Cyanobacteria, often associated with algae, lichens and fungi form biological soil crusts. In this subproject, the diversity, structure and function of those crusts will be examined. Their community structure as the result of pattern formation shall be analysed in detail. The importance of large- and small-scale abiotic and biotic factors for pattern formation, i.e. macro-, meso- and microclimate, substrate, inter- and intraspecific competition, phycobiont selection and fungus-lichen interactions will be examined. In co-operation with the other subprojects, the effects of biological soil crusts on the diversity of other groups of organisms will be studied. Another main focus of this project will be to analyse the effects of biological crusts on the carbon-cycle. Since up to now the sink of c. 25% of the annual CO2 production is unknown, it is expected, that biological crusts on soil and rock, which cover wide areas in arid and semiarid ecosystems, will fill a major part of this knowledge gap.