Permafrost as a microbial habitat: extreme for the Earth, favorable in space

Cold adapted viable paleomicroorganisms, the only known organisms, survive `in situ' within permafrost at subzero temperatures over geological time and upon thawing resume the activities. Because the cryosphere existence is a common phenomenon in space, the terrestrial permafrost environment inhabited by microbes and their metabolically end products can be considered as a model of conditions of the other planets. Ancient microbial communities within the Earth permafrost provide a range of analogies for possible extraterrestrial ecosystems, which might be found at permafrost depths on other planets. The main econiches where the microorganisms may survive are the unfrozen water films enveloping soil particles and functions as a cryoprotector. This type of rigidly associated `liquid' water has not been sublimated and may present in extraterrestrial cryosphere as a life indicator.