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Geobiosphere and Early Life


The biosphere is the part of planet Earth inhabited by life. It occurs in three overlapping zones: the lithosphere, comprising soils, sediments and sedimentary rocks; the hydrosphere, that part of the lithosphere either covered by water or containing water (within pores and fractures); and the atmosphere, the gaseous envelope surrounding the Earth. Most organisms rely on photosynthesis for their existence, either directly (primary producers) or indirectly via the food chain, and hence the greatest biomass inhabits the surface/near-surface lithosphere and shallow hydrosphere. Microorganisms make up a major component of the Earth's metabolism. Aerobic bacteria use free oxygen to degrade and metabolise labile organic substrates, but where oxygen cannot penetrate, for instance through fine grained sediments, anaerobic bacteria take over degradation using other terminal electron acceptors (oxidisiers) such as sulphate, nitrate, manganese iron and carbon dioxide. Anaerobes are the dominant inhabitants of the lithosphere. Their abundance in sedimentary basins shows a general decrease in increasing depth, eventually petering out as organic matter becomes too recalcitrant to be degraded or because water, nutrients and terminal electron acceptors cannot be supplied or because temperatures are too high.

It has been estimated that 90% of the prokaryotic (bacterial and archaeal) cells in Earth's biosphere exist in marine and terrestrial subsurface environments (Whitman et al. 1998). It has been posited that this Deep Biosphere harbours a greater biomass than the mass of all the living cells, prokaryotic and eukaryotic, in the surface regions of the biosphere (Onstott et al. 1999). These estimates of the extent of the Deep Biosphere have been made by projecting and extrapolating from data collected from a very limited number of boreholes in marine (for a review, see Parkes et al. 2000) and terrestrial (Hazen et al. 1991; Pedersen 1993, 2000) environments. In truth, we have only very limited data on which to base these estimates. The lower depth limit of the biosphere has not been reached in any borehole studies that have included a microbiological component, and the factors that control the abundance and activities of microbes at depth and the lower depth limit of life are still poorly understood. While the marine regions of the Deep Biosphere are now being systematically probed by the Integrated Ocean Drilling Program, the terrestrial Deep Biosphere is receiving somewhat less attention, and this is where ICDP must play the leading role.

 (by Brian Horsfield & Tom Kieft, 2005)