Tolerance To Drying And Freezing Stresses In Cyanobacteria, Nostoc Commune And Synechocystis Sp.PCC6803

Tolerance to and effects of the freezing stress in a desiccation-tolerant, terrestrial cyanobacterium, Nostoc commune, and in a desiccation-sensitive species, Synechocystis sp. PCC6803, were studied by measuring their photosynthetic activities and fluorescence emission spectra. The desiccation-tolerant species was more tolerant to the freezing stress than the desiccation-sensitive one, supporting the idea that tolerance to freezing stress is related to resistance to drying stress. Under freezing conditions, N. commune deactivated the photosystem (PS) II activity, which may protect the cells from photoinactivation. Light energy absorbed by PSII complexes was also dissipated to heat energy. Under freezing conditions, N. commune suffered cellular dehydration due to ice formation outside the cell. But still relatively high amounts of water remained in a liquid state inside the cells at -36# when the TV. commune colonies were almost fully wetted before freezing. High PSI activities at freezing temperatures are consistent with these results. Besides, 5% methanol enhanced the resistance to freezing stress in the sensitive species, and this effect seems to be related to maintenance of the PSI activity and energy transfer between pigment protein complexes by methanol.