NET PHOTOSYNTHESIS ACTIVATION OF A DESICCATED CYANOBACTERIUM WITHOUT LIQUID WATER IN HIGH AIR HUMIDITY ALONE - EXPERIMENTS WITH MICROCOLEUS-SOCIATUS ISOLATED FROM A DESERT SOIL CRUST

1. In previous studies it was shown that, in contrast to green algal l ichens and aerophilic green algae, dry cyanobacterial lichens and some free-living cyanobacteria which were also investigated were not able to reactivate photosynthetic metabolism solely by hydration in equilib rium with high air humidity. They needed liquid water to attain positi ve net photosynthesis. The same proved to be true for the cyanobacteri um Microcoleus sociatus in a soil crust of the Negev Desert when it wa s studied in its natural summer state of development. 2. In contrast, desiccated populations of the same organism in laboratory cultures att ained turgor and were able to photosynthesize in equilibrium with a re lative air humidity as low as 96% (15 degrees C). They reached about 3 0% of the maximum net photosynthesis without liquid water in almost wa ter-saturated air. Rates of photosynthesis were similar and no hystere sis effects occurred, when equilibrium in water potential with high ai r humidity was approached from the dry or from the wetted state. 3. Th is study shows that water vapour activation of net photosynthesis can not be attributed to green algae and to green algal lichens alone. Thi s capacity might be also important for productivity of desert soil cru sts which are formed by cyanobacteria.