Paddy Soil Flooding and Nonflooding Affect the Transcriptional Activity of Arsenic Methylation and Demethylation Communities

The intermediate product methylarsenite [MMA(III)] of arsenic (As) methylation can be methylated to dimethylarsenate (DMA), which causes rice straighthead disease via the As methylation enzyme (ArsM), demethylated to arsenite via the As demethylation enzyme (ArsI), or excreted from cells via the MMA(III) efflux enzyme (ArsP). Whereas As methylation is commonly reported in flooded soils, As demethylation is mostly mediated by aerobes. We used custom-built ROCker models (accuracies of 99.7–99.9%) to quantify the short-read sequences carrying As genes and investigate the variations in the transcriptional activity of the arsM, arsI, and arsP genes in flooded and nonflooded paddy soils. We revealed significantly (p < 0.05) greater transcriptional activity of the arsM and arsP genes in flooded than nonflooded soils, whereas the transcriptional activity of the arsI genes was comparable. MMA(III) demethylation in flooded soils is possibly coupled with denitrification, as revealed by the significantly (p < 0.05) positively correlated genes in terms of transcriptional activity. Moreover, we showed that microbes coexpressing the arsM and arsI genes were dominated by Actinomycetota and Pseudomonadota. This study sheds light on the active microbial communities involved in As methylation and demethylation in paddy soils and provides insights into the prevention of rice straighthead disease.