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Thioarsenates have recently been detected in rice and rice-based products, with particularly high contents in puffed rice cakes. Here, we show that puffing rice can cause almost complete transformation of dimethylarsenate (DMA) to dimethyldithioarsenate (DMDTA) and dimethylmonothioarsenate (DMMTA). Analysis of puffed rice cakes after 3 months of non-sealed storage at room temperature showed transformation of DMDTA mainly into DMMTA. From a food safety perspective, this likely represents an increased risk because DMMTA is highly cytotoxic and misidentified as non-regulated DMA by routine acid extractions. Analysis of 80 commercial puffed rice cakes confirmed widespread occurrence of thioarsenates. The sum of non-regulated, but potentially toxic DMMTA and DMDTA reached values up to 537 µg·kg -1 and 241 µg·kg -1 for generic and infant-labeled rice cakes, respectively. Our results highlight the importance of better understanding (de)thiolation processes along the rice cake-production chain and potentially revising current thresholds set for iAs to include DMMTA and DMDTA., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier Ltd. All rights reserved.)

Rice accumulates arsenic (As) when cultivated under flooded conditions in paddy soils threatening rice yield or its safety for human consumption, depending on As speciation. During long-term paddy use, repeated redox cycles systematically alter soil biogeochemistry and microbiology. In the present study, incubation experiments from a 2000-year-old paddy soil chronosequence revealed that As mobilization and speciation also change with paddy soil age. Younger paddies (≤100 years) showed the highest total As mobilization, with speciation dominated by carcinogenic inorganic oxyarsenic species and highly mobile inorganic thioarsenates. Inorganic thioarsenates formed by a high availability of reduced sulfur (S) due to low concentrations of reducible iron (Fe) and soil organic carbon (SOC). Long-term paddy use (>100 years) resulted in higher microbial activity and SOC, increasing the share of phytotoxic methylated As. Methylated oxyarsenic species are precursors for cytotoxic methylated thioarsenates. Methylated thioarsenates formed in soils of all ages being limited either by the availability of methylated As in young soils or that of reduced-S in older ones. The present study shows that via a linkage of As to the biogeochemistry of Fe, S, and C, paddy soil age can influence the kind and the extent of threat that As poses for rice cultivation., Competing Interests: Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Jose M. Leon Ninin reports financial support was provided by the German Academic Scholarship Foundation. Britta Planer-Friedrich reports financial support was provided by the Federal Ministry of Education and Research., (Copyright © 2023 Elsevier B.V. All rights reserved.)

Inorganic and methylated thioarsenates have recently been reported to form in paddy soil pore waters and accumulate in rice grains. Among them, dimethylmonothioarsenate (DMMTA) is particularly relevant because of its high cytotoxicity and potential misidentification as nonregulated dimethylarsenate (DMA). Studying DMMTA uptake and flag leaf, grain, and husk accumulation in rice plants during grain filling, substantial dethiolation to DMA was observed with only 8.0 ± 0.1, 9.1 ± 0.6, and 1.4 ± 0.2% DMMTA remaining, respectively. More surprisingly, similar shares of DMMTA were observed in control experiments with DMA, indicating in planta DMA thiolation. Exposure of different rice seedling varieties to not only DMA but also to arsenite and monomethylarsenate (MMA) revealed in planta thiolation as a common process in rice. Up to 35 ± 7% DMA thiolation was further observed in the shoots and roots of the model plant Arabidopsis thaliana . Parameters determining the ratio and kinetics of thiolation versus dethiolation are unknown, yet, but less DMA thiolation in glutathione-deficient mutants compared to wild-type plants suggested glutathione concentration as one potential parameter. Our results demonstrate that pore water is not the only source for thioarsenates in rice grains and that especially the currently nonregulated DMA needs to be monitored as a potential precursor of DMMTA formation inside rice plants.