Biological soil crusts as unique microecosystem represent a suitable model system to address taxonomy and cryptic diversity of microalgal key players

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On a global scale, biological soil crusts (BSC) are the most productive microbial biomass in arid and other extreme regions with various microalgal taxa as key components. BSCs are formed by different living organisms and their by-products, creating a microecosystem that exerts ecologically important, multi-functional roles in primary production, nitrogen fixation, mineralization, water retention and stabilization of soils. The systematics of microalgae is currently changing from a morphological to a molecular phylogenetic species concept. The morphological concept received a lot of criticism because the underlying organisms often lack distinct morphological characters. Therefore, both systems disagree in taxonomical classification and species delimitation: a fundamental problem that needs to be addressed. The microalgal genera Coccomyxa and Stichococcus can be found in BSCs all over the world, indicating ecological key functions. However, their taxonomy is still not resolved, which hamper, for example, any biodiversity estimation. The revision of Coccomyxa and Stichococcus addresses two points of the overarching goals within the SPP 1991 Taxon-Omics: I. discovery and delimitation of species; II. speeding up the naming process and generating identification tools, classifications, species inventories, or monographs. A polyphasic approach based on molecular data analyzed by modern algorithms and supported by morphological, ecophysiological and biochemical characteristics will result in a robust taxonomy as well as in the discovery and definition of new species. Therefore, existing strains from culture collections will be ordered and new strains of the respective genera will be isolated from BSCs along a gradient of coastal dunes at the Baltic Sea. Where missing, the highly variable ITS region and the chloroplast marker rbcL will be sequenced. Based on these data phylogenetic trees will be calculated and modern algorithms (like GYMC and PTP) for species delimitation will be applied. The results will be supported by chemotaxonomic markers (low molecular weight carbohydrates), habitat information, ecophysiological and morphological characteristics. This polyphasic approach will give strong evidence for a taxonomic revision of Coccomyxa and Stichococcus, thereby speeding up the naming process by defining species limits; new species will be described and, if necessary, morphological species merged. For future studies a barcode tool based on next-generation sequencing will be developed that allows a rapid and precise biodiversity estimation of the two important key organisms in BSCs: Coccomyxa and Stichococcus.