Comparative analysis of uranium(VI) reduction by a sulfate- and an iron-reducing bacterium

The safe disposal of high-level radioactive waste represents a significant scientific and societal challenge. According to geological, geochemical, and geophysical properties, clay formations represent a suitable host rock for the long-term storage of this waste. However, for a comprehensive safety assessment, the influence of naturally occurring microorganisms in clay rock and in the backfill material bentonite must be taken into account. Desulfosporosinus species play a crucial role in the community of sulfate-reducing bacteria present in clay rock and bentonite.[1,2] Desulfosporosinus hippei DSM 8344T is a close relative of the isolated species and was originally found in permafrost soils.[3] Desulfitobacterium sp. G1-2 has been isolated from bentonite samples and is an important representative of iron-reducing bacteria. As members of the microbial community from deep geological layers, these strains were selected to get a more profound knowledge about their interactions with U(VI). During time-dependent experiments in bicarbonate buffer (30 mM, 100 µM U(VI)), Desulfitobacterium sp. G1-2 showed a removal of up to 80% within 5 days. UV/Vis studies of the dissolved cell pellets verified the formation of U(IV) during the process. In contrast to these findings, Desulfosporosinus hippei DSM 8344T was not able to reduce U(VI) in the presence of bicarbonate. Therefore, experiments in artificial Opalinus Clay pore water [4] (100 µM U(VI), pH 5.5) were conducted. Determinations of the U concentrations showed a removal of up to 80% of the radionuclide from the supernatants within only 48 h. UV/Vis studies of the dissolved cell pellets provided clear proof of a partially reduction of U(VI) to U(IV), although bands of U(VI) were also still observable. These findings propose a combined association-reduction process as a possible interaction mechanism for this microorganism. TEM images combined with EDX analysis revealed the presence of two different U-containing aggregates inside cells of Desulfitobacterium sp. G1-2. Furthermore, cells of Desulfosporosinus hippei DSM 8344T released membrane vesicles as a possible defense mechanism against encrustation by U precipitates on the cell surface. However, cells showed almost no uptake of U. In this study, different analytical methods were used to better understand the U(VI) reduction by sulfate- and iron-reducing bacteria. Significant differences in the occurring mechanisms were evident between both microorganisms, highlighting the importance of studies on the U(VI) interactions of different microorganisms present in clay rock. Moreover, these results contribute to a safety concept for a nuclear repository in clay formations and for final disposal sites using bentonite as backfill material. 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.