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Microbe-mediated arsenic (As) biotransformation in paddy soils determines the fate of As in soils and its availability to rice plants, yet little is known about the microbial communities involved in As biotransformation. Here, we revealed wide distribution, high diversity, and abundance of arsenite (As(III)) oxidase genes (aioA), respiratory arsenate (As(V)) reductase genes (arrA), As(V) reductase genes (arsC), and As(III) S-adenosylmethionine methyltransferase genes (arsM) in 13 paddy soils collected across Southern China. Sequences grouped with As biotransformation genes are mainly from rice rhizosphere bacteria, such as some Proteobacteria, Gemmatimonadales, and Firmicutes. A significant correlation of gene abundance between arsC and arsM suggests that the two genes coexist well in the microbial As resistance system. Redundancy analysis (RDA) indicated that soil pH, EC, total C, N, As, and Fe, C/N ratio, SO42--S, NO3--N, and NH4+-N were the key factors driving diverse microbial community compositions. This study for the first time provides an overall picture of microbial communities involved in As biotransformation in paddy soils, and considering the wide distribution of paddy fields in the world, it also provides insights into the critical role of paddy fields in the As biogeochemical cycle. [ABSTRACT FROM AUTHOR]

Organic manures may contain high levels of arsenic (As) due to the use of As-containing growth-promoting substances in animal feed. To develop a bioremediation strategy to remove As from organic waste, Bacillus subtilis 168, a bacterial strain which can grow at high temperature but is unable to methylate and volatilize As, was genetically engineered to express the arsenite S-adenosylmethionine methyltransferase gene (CmarsM) from the thermophilic alga Cyanidioschyzon merolae. The genetically engineered B. subtilis 168 converted most of the inorganic As in the medium into dimethylarsenate and trimethylarsine oxide within 48 h and volatized substantial amounts of dimethylarsine and trimethylarsine. The rate of As methylation and volatilization increased with temperature from 37 to 50°C. When inoculated into an As-contaminated organic manure composted at 50°C, the modified strain significantly enhanced As volatilization. This study provides a proof of concept of using genetically engineered microorganisms for bioremediation of As-contaminated organic waste during composting. [ABSTRACT FROM AUTHOR]