Water Deficiency

All reviews on effects of freezing on microbial cells (Mazur 1980; Kushner 1981; Vorobyova et al. 1997) put the main emphasis on the state of water inside and immediately outside the cells. The formation of intracellular ice crystals is considered a critical

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o factor affecting survival of frozen cells, and cold-resistance is normally attributed to intracellular antifreeze compounds preventing the formation of crystals.

Some liquid water exists in soils at temperatures below freezing (Ershov 1998). The thickness of such quasi-liquid water film was calculated to be ca. 50 nm (Anderson 1967). Such thin water film could cover only a fraction of cells or form an external unfrozen water shell around the bacterial cell, but cannot provide continuous water channels to move around the icy space. Wolfe et al. (2002) used several reasonable assumptions about the geometry of surface and thermodynamic variables to derive the following simple equation relating unfrozen water content (UW) to freezing temperature (AT):

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UW = 3x 1018 ln-moleculesm-2 = 5x 10-6 ln-molesm-2. (9.1)

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This equation agreed well with experimental data (Romanovsky and Osterkamp 2000) on unfrozen water content in Sagwon site (Fig. 9.7). We plotted on the same figure the temperature-dependent content of water vapor over ice, assuming that some permafrost microorganisms could acquire water from the gas phase through aquaporins; these specialized water-transporting channels in membrane play an important role in microbial freeze-resistance (Tanghe et al. 2006). The content of unfrozen water declines abruptly just below the freezing point and then decreases slowly with further cooling, while humidity (water vapor content) displays uniform decline within the entire temperature range above and below the freezing point.

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