Life in deep subsurface
Résumé
Life extends far deeper into the Earth's subsurface than presumed possible 30 years ago. In the past, it was assumed that life is a surface phenomenon, and that even ``hardy prokaryotic types'' are not capable of living deeper than tens of meters below the surface [1]. In the 1990s, it became apparent that genetically and metabolically diverse microbial communities existed under highly reducing conditions in the deep subsurface [2]. Today we know that life in the deep subsurface is ubiquitous and comprises a large proportion of the biomass on Earth [3]. Many questions concerning life in the deep remain unanswered. What is the lower depth limit of the deep biosphere? Which energy sources are fueling these communities? How are genetic diversity and functional activity linked to geochemical factors? What we know is that the deep subsurface is an extreme environment and that the microorganisms living here have developed numerous mechanisms to deal with high pressure and temperature, limited energy and nutrient availability, extreme acidity and alkalinity, metal toxicity, and radioactivity [4].