The subsurface biosphere represents one of the final frontiers on Earth and may provide a model for how life can survive on other planets. While focusing on terrestrial subsurface and depths of over 3,000 meters underground, we present and apply a variety of computational tools and techniques for exploring the deep biosphere. Life at this depth is scarce and nutrients are often limited to hydrogen, sulfate, and single carbon compounds such as methane, carbon monoxide and carbon dioxide. Metagenomics and other sequencing techniques shed light onto how subsurface microorganisms transform these limited nutrients into energy and survive underground.
When appropriate, such methods are combined with geochemical measurements and thermodynamic predictions to provide the most accurate picture of life underground. In that direction, we find that the South African subsurface fluids exhibit a spectrum of redox conditions (influenced by the origin and age of the subsurface fluids) that directly affect the microbial community composition and function. Although these large-scale community shifts are believed to occur over long periods underground, we also present evidence for an adaptive methane oxidizing community that responds to changes in geochemistry over relatively short periods. Combined, these results provide a picture of how microbial communities function in the terrestrial subsurface as well as a theoretical framework for understanding the selective pressures these organisms face.