Impact of anoxic conditions, uranium(VI) and organic phosphate substrate on the biogeochemical potential of the indigenous bacterial community of bentonite

Uranium (U) is the most hazardous radionuclide in nuclear waste and its harmful effects depend on its mobility and bioavailability. Microorganisms can affect the speciation of radionuclides and their migration in Deep Geological Repositories (DGR) for high level radioactive waste (HLW) storage. Consequently, a better understanding of microbe-radionuclide interactions within a DGR concept is essential for a safe storage. With that in mind, bentonite microcosms amended with uranyl nitrate and glycerol-2-phosphate were incubated for six months under anoxic conditions. Post-incubation 16S rRNA gene sequencing revealed high microbial diversities including glycerol oxidizers such as Clostridium and Desulfovibrio and nitrate reducers (Limnobacter and Brevundimonas). In addition, uranium-reducing bacteria (Desulfovibrio and Pseudomonas) were highly enriched in glycerol-2-phosphate‑uranium amended microcosms. These bacteria may contribute to uranium immobilization through enzymatic reduction and/or biomineralization. Scanning electron microscopy of colored spots on the surface of the bentonite in the microcosms indicated the probable formation of Mn(IV) oxides likely through the activity of Mn(II)-oxidizing microbes. This could affect the biogeochemical cycle of U(VI) by concentrating and immobilizing this element in the bentonites. Finally, X-ray diffraction determined a high structural stability of bentonites. The outputs of this study help to predict the impact of microbial activity (e.g. smectite alteration, metal corrosion, and radionuclides mobilization) on the long-term performance of a DGR and to develop appropriate waste treatments, remediation, and management strategies.