Effect of exogenous extracellular polysaccharides on the desiccation and freezing tolerance of rock-inhabiting phototrophic microorganisms.

Two major stresses that threaten rock-inhabiting microbial communities are desiccation and freezing; both result in a loss of liquid water in the cells. The mechanisms necessary to tolerate these extremes may be similar, but are not well understood. In both cases extracellular polysaccharides (EPS) seem to play an important role. This study examines whether the EPS released by a rock-inhabiting phototroph can have a protective effect on other members of similar and neighboring microbial communities. This interaction was modeled by adding EPS isolated from the cryptoendolithic cyanobacterium Nostoc sp. to cells of the cryptoendolithic green alga Chlorella sp. and to cells of the epilithic cyanobacterium Chroococcidiopsis sp. The cells were then subjected to desiccation and freezing and the survival rates were determined by vital staining, using membrane integrity as a measure of viability. The results clearly demonstrate the importance of exogenous EPS in the desiccation tolerance of both species, while mixed results were found for the freezing trials.