Biotic crusts are communities of microorganisms such as algae, fungi, and bacteria, that together with lichens and bryophytes, aggregate soil in semi-arid and arid habitats world-wide. A survey of biotic crust algae from several biotic crust sites in western North America is currently underway. One aspect of this project is to assess the diversity of green algae from the soil samples, using both morphological and molecular data. At present, there are over 200 isolates of green algae in hand. Based on morphological, the isolates have been provisionally assigned to genera in three green algal classes: the Charophyceae, Chlorophyceae, and Trebouxiophyceae. A small proportion of the isolates have been sequenced thus far. Phylogenetic analyses using the molecular data can be used to demonstrate that the phylogenetic diversity of the desert green algae is very high. In addition, a significant proportion of the isolates are not closely related to published sequences of other green algae. In each of the Classes, there appear to be distinct clades of desert algae. Most interesting is the association of clades of desert algae to aquatic sister taxa. These pairs of taxa offer an interesting comparison between aquatic and desert taxa with regard to physiological adaptations to desert-living. A project to explore possible physiological adaptations is underway.

Key words: 18S rDNA, biotic crusts, Chlorophyta, desert