Effect of temperature and pressure on proliferation of microorganisms in bentonite

Bentonite hosts a very diverse spectrum of indigenous microorganisms with the potential to influence the long-term stability of deep geological repositories (DGR). It is therefore essential to understand factors determining the survivability and proliferation of microorganisms under repository conditions. In the present contribution, we describe the results of two experiments performed on Czech bentonite Cerny Vrch (BCV), that focused on the separate effect of temperature and pressure exposure as two of the key factors influencing the survival and proliferation of bacteria in future DGR. Water-dispersed bentonite suspension (200 g/L) was used purposefully for these experiments to boost microbial activity and to better visualize the microbial response to studied factors. Bentonite suspensions were exposed to range of temperatures for 4 weeks continuously under anaerobic conditions and samples were kept at lab temperature for 2 more weeks to check the ability of spores to regrowth after the temperature exposure. We showed, that only temperature 90 C was limiting for the microbial activity and subsequent proliferation. On the other hand, in experiment with bentonite suspensions exposed to external pressure of 10, 12, 15 MPa in the same 4+2 weeks design, we demonstrated, that the pressure itself does not influence the overall microbial growth and proliferation, but possibly has some effect on the microbial community structure. The pressure evolution during bentonite swelling thus cannot be considered as the key factor responsible for the microbial suppression observed in highly compacted bentonites, but other interlinked factors such as pore space and diffusion limitation, lower water activity, or changes in nutrient concentration are probably responsible for this effect.