In this review, experimental data on microbial activity in permafrost and other frozen media are summarized. By a deeply rooted and fair tradition, all manifestations of life are intimately associated with the presence of free water. The search for extraterrestrial life is ultimately associated with spotting of large aquatic reservoirs on other planets, "rivers" or "oceans" being the most probable loci accommodating life. In terrestrial studies of permafrost and other cold habitats a similar trend absolutely dominates, with the primary objective of detecting any form of liquid water: brine solutions, vein water in ice, unfrozen water in permafrost. Psychrophilic microorganisms are grown in supercooled liquid media containing high concentration of antifreezes.
The main message of the author is that macroscopically discerned liquid water is not an absolute prerequisite for microbial metabolic activity below the freezing point. Cultivation of psychrophilic microorganisms can be successfully done by using solid frozen media like frozen powder or thin films which allow gas exchange and provide solid support for slowly growing cells. The cryogenic planets in the Solar system could also have spots of biological activity outside extensive bodies of liquid water. More important seem to be continuous-delivery energy sources, such as flux of volatile compounds combined with the presence of adequate electron acceptors.
Permafrost and frozen tundra soils can no longer be considered as a depository of dormant organisms. Adequate conditions for life functions are provided by non-zero gas permeability, the presence of unfrozen water and a supply of mobile oxidizea-ble compounds. Contrary to sea ice, which has a relatively simple and "young" microbial community with easily domesticated members, the permafrost community is more complex, containing active and dormant populations, culturable and uncul-turable species with unknown growth requirements. Probably, fungi including mycelial organisms and dimorphic yeasts are more resistant to hostile permafrost environment and display more vigorous growth in frozen habitats than bacteria.
Acknowledgements This research was supported by the NSF grant MCB-0348681. The author thanks Dr. V. Romanovsky for permafrost sampling. Drs. J. Fell, J.P. Sampaio, N. Ivanushkina and S.M. Ozerskaya provided valuable assistance in preliminary identification of isolated fungi and yeasts.
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