TAXONOMIC COMPOSITION AND PHOTOSYNTHETIC CHARACTERISTICS OF THE BIOLOGICAL SOIL CRUSTS COVERING SAND DUNES IN THE WESTERN NEGEV DESERT

1. We report laboratory studies that aim to characterize the microphyte community composition and metabolic features of crusts covering dunes in the Negev. 2. The crusts are formed by at least one moss, four blue-green (Cyanobacteria) and two green (Chlorophyta) algal species with Microcoleus sociatus being dominant. The sheaths of the latter procaryotic alga, together with a contribution by moss rhizoides and protonemata, are responsible for stability of the topsoil crusts. 3. Following moistening of the dry crust, CO2 release took place, even in the light, until positive net photosynthesis was achieved. This delay was mainly due to the high CO2 buffer capacity of the soil solution. In moist crusts, growth of the microphytes soon took place, and CO2 assimilation increased continuously. Net photosynthesis seemed to be adapted to relatively low light and temperature conditions. 4. Maximal photosynthesis was till possible with a crust water content equivalent to precipitation of 0-2-0.3 mm, but was suppressed at less than 0.1 mm. Dew and fog inhibition would be expected to allow photosynthetic activity and growth. Chlorophyll-related maximal rates of carbon gain were of similar magnitude to leaves of arido-active phanerogamous Negev plants. Area-related maximal rates reach more than 20% of desert shrubs. Thus, in the short term, carbon input into the ecosystem through the soil crusts can become substantial.