Rice (Oryza sativa L.) is a staple food for more than half of the world’s population; however, it accumulates 10 times more toxic arsenic (As) in its grains than other cereals. Commonly, only inorganic and methylated As species are considered to be important in paddy soils, even though sulfate-reducing, methanogenic conditions are reported in paddy soil pore-water. Thioarsenates, As species where oxygen atoms were replaced by sulfur atoms, have been found under sulfate-reducing conditions in geothermal and terrestrial environments before but have never been reported in paddy fields up to now.
The aim of this study was to investigate the occurrence of inorganic and methylated thioarsenates in paddy fields and to study their transformation, uptake, accumulation, and translocation by rice plants.
Methylated and inorganic thioarsenates were detected in the pore-water of a large variety of paddy soils from different origins and throughout the whole growing season. By determining the thiolation potential in anoxic lab incubations, the soil pH was found to be an easy to measure indicator for the formation of methylated or inorganic thioarsenates. Compared to inorganic or methylated As species, methylated thioarsenates were sorbed less to Fe(oxyhydr)oxides making them highly mobile in the rhizosphere and available for plant uptake. Hydroponically grown rice plants were taking up and transporting thioarsenates to different extends showing the potential for their accumulation in rice grains.
Altogether, we demonstrated the importance of thioarsenates in paddy soils and for the uptake, translocation, and accumulation in rice plants. Thus, adequate analytical methods that can detect thioarsenates should be included in further studies and their contribution to As accumulation in rice grains should be evaluated, especially as the highly toxic dimethlymonothioarsenate (DMMTA) has already been detected in rice grains before.