Microbial analysis of Tsukinuno mine bentonite as a natural analogue for a nuclear waste repository

Bentonite is a key component of many proposed deep geological repository (DGR) designs. Studying indigenous microorganisms present in naturally occurring bentonite is important for assessing the stability of a future DGR. In the context of a DGR, sulfate-reducing bacteria are of particular interest due to their potential contributions of microbiologically influenced corrosion, which could lead to deterioration of engineered DGR components. Because the intended lifetime of a DGR is upwards of 100,000 years, the study of natural analogues of a DGR helps understand microbial communities that could be expected within well-established bentonite deposits. In this study, four bentonite cores were drilled and recovered from the Tsukinuno mine (Japan) for microbiological and geochemical analysis. Results demonstrated relatively low abundances of culturable microorganisms, especially culturable heterotrophic sulfate-reducing bacteria that were only detected in approximately half of the samples. Instead, microbial communities included representation from putative sulfur-oxidizing, hydrogen-oxidizing, and denitrifying genera Thiobacillus , Hydrogenophaga , and Pseudomonas , and family Comamonadaceae. Additionally, both phospholipid fatty acids and deoxyribonucleic acids (DNA) associated with eukaryotes were detected at low abundance in several samples. In agreement with taxa potentially capable of autotrophic growth, an analysis of natural organic matter suggested very low concentrations of organic carbon, with the majority being in highly diagenetically altered and non-bioavailable forms. Together, the results suggest that future DGR bentonite could similarly host low abundances of mostly autotrophic microorganisms.