Extracellular polysaccharide of Nostoc commune (cyanobacteria) inhibits fusion of membrane vesicles during desiccation

Cells of the cyanobacterium Nostoc commune secrete a complex, high molecular weight, extracellular polysaccharide (EPS) which accumulates to more than 60% of the dry weight of colonies, The EPS was purified from the clonal isolate N. commune DRH1. The midpoint of the membrane phase transition (T-m) of desiccated cells of N. commune CHEN was low (T-m dry = 8 degrees C) and was comparable to the T-m of rehydrated cells ((T-m)(wet) = 6 degrees C). The EPS was not responsible for the depression of T-m. However, the EPS, at low concentrations, inhibited specifically the fusion of phosphatidylcholine membrane vesicles when they were dried in vitro at 0% relative humidity (-400 MPa). Low concentrations of a trehalose:sucrose mixture, in a molar ratio which corresponded with that present in cells in vivo, together with small amounts of the EPS, were efficient in preventing leakage of carboxyfloroscein (CF) from membrane vesicles. Freeze-fracture electron microscopy resolved complex changes in the structure of the EPS and the outer membrane in response to rehydration of desiccated cells. The capacity of the EPS to prevent membrane fusion, the maintenance of a low T-m dry in desiccated cells, and the changes in rheological properties of the EPS in response to water availability, constitute what are likely important mechanisms for desiccation tolerance in this cyanobacterium.