Photosynthetic recovery following desiccation of desert green algae (Chlorophyta) and their aquatic relatives

Recent molecular data suggest that desert green algae have evolved from freshwater ancestors at least 14 times in three major classes (Chlorophyceae, Trebouxiophyceae and Charophyceae), offering a unique opportunity to study the adaptation of photosynthetic organisms to life on land in a comparative phylogenetic framework. We examined the photorecovery of phylogenetically matched desert and aquatic algae after desiccation in darkness and under illumination. Desert algae survived desiccation for at least 4 weeks when dried in darkness, and recovered high levels of photosynthetic quantum yield within 1 h of rehydration in darkness. However, when 4 weeks of desiccation was accompanied by illumination, three of six desert taxa lost their ability to recover quantum yield during rehydration in the dark. Aquatic algae, in contrast, recovered very little during dark rehydration following even just 24 h of desiccation. Re-illuminating rehydrated algae produced a nearly complete recovery of quantum yield in all desert and two of five aquatic taxa. These contrasts provide physiological evidence that desert green algae possess mechanisms for photosynthetic recovery after desiccation distinct from those in aquatic relatives, corroborating molecular evidence that they are not happenstance, short-term visitors from aquatic environments. Photosensitivity during desiccation among desert algae further suggests that they may reside in protected microsites within crusts, and species specificity of photosensitivity suggests that disturbances physically disrupting crusts could lead to shifts or losses of taxonomic diversity within these habitats.