Summary

The thin layer known as the Earth’s crust contains an even thinner layer that is suitable for sustaining life. The boundary conditions that define where we can find life in the deep subsurface depend on the physical space and water availability, accessibility to suitable substrates for energy and growth, in addition to tolerable temperature and pressure. The prevailing rather extreme conditions in the deep subsurface dictate that it is mainly inhabited by prokaryotic microorganisms. In fact, most of Earth’s microbial biomass is dwelling in the deep subsurface. Understanding the microbial diversity, dynamics and ecology in the Earth’s crust provides knowledge on the role of microbes in global elemental cycling. These capacities are investigated with cultivation-based experiments or molecular biological methods. Using genetic material of the microbes to reveal insights into metabolic processes, community structure and evolutionary history of microorganisms is state-of the-art in the deep biosphere research. Upgraded cultivation methods, such as those mimicking the physical and chemical conditions of the deep subsurface, and especially the combination of different methodologies have proven to be a useful approach in unraveling the way of microbial life in the depths. Partly the same fundamentals that define the habitability in the deep subsurface, dictate the potential of finding life outside Earth. Thus, studying the deep biosphere may help us to understand and explore the potential of microbial life to survive and thrive in extreme environments and therefore be useful for future space missions trying to find life on other planetary objects. The study can also help us draw conclusions on our own planet’s history and how life originated on early Earth.