Taxonomic composition and photosynthetic characteristics of the "biological soil crusts' covering sand dunes in the western Negev Desert

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. 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. Maximal photosynthesis was still possible with a crust water content equivalent to precipitation of 0.2-0.3 mm, but was suppressed <0.1 mm. Dew and fog inhibition would 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 >20% of desert shrubs. In the short term, carbon input into the ecosystem through the soil crusts can become substantial. -from Authors