Molybdate Causes Target and Nontarget Inhibitory Effects on Microbial Arsenic Methylation in Paddy Soil

Microbial arsenic (As) methylation is enhanced in flooded paddy soils, producing mainly dimethylarsenic (DMA), which can cause rice straighthead disease and large yield losses. Sulfate-reducing bacteria (SRB) have been implicated as primary As methylators, largely based on experiments employing molybdate as a selective inhibitor of SRB. However, the specificity of molybdate inhibition on SRB- and non-SRB-mediated As methylation remains inadequately evaluated. In this study, we showed that molybdate addition at 10 mmol kg –1 suppressed DMA production in flooded paddy soil by 41%, concomitant with decreased transcription of genes encoding dissimilatory sulfite reductase (by 78%) and arsenite S -adenosylmethionine methyltransferase ( arsM, by 28%). Metatranscriptomic analysis showed that molybdate additions significantly suppressed the expression of 12 and 31 arsMs hosted by SRB and non-SRB, respectively. To decipher the effects of molybdate on As methylation, we performed pure culture experiments with representative SRB and non-SRB fermentative bacteria isolated from paddy soils. Molybdate specifically inhibited the growth of SRB and completely suppressed its As methylation. Molybdate also caused partial inhibition of As methylation by fermentative bacteria without affecting their growth. Integrated transcriptomic and targeted metabolomic analyses revealed that molybdate suppressed the As(III)-induced transcription of arsM and trx [encoding thioredoxin (Trx) required as a reductant for As methylation] and the biosynthesis of methyl donor S -adenosylmethionine (SAM), contributing to the partial inhibition of As methylation in non-SRB. These findings indicate that, although molybdate inhibits SRB growth specifically and the associated As methylation, it can also cause nontarget effects on As methylation mediated by fermentative bacteria, which should be taken into account when interpreting microbial contributions to As methylation in flooded paddy soils.