Microbial Community Analysis of Opalinus Clay Drill Core Samples from the Mont Terri Underground research Laboratory, Switzerland

International audienceOpalinus Clay is a candidate host rock for a high-level radioactive waste repository in Switzerland. Microbial metabolism and its by-products could affect the physical and (geo)chemical conditions in such a repository. This study investigated the occurrence of indigenous microbes, their community size and-structure in an Opalinus Clay core from the Mont Terri Underground Research Laboratory, Switzerland, drilled with aseptic techniques. Core sub-samples were distributed to five laboratories, where they were analysed with microscopy, culture-and molecular biology techniques. Evidence supporting a viable microbial community in Opalinus Clay included: five positive culture results (including for sulphate-reducing bacteria, SRB) out of 20 culture attempts; extraction of 64 ng phospholipid fatty acids (PLFA) per g clay, suggesting the presence of viable cells; detection of PLFA biomarkers for anaerobic Gram-negative bacteria and SRB; and the presence of sufficient nutrients to support growth of indigenous and non-indigenous microorganisms for two months. Evidence against a thriving microbial community in Opalinus Clay included: 15 negative culture results out of 20 attempts; lack of cells by application of microscope techniques (phase contrast, AODC, CARD-FISH); consistent failure to extract PCR-amplifiable DNA from the core; the presence of 14 times higher amounts of lipids indicative of cell debris than those indicative of viable cells (PFLA); very small pore sizes; and very low water content. The combined results of this study (and evidence from other studies in comparable environments) suggest that unperturbed Opalinus Clay appears to contain only a small viable microbial community, which is probably metabolically almost inactive (dormant), due to space and water restrictions. However, any disturbances that would provide space, water and nutrients, as would be the case during repository excavation and construction, could revive the dormant organisms, unless the host rock was kept intact