Nostoc

Abstract: The cyanobacterium Nostoc commune is adapted to the terrestrial environment and has a cosmopolitan distribution. In this study, the role of extracellular polysaccharides (EPS) in the desiccation tolerance of photosynthesis in N. commune was examined. Although photosynthetic O2 evolution was not detected in desiccated colonies, the ability of the cells to evolve O2 rapidly recovered after rehydration. The air-dried colonies contained approximately 10% (wt/wt) water, and field-isolated, natural colonies with EPS were highly water absorbent and were rapidly hydrated by atmospheric moisture. The cells embedded in EPS in Nostoc colonies were highly desiccation tolerant, and O2 evolution was not damaged by air drying. Although N. commune was determined to be a mesophilic cyanobacterium, the cells with EPS were heat tolerant in a desiccated state. EPS could be removed from cells by homogenizing colonies with a blender and filtering with coarse filter paper. This treatment to remove EPS did not damage Nostoc cells or their ability to evolve O2, but O2 evolution was significantly damaged by desiccation treatment of the EPS-depleted cells. Similar to the EPS-depleted cells, the laboratory culture strain KU002 had only small amount of EPS and was highly sensitive to desiccation. In the EPS-depleted cells, O2 evolution was also sensitive to freeze-thaw treatment. These results strongly suggest that EPS of N. commune is crucial for the stress tolerance of photosynthesis during desiccation and during freezing and thawing.

Abstract: Drying of cells leads to damage resulting from crowding of cytoplasmic components, condensation of the nucleoid, increases in the Tm of membrane phase transitions, and imposition of stress upon cell walls. Prolonged desiccation leads to oxidation of proteins, DNA and membrane components through metal-dependent Fenton reactions, while Maillard reactions generate cross-linked products between the carbonyl groups of reducing sugars and the primary amines of nucleic acids and proteins. Although such damage restricts many organisms to aqueous environments, some, including many cyanobacteria, can tolerate the air-dried state for prolonged periods. Cyanobacteria in the Tintenstrich communities of exposed rock faces, Microcoleus and Lyngbya spp. in intertidal mats, chasmoendolithic Chroococcidiopsis spp. in the rocks of hot and cold deserts, and terrestrial epilithic crusts of Tolypothrix and Nostoc are examples that show a marked capacity to withstand the removal of their cellular water. For Nostoc commune, the ...