The deep subsurface biosphere has been regarded as an emerging topic in geo-bioscience and industry for the past few decades, and has been approached by terrestrial and seafloor drillings. Terrestrial sites have better proximity and greater relevance to the anthroposphere and technosphere, i.e., human habitats and societies, than do seafloor sites. Therefore, understanding the subterranean biosphere has more direct importance to issues related to a sustainable civilization, and issues such as formation/maturation of hydrocarbon reservoirs and ore deposits, disposal of radioactive wastes and carbon dioxide, and postulated association between seismogenic and microbial activities. Microbiological studies in the terrestrial deep subsurface have been prompted to respond to such human-related issues, and microbial life in sedimentary and crystalline rocks as well as pore-filling fluids has been studied to evaluate rock stability and (im) mobilization of redox-sensitive elements/nuclides, for instance. This is in contrast to subseafloor microbiology, which focuses more on microbial interactions with hydrothermal circulation, relevant biogeochemical processes including gas hydrate formation, associated diversity of life, and modern analogs of origin-of-life.
Avoiding man-induced contamination of cored samples and pumped fluids has been a microbiological issue. Technical (both instrumental and operational) measures to minimize contamination were first developed in subterranean microbiology, because of easier accesses to test sites for repetition, evaluation, improvement, etc. of attempted measures on land. Then, anti-contamination expertise was introduced into subseafloor practices, and anti-contamination protocols and facilities are now better developed by the Integrated Ocean Drilling Program (IODP) than the International Continental Scientific Drilling Program (ICDP). Newly developed techniques are also applied to measure/monitor geological and geochemical parameters that are used to characterize microbial habitats and processes occurring there.
Lessons from subterranean microbiology are directly applicable to subglacial microbiology that may retrieve microbial life from sub-million-year-old ice cores, although additional measures are needed for glacier drilling. Because land and icy surfaces are common in Earth-like planets or potentially life-bearing satellites, lessons (experiences and expertise) from subterranean microbiology should be applicable to astrobiological searches for extraterrestrial life.
