Interactions of halophilic microorganisms with uranium

Rock salt formations are considered as potential host rocks for the long-term storage of highly radioactive waste in a deep geological repository. Next to bacteria and fungi, extremely halophilic archaea, e.g. Halobacterium species, are predominantly present in 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 and the moderately halophilic bacterium Brachybacterium sp. G1 with uranium, one of the major radionuclides of concern in the geological repository of radioactive wastes, were investigated in detail in batch experiments. Furthermore, a multi-spectroscopic and -microscopic approach was used to reveal these interaction mechanisms on a molecular level. The two microorganisms exhibited different association characteristics with uranium. Brachybacterium sp. G1 cells sorbed uranium within a short time, whereas a multistage bioassociation process occurred with the archaeon. In situ attenuated total reflection Fourier-transform infrared spectroscopy, time-resolved laser-induced fluorescence spectroscopy and X-ray absorption spectroscopy were applied to elucidate the U(VI) bioassociation behavior. By using these spectroscopic tools the formation of U(VI) phosphate mineral, such as meta-autunite, by the Halobacterium species was demonstrated. These findings highlight 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 nuclear waste.