Rice, a staple food for half of the world’s population, takes up approximately 10 times more Arsenic (As) than other cereals and contributes to human As exposure. Arsenic is ubiquitously present in soils and mobilized during rice cultivation on flooded paddy soils by reductive dissolution of iron minerals. Recently, methylated thioarsenates (HAsS_xO_4−(x+y)(CH₃)_y²⁻, x;y =1-2) and inorganic thioarsenates (HAsS_xO_4−x²⁻, x =1-4) have been reported to occur in paddy fields besides the better-known oxyarsenates. Inorganic thioarsenates are structural analogues to arsenate and form under sulfur-reducing conditions from arsenite via OH-/SH- ligand exchange and addition of zerovalent sulfur. Methylated thioarsenates form via ligand exchange of OH-/SH- in mono- (MMA) or dimethylarsenate (DMA) molecules. Sulfur (S) addition is one driver for the occurrence of thiolation, even though naturally occurring S concentrations are sufficient for As thiolation in paddy soils. Sulfur fertilization has recently been suggested to lower grain As concentration, however, no conclusive data have been obtained so far.

Even though methylated thioarsenates are highly toxic for humans, no data about uptake, transformation or translocation of methylated thioarsenates in plants are available until now. Using hydroponic experiments with 20-day-old rice plants, we showed that monomethylmonothioarsenate (MMMTA), dimethylmonothioarsenate (DMMTA), and monothioarsenate (MTA) were taken up by rice roots and could be detected in the xylem. Total arsenic translocation from roots to shoots was higher for plants exposed to DMMTA, MTA, and DMA compared to MMMTA and MMA. Our experiments revealed that all thioarsenates were partially transformed in the presence of rice roots, but processes and extents differed. MMMTA was subject to abiotic oxidation and largely dethiolated to MMA outside the plant by root oxygen loss (ROL). DMMTA and MTA were not oxidized by ROL. Inside roots, MTA was rapidly enzymatically reduced to arsenite, while DMMTA transformation to DMA was not enzymatically catalyzed and much slower.

While MMMTA and MTA seem to be less critical for food safety, based on current knowledge, DMMTA behaves differently as uptake and total As translocation are high. Although DMMTA is mostly transformed to DMA, it is the only As species besides arsenite, arsenate, MMA and DMA that has ever been detected in rice grains before. DMMTA is highly toxic, comparable to inorganic As, but co-determined as DMA using common acid extraction for rice grains as the acid converts DMMTA to DMA. This is especially problematic, as DMA is not considered toxic and exempt from food guidelines. Contribution of DMMTA and total As in rice grains especially after S fertilization needs to be further investigated and potentially both standard extraction methods and guidelines need to be adapted.