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Microbial crusts are present on surfaces of soils throughout the world. A key feature of these crusts in arid zones is the abundance of filamentous sheath-forming and polysaccharide-excreting cyanobacteria. Several isolates of cyanobacteria were prepared from crust samples (Nizzana sand dunes, north-western Negev Desert, Israel). Optimal growth conditions for two such isolates of Microcoleus sp. were defined, and the role of the excreted polysaccharides in affecting the hydrological properties of crust-covered sand dunes was studied. Experiments with the native crust microbial population demonstrated the possibility of net primary productivity at both high relative air humidities and low moisture content.