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.

a b s t r a c tThe predicted Exigobacterium sibiricum bacterirhodopsin gene was amplified from an ancient Siberian permafrost sample. The protein bacteriorhodopsin from Exiguobacterium sibiricum (ESR) encoded by this gene was expressed in Escherichia coli membrane. ESR bound all-trans-retinal and displayed an absorbance maximum at 534 nm without dark adaptation. The ESR photocycle is characterized by fast formation of an M intermediate and the presence of a significant amount of an O intermediate. Proteoliposomes with ESR incorporated transport protons in an outward direction leading to medium acidification. Proton uptake at the cytoplasmic surface of these organelles precedes proton release and coincides with M decay/O rise of the ESR.

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54 strains of viable green algae and 26 strains of viable cyanobacteria were recovered from 128 and 56 samples collected from Siberian and Antarctic permafrost, respectively, with ages from modern to a few million years old. Although species of unicellular green algae belonged to Chlorococcales were subdominant inside permafrost, green algae Pedinomonas sp. were observed in Antarctic permafrost. Filamentous cyanobacteria of Oscillatoriales, Nostocales were just found in Siberian permafrost. Algal biomass in the permanently frozen sediments, expressed as concentration of chlorophyll a, was 0.06–0.46 μg g−1. The number of viable algal cells varied between <102 and 9×103 cfu g−1, but the number of viable bacterial cells was usually higher from 102 to 9.2×105 cfu g−1. Frozen but viable permafrost algae have preserved their morphological characteristics and photosynthetic apparatus in the dark permafrost. In the laboratory, they restored their photosynthetic activity, growth and development in favourable conditions at positive temperatures and with the availability of water and light. The discovery of ancient viable algae within permafrost reflects their ability to tolerate long-term freezing. In this study, the tolerance of algae and cyanobacteria to freezing, thawing and freezing–drying stresses was evaluated by short-term (days to months) low-temperature experiments. Results indicate that viable permafrost microorganisms demonstrate resistance to such stresses. Apart from their ecological importance, the bacterial and algal species found in permafrost have become the focus for novel biotechnology, as well as being considered proxies for possible life forms on cryogenic extraterrestrial bodies.

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