Sulfate addition and rising temperature promote arsenic methylation and the formation of methylated thioarsenates in paddy soils

Microbe-mediated arsenic (As) methylation is enhanced when paddy soils are flooded, producing primarily dimethylarsinic acid (DMA) that is phytotoxic and can induce rice straighthead disease. Thiolation of DMA produces methylated thioarsenate compounds such as di-methylated monothioarsenate (DMMTA) and di-methylated dithioarsenate (DMDTA), the former is highly toxic to humans. In the present study, we investigated the factors that influenced As methylation and thiolation in two paddy soils in which rice crops showed straighthead disease symptoms and contained high levels of DMA in the grains. Under flooded incubation conditions, the concentrations of various methylated and thio-methylated As species in the soil porewater increased rapidly when the redox potential decreased from approximately 0 to −100 mV, and their concentrations declined thereafter. Sulfate addition increased the copy numbers of dissimilatory sulfite reductase β-subunit (dsrB) genes and arsenite S-adenosylmethionine methyltransferase genes (arsM), and the production of DMA, DMMTA and DMDTA. Rising temperature also increased dsrB and arsM copy numbers, increased and advanced the peak concentrations of DMA, DMMTA and DMDTA production. Sulfate addition produced larger effects on dsrB and arsM copy numbers and As methylation at the higher temperature. There were significant correlations between dsrB and arsMcopy numbers, and between DMA and DMMTA or DMDTA concentrations, with DMMTA and DMDTA on average representing 36% and 6% of DMA, respectively. These results suggest that sulfate reducing bacteria are the key microbial group driving both the As methylation and thiolation processes in flooded paddy soils.