Life in the Deep Sub-Seafloor Biosphere

Abstract

The interest in the origin of life on earth and the prospects of life on other planets have accelerated in recent years and have gained great public awareness. Whereas life has been explored and found everywhere on the surface of our own planet, the deep subsurface was until recently considered to be influenced only by abiotic processes. The discovery of micro-organisms in several million year old sedimentary deposits and even in basement rock has profoundly changed our perspective of the limits of living organisms. It is now clear that processes in the geosphere may provide the driving force for life and that, vice versa, the sub-surface biosphere has a large impact on present geological processes. The emerging fluids of cold seeps in subduction zones along continental margins are chemically modified by deep bacteria, just like the fluids of hydro-thermal vents along mid-oceanic ridges are modified by geo-thermal processes. Both fluid flows have a major influence on modern ocean chemistry. The slow biological degradation of organic carbon in the deep sub-surface and the formation of huge quantities of methane have a large-scale impact on sea-floor processes, such as the accumulation of gashydrates or the formation of carbonate mounds.