| Summary: | Filamentous cyanobacteria are the key organisms in biological soil crust formation in all biomes of the world. However, especially in temperate, arctic, and high alpine regions, as well as in few dry Savannah ecosystems, filamentous green algae may act in a similar role. Here, we give an overview on the role, diversity, and biogeography of cyanobacteria and eukaryotic algae in biocrusts from all climatic regions and continents of the Earth. We refer to the species level wherever this is possible. Currently, there have been 320 species of cyanobacteria and more than 350 species of eukaryotic algae described from biocrusts. Despite this high diversity, only a minority of the cyanobacterial and algal species found is responsible for the bulk of biocrust formation. Others likely are opportunistic, utilizing the habitat created by biocrusts in the harsh regions of the Earth where habitable space is rare. We also discuss methods for the sampling and identification of biocrust algae and cyanobacteria.
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