Survivability and proliferation of microorganisms in bentonite with implication to radioactive waste geological disposal: strong effect of temperature and negligible effect of pressure

Abstract

As bentonite hosts a diverse spectrum of indigenous microorganisms with the potential to influence the long-term stability of deep geological repositories, it is essential to understand the factors influencing microbial activity under repository conditions. Here, we focus on two factors, i.e., temperature and swelling pressure, using a suspension of Cerny Vrch bentonite to boost microbial activity and evaluate microbial response. Suspensions were exposed either to different pressures (10, 12 and 15 MPa; to simulate the effect of swelling pressure) or elevated temperatures (60, 70, 80 and 90 °C; to simulate the effect of cannister heating) for four weeks. Each treatment was followed by a period of anaerobic incubation at atmospheric pressure/laboratory temperature to assess microbial recovery after treatment. Microbial load and community structure were then estimated using molecular-genetic methods, with presence of living cells confirmed through microscopic analysis. Our study demonstrated that discrete application of pressure did not influence on overall microbial activity or proliferation, implying that pressure evolution during bentonite swelling is not the critical factor responsible for microbial suppression in saturated bentonites. However, pressure treatment caused significant shifts in microbial community structure. We also demonstrated that microbial activity decreased with increasing temperature, and that heat treatment strongly influenced bentonite microbial community structure, with several thermophilic taxa identified. A temperature of 90 °C proved to be limiting for microbial activity and proliferation in all bentonite suspensions. Our study emphasizes the crucial role of a deep understanding of microbial activity under repository-relevant conditions in identifying possible strategies to mitigate the microbial potential within the deep geological repository and increase its long-term stability and safety.

Dramatic loss of microbial viability in bentonite exposed to heat and gamma radiation: Implications for deep geological repository

“…In our study, both enrichment cultures in different media and natural incubations of bentonite suspensions were included to test which cultivation conditions were more suitable for demonstrating effects on microbial survivability. While natural incubations were successfully used in our previous experiments (Kašpar et al 2021;Bartak et al 2023) to assess the reaction of bentonite microorganisms to elevated pressure and temperature, the enrichment culture approach was clearly superior to natural incubations for detecting microbial survivability in the present study. This nding agrees well with previous studies, where excess of available nutrients promoted microbial recovery following harsh treatment, e.g.…”

Insights into the Impact of Physicochemical and Microbiological Parameters on the Safety Performance of Deep Geological Repositories

“…The activity absence of IRB and SRB at 60 • C, or higher temperatures, is of particular interest related to the MIC of metal canisters previously mentioned, as this fact would retard the MIC on the canister surface. Additionally, Bartak et al [157] found a decline in microbial activity as temperature rose, and that thermal treatment significantly impacted the microbial community composition within the bentonite, identifying various thermophilic species (e.g., Caldinitratiruptor and Brockia) or thermotolerant spore-forming genera (e.g., Thermincola and Bacillus). Moreover, they determined a threshold temperature of 90 • C to inhibit microbial activity and growth across all the tested bentonite suspensions.…”