Though only about 1 percent of available organic carbon reaches the seafloor, it has been estimate that upwards of 60% of all prokaryotic cells on earth live in ocean sediments (Whiticar et al. 1998).  They are found wherever there are sediments, from continental margins to the deep ocean, though the nature of the communities can change with location (D'Hondt et al. 2002).  The existence of the communities has been inferred for some time, but new discoveries continue to extend the depths to which the communities exist and expand our knowledge of their biosphere and its interactions with the ocean and ocean sediments.

            From at least the 1970s, the existence of microorganisms living in the sea floor sediments had been inferred from the release of sulfate and methane gas from oceanic sediment cores (Parkes et al. 2000).  It was assumed that the gases were the products on anaerobic breaking down of organic carbon being buried in the sediment.  It was conjectured that aerobic respiration dominated in the top few millimeters to meter of sediment and methanogenesis and sulfate reduction (Joergenson 1982).  Sulfate reduction was thought to be the final step of the oxidation of organic matter in the sediments (Joergenson 1982).  Since then, our knowledge of the processes and organisms involved in the oxidation of organic carbon has greatly increased.  From known data, methanogenesis is the final step in the oxidation of organic carbon (Parkes et al. 2000).  However, it now appears, that the location of the sediment, whether in the open ocean or along the continental margins, has a significant affect on whether sulfate reduction or methanogenesis is the dominant energy provider for the community.  The depth to which living cells have been recovered has extended to 800 plus meters (Parkes et al. 2000, Schippers et al. 2005), with extrapolated populations reaching even further into the sediment.

            This large expansion of the biosphere has serious implications for nutrient cycling in the ocean and the global carbon budget.  The scientific community is also forced to re-evaluate other possible sources of energy in areas formerly overlooked.