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In this paper we demonstrate to what extent lichens are able to profit from snow as a water source for rehydration under the extreme environmental conditions in Continental Antarctica. Net photosynthesis of the foliose macrolichen Umbilicaria aprina occurred at subzero temperatures in the field even when snow was the only water source. Laboratory experiments revealed that in dry thalli of U. aprina, metabolic activity was initiated during rehydration from snow at subzero temperatures. The degree of rehydration of dry thalli of U. aprina from snow at subzero temperatures was strictly dependent on the temperature, with the final water content decreasing with decreasing temperature. In the crustose epilithic lichen Buellia frigida overheating of the rock surface and the black thallus by strong insolution leads to snow melt and therefore to an activation of photosynthetic activity by liquid water at a small-scale. This phenomenon occurred even if air temperature was down to -10 degrees C. It was demonstrated that activation of different thalli depended on the micro-topography of the rock surface. The possible influence of global climatic changes on water availability and the consequences for lichens in polar desert ecosystems are discussed.