Interactions of an extremely halophilic archaeon relevant in nuclear waste repositories in rock salt with uranium

Rock salt is a potential host rock for the final storage of radioactive waste in a deep geological formation. Indigenous microorganisms and their interactions with radionuclides must be considered for the safety performance of the repository, considering the worst case scenario, the release and subsequent migration of radionuclides. Therefore, the extremely halophilic microorganism Halobacterium noricense DSM 15987T, which occurs naturally in the potential host formation rock salt, was used to study its interactions with uranium.
A time-dependent sorption experiment showed that bioassociation of uranium onto cells of H. noricense DSM 15987T is not only a sorption process; i.e. fast sorption within the first hours until reaching a stable equilibrium state. The obtained kinetic data showed a multistage process with fast sorption during the first two hours of exposure time. Over the next hours, an increasing amount of uranium was detectable in the supernatant, implying that the uranium already sorbed was again released from the cells. Subsequently, the amount of bioassociated uranium increased very slowly until a maximum sorption of 80% was reached after 48 h. To investigate this multistage bioassociation process on archaeal cells in detail several spectroscopic as well as microscopic methods were applied. With in situ attenuated total reflection fourier-transform infrared spectroscopy the initial sorption process of uranium to cells of H. noricense DSM 15987T within the first two hours was proven. Results showed that the radionuclide binds to carboxylic as well as to phosphate groups simultaneously within the first two hours of incubation time. Additionally, cryo time-resolved laser-induced fluorescence spectroscopic investigations were performed, which showed the involvement of polynuclear carboxylate species and the presence of a meta-autunite like mineral. By using scanning electron microscopy coupled with energy dispersive X-ray spectroscopy, uranium could be localized on the cell surface of the halophilic archaeon within the first sorption phase and later in the biomineral-like agglomerates. Hence, our study showed that uranium can be immobilized by halophilic archaea via biomineralization and bioassociation, which might influence the further migration of the actinide.