Metabolism-dependent bioaccumulation of uranium by Rhodosporidium toruloides isolated from the flooding water of a former uranium mine

Remediation of former uranium mining sites represents one of the biggest challenges worldwide that have to be solved in this century. The former uranium mine Königstein (Germany) displays one of these sites and is currently remediated by controlled flooding of the underground. The flooding water is cleaned up by a conventional chemical waste water treatment plant. During the last years, the search of alternative strategies involving environmentally sustainable treatments has started. Bioremediation, the use of microorganisms to clean up polluted sites in the environment, is considered one of the best alternative. By means of culture-dependent methods, we isolated an indigenous yeast strain, KS5 (Rhodosporidium toruloides), directly from the flooding water and investigated its interactions with uranium(VI). Our results highlight distinct adaptive mechanisms towards high uranium concentrations on the one hand, and complex interaction mechanisms on the other. The cells of the strain KS5 exhibit high uranium tolerance being able to grow up to 5 mM, and also high ability to accumulate this radionuclide (350 mg uranium/g dry biomass in 48 hours). The removal of uranium by KS5 displays a temperature- and cell viability-dependent process. By STEM investigations we observed that uranium was removed by two mechanisms, inactive biosorption and active bioaccumulation. EXAFS analysis revealed that the molecular speciation of uranium associated with the cells is similar to that of meta-autunite-like minerals. The present study highlights the potential of KS5 as a representative of indigenous species which might play a key role in bioremediation of uranium-contaminated sites.