Microscopic and spectroscopic investigations of uranium(VI) reduction by <i>Desulfosporosinus hippei</i> DSM 8344

Abstract. Clay formations are potential host rocks for the long-term storage of high-level radioactive waste in a deep geological repository. Bentonites are supposed to serve as backfill material, not only for a final disposal site in clay formations but also in crystalline rock. For a long-term safety assessment, various aspects must be taken into account. Besides geological, geochemical and geophysical considerations, naturally occurring microorganisms also play a crucial part in the environment of such a repository. In the event of a worst-case scenario when water enters the disposal site, they can interact with the radionuclides and change for example the chemical speciation or the oxidation state (Lloyd et al., 2002). Desulfosporosinus spp. are an important representative of anaerobic, sulfate-reducing microorganisms, which are present in clay formations as well as in bentonites. Various studies have shown that they play a major role in the microbial communities of these surroundings (Bagnoud et al., 2016; Matschiavelli et al., 2019). A closely related microorganism to the isolated species is Desulfosporosinus hippei DSM 8344, which was originally found in permafrost soil (Vatsurina et al., 2008). This bacterium was used to investigate its interactions with uranium(VI) especially regarding the reduction to the less mobile uranium(IV). Time-dependent reduction experiments in artificial Opalinus Clay pore water (Wersin et al., 2011) (100 µM uranium(VI), pH 5.5) showed the removal of about 80 % of the uranium(VI) from the supernatants within 48 h. Corresponding UV/Vis measurements of the dissolved cell pellets exhibited an increasing proportion of uranium(IV) in the cell-bound uranium. Calculations with the inclusion of extinction coefficients led to a ratio of 39 % uranium(IV) after 1 week. Therefore, a combined sorption-reduction process is a possible interaction mechanism. Time-resolved laser-induced luminescence spectroscopy verified the presence of two uranium(VI) species in the supernatant. A comparison with reference spectra led to an assignment to a uranyl(VI) lactate and a uranyl(VI) carbonate complex. The species distribution showed a decrease of the proportion of the lactate species with time, whereas the proportion of the carbonate species remained almost constant. Uranium aggregates are formed on the cell surface during the process, as determined by transmission electron microscopy (TEM). Furthermore, uranium occurs inside and outside the cells as well as vesicles containing uranium. These findings help to close existing gaps in a comprehensive safeguard 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).