Permafrost is defined as a subsurface frozen layer, primarily soil or rock, which remains frozen for more than 2 years. The age of permafrost ranges from a few thousand years up to 2-3 million years and even older in Antarctica. Permafrost makes up more than 20% of the land surface of the Earth, including 82% of Alaska, 50% of Russia and Canada, 20% of China, and most of the surface of Antarctica (Harris 1986; Williams and Smith 1989). Permafrost underlies the glaciers and soils of polar and alpine regions. Permafrost soils contain about 20-70% of ice and 1-7% of unfrozen water in the form of salt solutions with low water activity (aw = 0.85) (Gilichinsky et al. 1993). Since life depends upon liquid water, permafrost is one of the most extreme environments on the Earth. In addition, permafrost is characterised by constant negative temperature, inaccessibility of nutrient supplies, and complete darkness. It is surprising to discover photoautotrophic microorganisms which need to use light energy to drive their metabolic reactions within permafrost sediments. Because of the difficulty of studying permafrost in an undisturbed form, interactions among the organisms that live in it are not yet well understood. Arctic Permafrost

The study sites have been located on Kolyma lowland, Northeast Russia (67-70°N, 152-162°E). The Arctic permafrost represents an anaerobic oligotrophic environment with a mean annual temperature of -10°C, redox potential Eh = +40 to -250), and an organic carbon content in the range 0.05-7% (Gilichinsky 2002). Nitrogen in form of NH4+, NO2-, or NO3- was determined (Janssen and Bock 1994). A total of 293 permafrost samples differentiated in lithology, genesis, and physico-chemical properties were screened for the presence of photosynthetic microorganisms. The distances between boreholes ranged from 50 to 300 km. The deepest sample was from a depth of 61 m, and the oldest sample was 3 million years old. The permafrost samples were hydrocarbonate-calcium fresh composition with a low salinity and neutral pH, and of marine origin with significantly higher salinity and dominance of ions Na+ and Cl-. Antarctic Permafrost

The study areas have been located in the McMurdo Dry Valleys of Southern Victoria Land, Antarctica (77-78°S, 160-163°E). The temperature of the Antarctic permafrost varies from -18.5°C (Taylor Valley) through -24°C (Beacon Valley) to -27°C (Mt. Feather) (Gilichinsky et al. 2007b). The Antarctic permafrost is of fresh-water genesis, with the alkaline pH, low clay content and organic matter often close to zero (0.05-0.25%) (Wilson et al. 1996). Since the Antarctic permafrost has a low buffering capacity, the soil pH is sensitive to the total accumulation of soil salts (Campbell and Claridge 1987). A total of 56 permafrost samples were analysed for presence of cyanobacteria and green algae. The Antarctic samples were not so anaerobic (redox potential Eh = + 260 to + 480), and the gaseous phase contained oxygen, nitrogen, methane, carbon dioxide, etc. (Rivkina and Gilichinsky 1996; Wilson et al. 1996).

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