Naturally occurring physical and chemical constraints of life and the biosphere

Deep-sea and deep-subsurface environments have been recognized to be among the most extreme biotopes potentially placed very close to an interface between the habitable and the uninhabitable terrains for life on Earth. The concept of habitability appears difficult to define, particularly in terms of an astrobiological perspective. Nevertheless, it is widely accepted that the harshest habitats for life, such as deep-sea and deep-subsurface environments in this ‘highly habitable’ planet, the Earth, may be approximated to the most plausible environments for extraterrestrial life in some ‘hardly habitable’ planets and moons of our Solar System. Thus, to understand the limits of life and the biosphere in the deep-sea and deep-subsurface environments of the Earth could be a key for elucidating the potential habitability of extraterrestrial life in the Universe. In this chapter, the possible factors that limit life and the biosphere on the Earth are overviewed and discussed from insights gained from the recent biogeochemical and geomicrobiological explorations in the deep-sea and deep-subsurface biosphere.

In the deep-sea and deep-subsurface environments many physical and chemical parameters limiting the activities of microbial life have been elucidated. The best example is temperature. In the terrestrial and oceanic surface environments, liquid water boils at around 100°C, while with an increasing pressure (hydrostatic), liquid water can be present at up to 373°C for pure water and 407°C for seawater (critical points) (Bischoff and Rosenbauer,1988).