Rice is one of the most important sources for human arsenic (As) uptake. The high As accumulation in rice is explained by its growth under flooded, reducing conditions, where As prevails as highly mobile arsenite (H3AsIIIO3). The anoxic conditions of soils also promote methylation by rhizosphere bacteria. In an effort to reduce As accumulation in rice, major advances have recently been made in understanding the uptake, translocation, and detoxification pathways of the oxyanions. In contrast to the oxyanions, thioarsenates have never been studied in paddy soils before. Thioarsenates (HAsVS-IInO4-n2-, n=1-4) form from arsenite under sulfate-reducing conditions and are structural analogues to arsenate.
Studying paddy soils in Italy and France, we have been able to prove natural occurrence of mono- and dithioarsenate for the first time. Thioarsenates contributed up to 10% to total As which is comparable to concentrations measured for mono- and dimethylarsenate-species typically considered in paddy soil. We furthermore detected up to 3% mixed methylthiolated arsenates. Laboratory incubation experiments with different types of soil showed that acetate addition and high sulfate concentrations promoted thioarsenate formation. In low iron soils, we even detected trithioarsenate and the sum of thioarsenates reached up to 60% of total As. On a temporal scale, thiolation preceded methylation. The effect that As thiolation in paddy soil will have on the As content in rice grains, is unknown, yet. Preliminary results show, that monothioarsenate is more toxic for rice than arsenate, but less toxic than arsenite. Whether monothioarsenate is taken up intact in the rice plant is still unclear, but rice is capable of reducing monothioarsenate to arsenite.
With the now proven occurrence of thioarsenates in paddy soils, their uptake, transformation, translocation, and accumulation in rice plants and their grains urgently need to be addressed.