Dehydration and freezing resistance of lichenized fungi

Abstract

The poikilohydrous nature of lichens provides them the ability to resist low temperature, deep dehydration and deficit in light irradiance. The process of water uptake can be correlated with the resistance for dehydration below water percolation threshold (fractal exponent characteristic for approximately two-dimensional lattice). Gaseous phase hydration kinetics presents tightly bound water and mobile loosely bound water fractions, differentiated in hydration/dehydration rate and in proximity to thallus surfaces. Some lichens have the ability to hydrate from snow to the level sufficient for activation of photosynthesis. Freezing tolerance of lichens combines resistance to low temperature and duration of freezing. Antarctic lichens reveal the lowest temperature of net photosynthesis. In number of species, water supercools;however, some lichen taxa tolerate ice nucleation activity (INA) for temperatures at which the active photosynthesis process still occurs. A number of factors contribute the lichen freezing resistance. The decrease in hydration rate of freezing loosely bound water pool; active transfer of freezing, loosely bound water pool to non-freezing one; presence of tightly bound nonfreezing water fraction; and non-cooperative immobilization of supercooled bound water are important factors to the lichen freezing resistance together in the, at higher hydration levels INA for supercooled bound water, diffusion-induced migration of supercooled water molecules to ice microcrystallites (which is not phase growth in thermodynamic system but is implied by the structure). Freezing resistance and dehydration resistance direct lichens for testing Panspermia theory. Several endolithic lichens may survive the space conditions; however, their survival during re-entry from the space was not yet demonstrated.