Microbial diversity and activity in rock salt formations

Rock salt formations are considered as potential host rocks for the long-term storage of highly radioactive waste in a deep geological repository. A combination of culture-dependent and culture-independent methods was used to investigate the microbial diversity in rock salt. Extremely halophilic archaea, e.g. Halobacterium species, dominate this habitat. For long-term risk assessment it is of high interest to study how these microorganisms can interact with radionuclides if released from the waste repository. Therefore, the interactions of the extremely halophilic archaeon Halobacterium noricense DSM 15987T with uranium, one of the major radionuclides of concern in the geological repository, were investigated in detail in batch experiments. A multi-spectroscopic and microscopic approach was used to decipher the interaction mechanisms on a molecular level. H. noricense DSM 15987T showed a multistage bioassociation of uranium. By using time-resolved laser-induced fluorescence spectroscopy and X-ray absorption spectroscopy the formation of U(VI) phosphate minerals, such as meta-autunite, was observed. Furthermore, the presence of U(VI)-phosphate mineral could be visualized by scanning electron microscopy. These findings highlight the potential significance of the microbial life in deep geological hypersaline environments and offer new insights into the microbe-actinide interactions at highly saline conditions relevant to the disposal of highly radioactive waste as well as bioremediation.