Shifts in bentonite bacterial community and mineralogy in response to uranium and glycerol-2-phosphate exposure

The multi-barrier deep geological repository system is currently considered as one of the safest option for the disposal of high-level radioactive wastes. Indigenous microorganisms of bentonites may affect the structure and stability of these clays through Fe-containing minerals biotransformation and radionuclides mobilization. The present work aimed to investigate the behavior of bentonite and its bacterial community in the case of a uranium leakage from the waste containers. Hence, bentonite microcosms were amended with uranyl nitrate (U) and glycerol-2-phosphate (G2P) and incubated aerobically for 6 months. Next generation 16S rRNA gene sequencing revealed that the bacterial populations of all treated microcosms were dominated by Actinobacteria and Proteobacteria, accounting for >50% of the community. Additionally, G2P and nitrate had a remarkable effect on the bacterial diversity of bentonites by the enrichment of bacteria involved in the nitrogen and carbon biogeochemical cycles (e.g. Azotobacter). A significant presence of sulfate-reducing bacteria such as Desulfonauticus and Desulfomicrobium were detected in the U-treated microcosms. The actinobacteria Amycolatopsis was enriched in G2P‑uranium amended bentonites. High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy analyses showed the capacity of Amycolatopsis and a bentonite consortium formed by Bradyrhizobium-Rhizobium and Pseudomonas to precipitate U as U phosphate mineral phases, probably due to the phosphatase activity. The different amendments did not affect the mineralogy of the bentonite pointing to a high structural stability. These results would help to predict the impact of microbial processes on the biogeochemical cycles of elements (N and U) within the bentonite barrier under repository relevant conditions and to determine the changes in the microbial community induced by a uranium release.This work was funded by the ERDF-financed Grant CGL2014-59616-R (80% funding by FEDER), (Ministerio de Ciencia e Innovación, España) as well as by an FPU 14/04263 (“Formación de Profesorado Universitario”) grant to the first author from the Spanish Ministry (Ministerio de Educación Cultura y Deporte, MECD)