The deep biosphere; background, news, and implications

During the last two decades increasing evidence for the existence of a deep biosphere in the terrestrial and marine subsurface environments has been reported. The detection of microbial populations or traces of microbial life in sediment deposits, formation water from oil wells, groundwater and hydrothermal fluids from continental springs and deep-sea vents, and continental and oceanic crustal rocks suggests that the subsurface biosphere is extensive with a biomass potentially similar to that of the surface biosphere. The wide range of geophysical and geochemical conditions in the subsurface environment opens for a large microbial diversity. The microbial activity may, on the other hand, have major influence on subsurface geochemical processes important for the chemical exchange between the lithosphere and the hydrosphere. While the predominately biogenic energy sources present in sediments give rise to a variety of chemoorganotrophic organisms, microbial life in crustal rocks where organic compounds are scarce may be based on chemolithotrophic organisms retrieving energy from reduced inorganic compounds from the lithosphere, such as hydrogen, reduced metals (e.g. iron/manganese) and sulfur compounds, while carbon dioxide serve as the main carbon source. This implies that our planet may harbour two biospheres, one that is sun driven and one that is earth driven. The continuing exploration of these extreme environments constantly expands the magnitude and conditions of the deep biosphere. Thus, the record of active microbial life in marine sediments was lately extended from 842m to a depth of 1626m. Similarly, evidence for microbial growth in a deep-sea hydrothermal venting area at significant higher temperatures than the currently known limit to life has recently also been reported. Documentation of microorganisms in altered seafloor and subseafloor basaltic and ultramafic rocks and fluids indicates moreover that water-rock reactions in the oceanic lithosphere support microbial life, even if it is uncertain to what extend these communities may be linked to the surface biosphere. The deep biosphere has clear implications for geochemical processes involving groundwater quality, carbon cycling and climate, and other global geochemical cycles. The biotechnological potential may be high, since many of the subsurface microbes are unknown and live in extreme environments defining the limits of life. The deep biosphere also raises fundamental questions concerning the origin and evolution of life, and the possibility for life on other planets.