The osmotic potential and effects of plasmolysis on photosynthetic oxygen evolution and chlorophyll fluorescence were studied in two Arctic Zygnema sp. (strain B, strain G) and two Antarctic Zygnema sp. (strain E, strain D). Antarctic strain D was newly characterized by rbcL sequence analysis in the present study. The two Antarctic strains, D and E, are most closely related and may represent different isolates of the same species, in contrast, strain B and G are separate lineages. Incipient plasmolysis in the cells was determined by light microscopy after incubating cells in sorbitol solutions ranging between 200 mM and 1000 mM sorbitol for 3, 6 and 24h. In Zygnema strain B and G incipient plasmolysis occurred at ~600 mM sorbitol solution (720 mOsmol kg(-1), ψ=-1.67 MPa) and in strains D and E at ~300 mM (318 mOsmol kg(-1), ψ=-0.8 MPa) sorbitol solution. Hechtian strands were visualized in all plasmolysed cells, which is particularly interesting, as these cells lack pores or plasmodesmata. Ultrastructural changes upon osmotic stress were a retraction of the condensed cytoplasm from the cell walls, damages to chloroplast and mitochondrial membranes, increasing numbers of plastoglobules in the chloroplasts and membrane enclosed particles in the extraplasmatic space. Maximum photosynthetic rates (P(max)) in light saturated range were between 145.5 μmol O(2) h(-1)mg(-1)Chl a in Zygnema G and 752.9 μmol O(2) h(-1)mg(-1)Chl a in Zygnema E. After incubation in 800 mM sorbitol for 3h P(max) decreased to the following percentage of the initial values: B: 16.3%, D: 16.8%, E: 26.1% and G: 35.0%. Osmotic stress (800 mM sorbitol) decreased maximum photochemical quantum yield of photosystem II (F(v)/F(m)) when compared to controls. Maximum values of relative electron transport rates of photosystem II (rETR(max)) decreased after incubation in 400 mM sorbitol in Zygnema D and E, while they decreased in Zygnema B and G only after incubation in 800 mM sorbitol. The kinetics of the rETR curves were similar for the Arctic strains Zygnema B and G, but distinct from the Antarctic strains Zygnema D and E, which were similar when compared with each other. This suggests that the investigated Arctic Zygnema sp. strains might be better adapted to tolerate osmotic water stress than the investigated strains from the Antarctic.