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.

淹没稻田土壤时，微生物介导的砷（As）甲基化会增强，主要产生具有植物毒性的二甲基ar砷酸（DMA），并可能诱发水稻直根病。DMA的硫醇化反应会生成甲基化的硫代砷酸盐化合物，例如二甲基化的一硫代砷酸盐（DMMTA）和二甲基化的二硫代砷酸盐（DMDTA），前者对人体有剧毒。在本研究中，我们调查了影响两种稻田土壤甲基化和硫醇化的因素，在稻田中，稻谷作物表现出直纹病症状，且谷物中的DMA含量较高。在淹没的孵化条件下，当氧化还原电势从大约0降至-100 mV时，土壤孔隙水中各种甲基化和硫代甲基化的As物种的浓度迅速增加 其浓度随后下降。硫酸盐的添加增加了异化亚硫酸盐还原酶β-亚基（dsrB）基因和亚砷酸S-腺苷甲硫氨酸甲基转移酶基因（arsM），以及DMA，DMMTA和DMDTA的产生。温度升高还增加了dsrB和arsM的拷贝数，增加并推进了DMA，DMMTA和DMDTA产生的峰值浓度。在较高温度下，添加硫酸盐对dsrB和arsM的拷贝数和As甲基化产生较大影响。dsrB和arsM之间存在显着相关性拷贝数，在DMA和DMMTA或DMDTA浓度之间，其中DMMTA和DMDTA平均分别代表DMA的36％和6％。这些结果表明，硫酸盐还原菌是淹没水稻土中驱动砷甲基化和硫醇化过程的关键微生物。