In addition to the life on the surface of the Earth and in its oceans, ecosystems have evolved deep under us in a realm coined the “deep biosphere” which stretches several kilometers down into the bedrock. Down there, the conditions are harsh and life is forced to adjust to a lifestyle that we at the surface would call extreme. One major difference to surface conditions is the lack of oxygen; a compound we take for granted and consider to be a prerequisite for survival but which subsurface life has to cope without.