New insights into uranium(VI) reduction by a sulfate-reducing bacterium relevant to nuclear waste disposal

For a comprehensive safety assessment of a nuclear repository, the influence of naturally occurring microorganisms from deep geological layers has to be taken into account. Clay rock represents a suitable host rock for the long-term storage of high-level radioactive waste with bentonite as backfill material. In the event of a worst-case scenario, water can enter the repository, solubilize components of the waste, and transport it into the surrounding barriers. In this case, microorganisms can interact with the radionuclides and thereby change the chemical speciation or induce redox reactions.
Desulfosporosinus species represent important members of anaerobic, sulfate-reducing bacteria present in both, clay rock and bentonite. They occur, among others, in the pore water of Opalinus Clay and in the Bavarian bentonite B25. [1,2] Desulfosporosinus hippei DSM 8344T is a close phylogenetic relative to an isolated bacterium from bentonite samples. [3] Therefore, this strain was selected to get a more profound insight into the uranium(VI) interactions, especially regarding the reduction to the less mobile uranium(IV).
Artificial Opalinus Clay pore water [4] served as background electrolyte for the reduction experiments (100 µM uranium(VI), pH 5.5), in which the uranium concentrations in the supernatants decreased rapidly. Time-resolved laser-induced fluorescence spectroscopy showed the presence of a uranyl lactate and a uranyl carbonate complex as aqueous species in the supernatant. While the proportion of the uranyl lactate complex decreased with the incubation time, the uranyl carbonate fraction remained almost constant.
UV/Vis studies of the dissolved cell pellets provided clear proof of a partial reduction of uranium(VI) to uranium(IV) of up to 39% in the samples. Therefore, a combined association-reduction process is a possible interaction mechanism.
TEM images showed the presence of uranium-containing aggregates on the cell surface. To mitigate encrustation, cells released membrane vesicles as a possible defense mechanism.
In addition, uranium(VI) reduction was confirmed by HERFD-XANES measurements. Moreover, uranium(V) could be detected as an intermediate, providing first evidence of the involvement of uranium(V) in uranium(VI) reduction by sulfate-reducing microorganisms. EXAFS measurements helped to identify different cell-related uranium species.
This study helps to improve the understanding of the complexity of uranium-microbe interactions relevant to the long-term storage of high-level radioactive waste in clay rock and therefore contributes to a safety concept for a nuclear repository in this host rock.
References:
[1] Bagnoud et al. (2016) Nat. Commun 7, 1–10.
[2] Matschiavelli et al. (2019) Environ. Sci. Technol. 53, 10514–10524.
[3] Vatsurina et al. (2008) Int. J. Syst. Evol. Microbiol. 58, 1228–1232.
[4] Wersin et al. (2011) Appl. Geochemistry 26, 931–953.