Comparative photophysiology of green algae isolated from desert microbiotic crusts and their close aquatic relatives

Background/Question/Methods

A key step in the evolution and diversification of advanced life was a transition from aquatic to terrestrial habitats.  Green plants are the largest lineage of photosynthetic eukaryotes to have made this transition, and the familiar embryophytes (higher plants, mosses, ferns) are the most extensively studied single lineage of terrestrial green plants. A broader understanding of the diversity of ways organisms in the green plant clade cope with terrestrial environmental stressors requires examination of multiple evolutionary lineages that have independently made the shift from water to land. We are working with diverse green algae isolated from desert crusts in the western U.S. that represent five such evolutionarily independent lineages that transitioned from water to land. These five desert lineages fall into four green algal classes, the Chlorophyceae, Trebouxiophyceae, Klebsormidiophyceae, and Zygnemophyceae. For these desert-derived algae (and paired close aquatic relatives), we used room temperature chlorophyll fluorescence to analyze induction of NPQ (non-photochemical quenching, as a measure of photoprotection) during exposure of dark-adapted algae to increasing levels of high light. We also analyzed chlorophyll fluorescence spectra obtained at 77K to explore state transitions associated with several hours high light exposure after dark adaptation. 

Results/Conclusions

We found that only a subset of the ten focal desert and aquatic algae exhibited the increase in 735nm/685nm fluorescence emission at 77K that would be expected based on higher green plant behavior upon state transitions toward state II in high light. We also found that room temperature fluorescence traces from the algae exhibited a variety of behaviors departing from those expected from higher plants, including, for example, that chlorophyll fluorescence during a saturating light flash was not maximal after overnight dark adaptation. Instead, for several taxa, maximal fluorescence during a saturating flash was observed after exposure to very low intensity red light, or during dark recovery after several hours of light exposure. We discuss these data within a phylogenetic context, considering adaptation of the desert algae to the terrestrial habitat.