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Biotransformation mediated by microbes can affect the biogeochemical cycle of arsenic. However, arsenic biotransformation mediated by earthworm-related microorganisms has not been well explored, especially the role played by earthworm skin microbiota. Herein, we reveal the profiles of arsenic biotransformation genes (ABGs) and elucidate the microbial communities of the earthworm gut, skin, and surrounding soil from five different soil environments in China. The relative abundance of ABGs in the earthworm skin microbiota, which were dominated by genes associated with arsenate reduction and transport, was approximately three times higher than that in the surrounding soil and earthworm gut microbiota. The composition and diversity of earthworm skin microbiota differed significantly from those of the soil and earthworm gut, comprising a core bacterial community with a relative abundance of 96% Firmicutes and a fungal community with relative abundances of 50% Ascomycota and 13% Mucoromycota. In addition, stochastic processes mainly contributed to the microbial community assembly across all samples. Moreover, fungal genera such as Vishniacozyma and Oomyces were important mediators of ABGs involved in the biogeochemical cycle of arsenic. This is the first study to investigate earthworm skin as a reservoir of microbial diversity in arsenic biotransformation. Our findings broaden the current scientific knowledge of the involvement of earthworms in the arsenic biogeochemical cycle. [Display omitted] • Profiles of ABG in the earthworm skin dominated by arsenate reduction and transport were observed. • The earthworm skin microbiota differed significantly from those of the soil and earthworm gut. • Stochastic processes strongly shaped the microbial community assembly of earthworm gut and skin. • The earthworm skin as a reservoir of microbial diversity played a vital role in arsenic biotransformation. [ABSTRACT FROM AUTHOR]

The biotransformation of arsenic mediated by microorganisms plays an important role in the arsenic biogeochemical cycle. However, the fate and biotransformation of arsenic in different soil fauna gut microbiota are largely unknown. Herein the effects of arsenic contamination on five types of soil fauna were compared by examining variations in arsenic bioaccumulation, gut microbiota, and arsenic biotransformation genes (ABGs). Significant difference was observed in the arsenic bioaccumulation across several fauna body tissues, and Metaphire californica had the highest arsenic bioaccumulation, with a value of 107 ± 1.41 mg kg−1. Arsenic exposure significantly altered overall patterns of ABGs; however, dominant genes involved in arsenic redox and other genes involved in arsenic methylation and demethylation were not significantly changed across animals. Except for M. californica , the abundance of ABGs in other animal guts firstly increased and then decreased with increasing arsenic concentrations. In addition, exposure of soil fauna to arsenic led to shifts in the unique gut-associated bacterial community, but the magnitude of these changes varied significantly across ecological groups of soil fauna. A good correlation between the gut bacterial communities and ABG profiles was observed, suggesting that gut microbiota plays important roles in the biotransformation of arsenic. Overall, these results provide a universal profiling of a microbial community capable of arsenic biotransformation in different fauna guts. Considering the global distribution of soil fauna in the terrestrial ecosystem, this finding broadens our understanding of the hidden role of soil fauna in the arsenic bioaccumulation and biogeochemical cycle. [Display omitted] • Arsenic bioaccumulation exhibited significant differences among different soil fauna. • Both bacterial communities and arsenic biotransformation genes shifted in different fauna guts. • Streptomycetaceae , Chthoniobacter, Bradyrhizobiaceae, Enterobacteriaceae as indicator species for arsenic were observed. [ABSTRACT FROM AUTHOR]