Antarctic permafrost soils have not received as much geocryological and biological study as has been devoted to the ice sheet, though the permafrost is more stable and older and inhabited by more microbes. This makes these soils potentially more informative and a more significant microbial repository than ice sheets. Due to the stability of the subsurface physicochemical regime, Antarctic permafrost is not an extreme environment but a balanced natural one. Up to 10(4) viable cells/g, whose age presumably corresponds to the longevity of the permanently frozen state of the sediments, have been isolated from Antarctic permafrost. Along with the microbes, metabolic by-products are preserved. This presumed natural cryopreservation makes it possible to observe what may be the oldest microbial communities on Earth. Here, we describe the Antarctic permafrost habitat and biodiversity and provide a model for martian ecosystems.

Deep subterranean layers may be regarded as the most stable environment for microorganisms where possible fluctuations should be explained by geological events only. The analysis of the total amount of microorganisms has revealed that in sedimentary deposits their number is only one order of magnitude lower than the same parameter in soil. Taking into account the depth of sediments the microbial biomass in subterranean rocks has to be considerably larger than that in soils. Permafrost is the most constant and stable environment among deep habitats. Microbial communities survive in permafrost for at least some millions of years. The diversity of organisms and of microbial activities after permafrost thawing displays distinct differences to those in soils. The abundance of the bacterial biomass assumed is comparable in frozen and unfrozen sediments. Therefore, the permanently low temperature in permafrost is a stabilizing factor that sustains life in deep cold biotopes. Studies of microbial communities in permafrost sediments of different lithology and age suggest that the level of subzero temperature and the duration of its influence define the ratio between the hypometabolic cells, readily reversible to proliferation, and the so‐called viable but non‐culturable cells (deep resting cells). To a certain extent, cell aggregates in the extracellular matrix may be regarded as an additional survival mechanism supporting the hypometabolic state of cells. There is indirect evidence for adaptive physiological and biochemical processes in microorganisms during the long‐term impact of cold.

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In permanently frozen rocks, water occurs in all the three phases and plays a dual role from the biological point of view. About 93-98% of it is in the solid state. This, alongside with negative temperatures, contributes to cell cryoconservation. The remaining 2-7% is in the unfrozen state and represents thin films enveloping organic-mineral particles. These films play the role of cryoprotectors against cell damage by ice crystals during geologically significant time. Electron microscope examinations of prokaryotes revealed the well preserved outer cell structures, specifically strong envelopes and capsules. The cells are resistant to water phase transitions through 0 degrees C, i.e. to the freezing-thawing stress. The exobiological implication of this phenomenon is determined by the fact that the Earth permafrost at first approximation can he considered as a model of e.g. the Mars one. The latter protects the cells against radiation and simultaneously serves as a cryoconservant. However, most important is the possible presence of unfrozen (= liquid) water as prerequisite for the development of microbial life forms.

The structure of individual cells in microbial populations in situ of the Arctic and Antarctic permafrost was studied by scanning and transmission electron microscopy methods and compared with that of cyst-like resting forms generated under special conditions by the non-spore-forming bacteria Arthrobacter and Micrococcus isolated from the permafrost. Electron microscopy examination of microorganisms in situ revealed several types of bacterial cells having no signs of damage, including "dwarf" curved forms similar to nanoforms. Intact bacterial cells in situ and frozen cultures of the permafrost isolates differed from vegetative cells by thickened cell walls, the altered structure of cytoplasm, and the compact nucleoid, and were similar in these features to cyst-like resting forms of non-spore-forming "permafrost" bacterial strains of Arthrobacter and Micrococcus spp. Cyst-like cells, being resistant to adverse external factors, are regarded as being responsible for survival of the non-spore-formers under prolonged exposure to subzero temperatures and can be a target to search for living microorganisms in natural environments both on the Earth and on extraterrestrial bodies.