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The discovery of the deep biosphere gives rise to new, fundamental questions about the evolution and distribution of life. Considering the broad range of temperatures and high pressures in the subsurface and the lack of fresh organic matter as a source of energy and nutrients, the existence and survival of an extensive deep biosphere are remarkable. Geofuels, i.e. gases that are generated by mineral-water interactions or derived from the deep crust, might be an alternative energy source in the absence of light and photosynthesis. So far, only little is known about the individual inhabitants of the deep biosphere, about their genetic diversity, metabolic capabilities and survival strategies. Moreover, the relationships between deep subsurface ecosystems and the surface world remain to be explored.
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