Importance of Thioarsenates for Rice Plants
Carolin Kerl (05/2016-08/2019)
Support: Britta Planer-Friedrich
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. Arsenic is ubiquitously present in the environment and mobilized in paddy fields during reductive dissolution of iron (Fe)-minerals under anoxic conditions in flooded paddy soils. Commonly, only inorganic arsenite and arsenate, as well as organic monomethylarsenate (MMAV) and dimethylarsenate (DMAV) are considered to be important As species 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 thesis 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. Additionally, the mobility of methylated thioarsenates in presence of Fe(oxyhydr)oxide was examined and it was tested whether roots covered with Fe(oxyhydr)oxide could be a barrier for the uptake of methylated thioarsenates in rice plants.
In the first study, 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 in Italian soils. The contributions of thioarsenates to total As concentrations were similar to methylated oxyarsenates. 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. Sulfur-fertilization increased thiolation and methylation in soil but lowered the total As concentration in the pore-water. Increased shares of methylated and thiolated As with decreasing total As indicated that mobility of thioarsenates in the rhizosphere could be higher than that of inorganic As.
The following two studies revealed that thioarsenates could be taken up and transported by hydroponically grown rice plants, nevertheless, differences were observed for the individual thioarsenates. Inorganic monothioarsenate (MTA) was taken up in rice roots and rapidly transformed to arsenite by a so far unknown enzyme, however, this transformation was not complete and MTA was detected in the xylem, as well as in roots and shoots. Higher translocation from roots to shoots, compared to the non-thiolated analog arsenate implied that different, so far unknown, enzymes and transporters were involved in the uptake, reduction, and translocation of MTA. Monomethylmonothioarsenate (MMMTA) is oxygensensitive and was partly transformed to its non-thiolated analog MMAV outside the rice rootby root oxygen loss. No transformation was detected inside the root cells and MMMTA was partly transported to the xylem. The overall As uptake and translocation of rice plants exposed to MMMTA was similar to MMAV, indicating effective As sequestration in roots, even though the exact mechanism for MMMTA sequestration remained unknown. Dimethylmonothioarsenate (DMMTA) was not transformed by root oxygen loss and taken up inside the rice roots. However, DMMTA was partly transformed to DMAV in roots but nontransformed DMMTA was partly transported to the xylem. The transformation of DMMTA was most likely a chemical disproportionation in the presence of glutathion to DMAV and dimethyldithioarsenate (DMDTA). High As accumulation in roots and shoots when plants were exposed to DMMTA revealed that the detoxification and translocation of DMMTA is clearly different from its non-thiolated analog DMAV.
The fourth study confirmed the assumption from the pore-water speciation in study 1 that methylated thioarsenates were sorbed less to Fe(oxyhydr)oxides than inorganic or methylated As species. Goethite-rich iron plaque formed around rice roots, was no barrier for the uptake of MMMTA and DMMTA in rice roots, as they were not sorbed effectively. Lab studies with goethite and ferrihydrite revealed that methylated thioarsenates have to be transformed to the non-thiolated analogs MMAV and DMAV prior to sorption. Especially DMMTA was poorly sorbed in all treatments due to its slow transformation to DMA explaining its high mobility in the rhizosphere.
Altogether, the four studies 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 DMMTA has already been detected in rice grains before.