Investigation of uranium(VI) reduction by the repository-relevant bacterium Desulfosporosinus hippei DSM 8344T

INTRODUCTION
For a comprehensive safety assessment regarding the deep geological disposal of high-level radioactive waste, various aspects have to be taken into account. Besides geological, geochemical, and geophysical properties, the influence of naturally occurring microorganisms in the surrounding host rock and backfill material play a crucial role in the environment of such a repository. Clay formations are potential host rocks for the long-term storage of this waste, whereas bentonites are supposed to serve as backfill material, not only for a final disposal site in clay formations but also in crystalline rock. In the event of a worst-case scenario, if water enters the disposal site, radionuclides can escape from the waste canisters and thus interact with the microorganisms. This can, for example, lead to changes in the chemical speciation or the oxidation state of the metal ions.
RESULTS & DISCUSSION
Under repository-relevant conditions, Desulfosporosinus spp. are important representatives of anaerobic, sulfate-reducing bacteria being present in clay formations as well as in bentonites. Various studies show that they are playing a major role in the microbial communities of these surroundings. A closely related microorganism to the isolated species is Desulfosporosinus hippei DSM 8344T. Therefore, this bacterium was used to investigate its interactions with uranium(VI) especially regarding the reduction to the less-mobile uranium(IV) having favorable properties like a reduced mobility.
Time-dependent reduction experiments showed the removal of about 80% of the uranium(VI) from the supernatants in artificial Opalinus Clay pore water (100 µM uranium(VI), pH 5.5) within 48 h. Corresponding UV/Vis measurements of the dissolved cell pellets provide clear proof of the formed uranium(IV). The proportion of this oxidation state in the cell-bound uranium increases up to 40% after one week. Therefore, a combined sorption-reduction process is a possible interaction mechanism.
Time-resolved laser-induced luminescence spectroscopy reveals the presence of two uranium(VI) species in the supernatant. A comparison with reference spectra leads to an assignment to a uranyl(VI) lactate and a uranyl(VI) carbonate complex. The species distribution shows a decrease of the proportion of the lactate species with time, whereas the proportion of the carbonate species remains almost constant.
Uranium aggregates are formed on the cell surface during the process, as determined by scanning transmission electron microscopy (STEM). Furthermore, cells release uranium-containing vesicles as a possible defense mechanism against cell encrustation.
CONCLUSIONS
The findings of this study help to close existing gaps in a comprehensive safeguards concept for a repository for high-level radioactive waste in clay rock. Moreover, this study provides new insights into the interactions of sulfate-reducing microorganisms with uranium(VI) and thus, contributes to new bioremediation approaches of radionuclide-contaminated environments, as well.