddy soils by enhancing the population of anaerobic As(III) oxidizers, offering a potential strategy to decrease As mobility in As-contaminated paddy soils.

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Microbial arsenic (As) methylation and volatilization are important processes controlling the As biogeochemical cycle in paddy soils. To further understand these processes, we isolated a novel bacterial strain, SM-1, from an As-contaminated paddy soil. SM-1 showed strong As methylation and volatilization abilities, converting almost all arsenite (10 µM) to dimethylarsenate and trimethylarsenic oxide in the medium and trimethylarsine gas into the headspace within 24 h, with trimethylarsine accounting for nearly half of the total As. On the basis of the 16S rRNA sequence, strain SM-1 represents a new species in a new genus within the family Cytophagaceae. Strain SM-1 is abundant in the paddy soil and inoculation of SM-1 greatly enhanced As methylation and volatilization in the soil. An arsenite methyltransferase gene (ArarsM) was cloned from SM-1. When expressed in Escherichia coli, ArArsM conferred the As methylation and volatilization abilities to E. coli and increased its resistance to arsenite. The high As methylation and volatilization abilities of SM-1 are likely attributed to an efficient ArArsM enzyme coupled with low arsenite efflux. These results suggest that strain SM-1 plays an important role in As methylation and volatilization in the paddy soil and has a great potential for As bioremediation.

c Inorganic arsenic (As) is highly toxic and ubiquitous in the environment. Inorganic As can be transformed by microbial methylation, which constitutes an important part of the As biogeochemical cycle. In this study, we investigated As biotransformation by Pseudomonas alcaligenes NBRC14159. P. alcaligenes was able to methylate arsenite [As(III)] rapidly to dimethylarsenate and small amounts of trimethylarsenic oxide. An arsenite S-adenosylmethionine methyltransferase, PaArsM, was identified and functionally characterized. PaArsM shares low similarities with other reported ArsM enzymes (<55%). When P. alcaligenes arsM gene (PaarsM) was disrupted, the mutant lost As methylation ability and became more sensitive to As(III). PaarsM was expressed in the absence of As(III) and the expression was further enhanced by As(III) exposure. Heterologous expression of PaarsM in an As-hypersensitive strain of Escherichia coli conferred As(III) resistance. Purified PaArsM protein methylated As(III) to dimethylarsenate as the main product in the medium and also produced dimethylarsine and trimethylarsine gases. We propose that PaArsM plays a role in As methylation and detoxification of As(III) and could be exploited in bioremediation of As-contaminated environments.