NATURE AND EXTENT OF THE DEEP BIOSPHERE

Explorations of subsurface life beneath seafloor and terrestrial settings benefit from common approaches and are beset with common challenges. The first glimpses of life at depth obtained early in the last century have now transformed into numerous efforts driven both by fundamental questions related to microbial ecology and by practical questions seeking to understand how we might control these microbes or how we depend upon them to perform biogeochemical processes of significance to the surface world. Numerous subsurface phenomena – natural and human induced - permit the conditions by which microbes can exploit a thermodynamic disequilibrium for deriving energy to conduct the activities associated with life. Studies so far find subsurface life most everywhere that permeable pore space is present and that the earth is not too hot. As microbiologists attempt to establish principles related to the diversity of subsurface microbial life, more information is being collected regarding Bacteria, Archaea, Eukarya, and viruses that survive in the deep Earth. An impressive array of tools, instruments, and techniques (e.g., drill ships, observatories, biogeophysics, molecular sciences and computational modeling) has been adapted to sample the subsurface biosphere and interrogate its life. We anticipate that further research will close knowledge gaps that deal with the role of viruses, whether evolution can occur without cell growth, and the nature of cell dormancy in the subsurface. The importance of “latent” redox systems that are too subtle to measure directly and the “rare” biosphere to sustain functional resilience in subsurface communities are yet to be fully examined. And we also expect new studies to clarify how subsurface microbes may influence processes of concern at the Earth’s surface such as how this life may contribute to the “slow” and “fast” carbon cycles.