Green algae are present in desert soils as components of microbiotic or cryptobiotic crust communities, along with lichens, cyanobacteria and other prokaryotes, non-lichenized fungi, and diatoms. Microbiotic crusts can be found in arid and semi-arid regions world-wide, on a large number of different soil types. Microbiotic crusts are ecologically important, playing a role in nutrient cycling and soil stabilization. There has been great interest in understanding the ecological function of and taxonomic composition of crust communities. The green algae that occur in crusts represent a diverse assemblage of taxa spanning three classes of algae, the Chlorophyceae, Trebouxiophyceae, and Charophyceae. They are genetically diverse, but are morphologically simple unicells or packets of cells. In an ongoing study of the diversity of microbiotic green algae from thirty localities in western North America, a large number of green algal taxa have been found, many of which represent new species and new genera. Some genera, such as Bracteacoccus, are heavily represented. Phylogenetic analyses using ribosomal RNA gene sequence data have been a powerful tool in understanding the diversity and evolution of desert crust green algae. These studies indicate that microbiotic crust green algae evolved from aquatic green algae at least five independent times. In addition, desert green algae are always found to be derived from freshwater, not marine green algae. Some lineages of green algae have a high proportion of desert taxa, while other lineages of green algae have no known desert representatives. Ongoing studies are addressing the ways in which desert green algae are adapted to living in these conditions, including adaptations to high light levels.

Key words: Chlorophyta, desert, diversity, microbiotic crusts, ribosomal rDNA