Microbial metabolism in bentonite clay: Saturation, desiccation and relative humidity

• High relative humidity extended microbial metabolism at clay-air interfaces
• High relative humidity suppressed microbial survival in desiccated bentonite
• Microbial activity, including sulphur reduction, decreased bentonite clay swell index
• Salinity suppressed desiccated and saturated microbial activity in bentonite

Within a Deep Geological Repository for used nuclear fuel storage, compacted bentonite clays are the candidate buffer due to their physical and rheological properties, and their ability to suppress microorganisms. This study focused on the potential for microbial metabolism at bentonite-air interfaces, the influence of relative humidity (RH) and the consequences of metabolic activity on bentonite. Microbial activity, determined by monitoring the concentration of evolved CO2, was sustained at desiccated bentonite-air interfaces at 75% RH (0.6 ppm CO2/min after 5 days of dessication) but was completely suppressed at 30% RH. Conversely, microbial survival was promoted in dry bentonite, with culturable cell survival up to 3 times higher at lower RH (30%) than higher RH (75%). It was also shown that, under water-saturated conditions, microbial sulphur reduction decreased the clay swell index of uncompacted bentonite, swelling approximately 2.7 cm/(g dry weight) less than controls. Notably, natural groundwater salinities were shown adequate to suppress all microbial activity under both saturated and desiccated conditions, confirming that a combination of high bentonite dry density and high salinity inhibits microbial activity, even in microenvironments like surface-air interfaces where swelling pressure limitations may be transiently compromised. Along with the applied need for this knowledge, this study also provided a fundamental opportunity to explore microbial activity in desiccated environments, and suggests that lower RH may promote rapid entry into a dormant cell state and thus more effective long-term adaptation.