Exploration of the marine subsurface biosphere

The Ocean Drilling Program (ODP) has provided a unique glimpse of another living world - the buried biosphere of deep-sea sediments and crust. Porewater geochemical studies of sediments recovered by ODP have shown that microbial activity occurs beneath the seafloor throughout much of the world. Direct counts of microbial cells in ODP samples have shown microbes to be ubiquitous in the subsurface realm of deep-sea sediments (Cragg et al., 1990; Thierstein and Störrlein, 1991; Parkes et al., 2000). Biogeochemical and microbiological studies have pushed the estimated maximum burial depth of the subseafloor sedimentary biosphere to more than 800 meters below seafloor (mbsf). The results of ODP studies suggest that subseafloor microbes may constitute a large and biogeochemically important portion of Earthly life. Extrapolation from direct cell counts of ODP samples suggests that the subsurface biosphere of seafloor sediments may constitute as much as one-third of the living biomass on Earth (Whitman et al., 1998). The activities of this subsurface biosphere affect long-term global biogeochemical cycles by mediating the diagenesis of marine sediments (Claypool and Kaplan, 1974) and the weathering of oceanic basalt (Fisk et al., 1998). Under certain conditions, the subsurface microbes may also occasionally affect the surface Earth on much shorter time scales. Intermittent release of methane (CH4) from marine sedimentary hydrates may have greatly affected the global climate and/or oceans at multiple times in Earth history (Dickens, 2001)