Light, Temperature, and Desiccation Effects on Photosynthetic Activity, and Drought-Induced Ultrastructural Changes in the Green Alga Klebsormidium dissectum (Streptophyta) from a High Alpine Soil Crust

Members of the cosmopolitan green algal genus Klebsormidium (Klebsormidiales, Streptophyta) are typical components of terrestrial microbiotic communities such as biological soil crusts, which have many important ecological functions. In the present study, Klebsormidium dissectum (Gay) Ettl & Gartner was isolated from a high alpine soil crust in the Tyrolean Alps, Austria. Physiological performance in terms of growth and photosynthesis was investigated under different controlled abiotic conditions and compared with ultrastructural changes under the treatments applied. K. dissectum showed very low light requirements as reflected in growth patterns and photosynthetic efficiency. Increasing temperatures from 5A degrees C to 40A degrees C led to different effects on respiratory oxygen consumption and photosynthetic oxygen evolution. While at low temperatures (5-10A degrees C), respiration was not detectable or on a very low level, photosynthesis was relatively high, Reversely, at the highest temperature, respiration was unaffected, and photosynthesis strongly inhibited pointing to strong differences in temperature sensitivity between both physiological processes. Although photosynthetic performance of K. dissectum was strongly affected under short-term desiccation and recovered only partly after rehydration, this species was capable to survive even 3 weeks at 5% relative air humidity. K. dissectum cells have a cell width of 5.6 A +/- 0.3 mu m and a cell length of 8.4 A +/- 2.0 mu m. Desiccated cells showed a strongly reduced cell width (46% of control) and cell length (65% of control). In addition, in desiccated cells, fewer mitochondria were stained by DIOC6, and damaged plasma membranes were detected by FM 1-43 staining. High-pressure freeze fixation as well as chemical fixation allowed visualizing ultrastructural changes caused by desiccation. In such cells, the nucleus and chloroplast were still visibly intact, but the extremely thin cell walls (75-180 nm) were substantially deformed. The cytoplasm appeared electron dense and mitochondria were altered. Although K. dissectum can be characterized as euryoecious species, all ecophysiological and ultrastructural data indicate susceptibility to desiccation. However, the steadily occurring fragmentation of filaments into smaller units leads to improved self protection and thus may represent a life strategy to better survive longer periods of drought in exposed alpine soil crusts.