Underground Research Laboratories: Windows into the Deep Subterranean Biosphere

The general approach for disposal of high-level radioactive wastes, mainly spent nuclear fuel, is to build deep underground repositories several 100 m below ground. Many countries construct underground research laboratories (URLs) for testing engineered concepts and study the characteristics of the proposed geology, geochemistry, and microbiology for the repository or repositories to be built. Microbiological investigations in and around URL sites, in Sweden, Finland, Canada, France, Switzerland, Japan, and Belgium, performed from the end of the 1980 decade to present date, are briefly reviewed. Methods to access the deep biosphere, to evaluate the contamination issue, and to detect microbial numbers, diversity, and activity are discussed. The advantage of sampling for microbiological research in URLs, compared to sampling from the ground surface is emphasized. A hypothesis on molecular hydrogen from very deep sources in Earth as the ultimate source of energy to sustain the observed microbes was formulated in the 1990 decade. The diffusion of hydrogen from the interior of earth is a very slow, but continuous process that can feed subterranean microbiomes with reducing power and thereby keep a life cycle, independent of solar energy, continuously running in deep subterranean environments.