Green algal species live as components of microbiotic crusts in many deserts of the world, enduring extremes of temperature, water availability, and, perhaps, light. Using a phylogenetic approach, we are investigating components of the photosynthetic physiology of Southwestern US desert green algae and aquatic sister species from three classes of green algae: the Chlorophyceae, Trebouxiophyceae, and Charophyceae. Because these desert algae are similar morphologically, sequence data from the 18S ribosomal RNA gene is required for identification of related pairs of aquatic and desert species. We hypothesize that the desert-living habit, which has evolved at least six times in the green algae, is associated with enhanced photoprotective mechanisms compared to photosynthetic physiology found in the aquatic habit. To date, analysis of chlorophyll fluorescence quenching in desert and aquatic sister species reveals that at least within the Chlorophyceae and Trebouxiophyceae, desert algae exhibit altered patterns of non-photochemical quenching (qNP) in response to light relative to their aquatic relatives. Such changes in qNP are indicative of altered energy dissipation mechanisms associated with the photosynthetic apparatus, and may prove to be a physiological feature common to desert algal lineages.

Key words: chlorophyll fluorescence, crust, desert, green algae, photosynthesis, phylogeny