Temperature-driven shifts in microbial reactions and community structure in bentonite under Fe(III)- and sulfate-reducing conditions: implications for deep geological repository performance.

Bentonite is a promising buffer material for the engineered barrier system in deep geological repositories (DGRs) of spent nuclear fuel. Beyond its physical barrier function, bentonite harbors microbial communities that can mediate biogeochemical interactions with surrounding materials, potentially impacting long-term repository stability. Over repository timescales, these microbes encounter a wide temperature range from radionuclide decay heat. In this study, we evaluated temperature effects on microbial processes and community dynamics under Fe(III)- and sulfate-reducing conditions. Microcosms of a Ca-type bentonite with lactate as an electron donor were incubated at 18-70°C. Results revealed a clear temperature-dependent shift in microbial reaction pathways. Low temperatures (18-30°C) exhibited lactate utilization via fermentation with active microbial reduction of both Fe(III) and sulfate. In contrast, high temperatures (40-70°C) favored lactate metabolism through incomplete oxidation to acetate followed by rapid sulfate reduction and comparatively lower Fe(III) reduction. Microbial community analysis identified common, metabolically flexible taxa across all temperatures, as well as temperature-dependent development of distinct sulfate- reducing bacteria, implying the ability of indigenous WRK microbes to survive and adapt to elevated temperatures. Our findings provide novel insights into bentonite microbial dynamics under repository-relevant conditions, advancing understanding of microbial interactions critical for long-term stability and safety of DGRs. IMPORTANCE: After the construction of the spent nuclear fuel (SNF) repository, the initial ambient environment of the bentonite barrier is subject to residual radioactive decay heat. Subsequently, the elevated temperature conditions within the repository (40-70°C) can persist for >1,000 years. Furthermore, a temperature gradient can be generated from the center of SNF storage area to the surface of the host rock, as the boundary of the SNF is at natural groundwater temperature (18-30°C). Given that SNF repositories must remain stable for over 100,000 years, understanding how microbial activity responds to these temperature conditions is critical for long-term repository safety.