The interplay between microbial metabolism and survival at surface-air interfaces : Implications for the Long-Term Storage of Used Nuclear Fuel

The Nuclear Waste Management Organization of Canada was mandated with the safe, long-term containment of the mounting nuclear industry waste (NWMO, 2005). The engineering of a multi-level barrier system in deep geological repositories to prevent radionuclide escape into groundwater involves the multidisciplinary collaboration of physicists, modelers, engineers, policy makers and legislators. This extreme environment is largely inhospitable to life nevertheless microbial ubiquity demands biological attention if we are to approach risk assessment with legislative integrity. Two decades of work driven by an engineer-turnedmicrobiologist (Stroes-Gascoyne et al., 2010) provided extensive data to support highly compacted bentonite clay for microbial suppression during repository saturation. However, interfaces still demand microbiological attention (Wolfaardt and Korber, 2012), and desiccated bentonite interfaces are thus the focus of this research. The foundational work (Chapter 2) developed tools for evaluating microbial activity in saturated and desiccated bentonite. Novel aspects included the evaluation of precipitated sulphides with fluorometry, and the use of closed loop CO2 measurements to assess microbial metabolism at bentonite-air interfaces. These tools were employed (Chapter 3) to measure the influence of relative humidity (RH) on persistent microbial metabolic activity at bentonite-air interfaces, and the influence of metabolic activity on the clay matrix. It was shown that the combination of bentonite interfaces and high RH stimulated the metabolic persistence of microbial communities during desiccation, despite high RH inhibiting the long-term survival of viable communities. A number of hypotheses were built on these observations (Chapter 4). These extended the observation of the interactive influence of high RH and bentonite to another hygroscopic matrix, polyethylene glycol, confirming that hygroscopic interfaces and high RH improve microbial access to water vapor at surface-air interfaces. It was also shown that oligotrophy increases biofilm desiccation-resilience in a desiccation-tolerant species (Arthrobacter), but not a desiccation-sensitive species (Pseudomonas). An appreciation of the microbial ability to harness their matrix for access to water vapor, and the interaction of this metabolic persistence with long-term survival, is applicable in the modeling and engineering of these industrial environments, and is an intriguing picture of the response of these versatile organisms to the vast challenges within which they so remarkably persist.