However, not all habitable environments on Earth are equally suitable for the support of such complex ecosystems. A great variation in abiotic conditions is found with respect to factors such as temperature, pH, water availability, pressure, etc. Compared to the kind of environments described above, a hot spring with a temperature above 80o C and a pH below 2 may surely be considered an environment hostile to life. The same is true for a soda lake saturated with salt, in which the availability of water is strongly reduced, and in addition the pH of 10-11 or higher severely stresses any organism that attempts to colonize it. There are many different kinds of such “extreme environments”. Some are geographically restricted, such as the black smoker deep-sea hydrothermal vents (presenting a combination of high temperature and high hydrostatic
pressure), or acidic hot springs. Others are far more abundant, such as, the deep-sea
environment with its low temperature (1-4o
C) and its several hundreds of atmospheres
of hydrostatic pressure. Another case is the hot subsurface environment whose
exploration has started only a few years ago, and which now appears to be inhabited by
a variety of heat- and pressure-tolerant microorganisms.