Photoprotection in Diverse, Desiccation-Tolerant, Desert Green Algae and Their Close Aquatic Relatives

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The objective of this research is to determine the process(es) underlying the mechanism(s) protecting the photosynthetic apparatus of desert green algae during desiccation and rehydration. A key step in the evolution of life on Earth was the move from living in water to surviving on land. Organisms making that leap faced high light levels, extreme temperatures, and desiccation. Flowering plants, conifers, mosses, and ferns are all descended from a single green algal ancestor that successfully invaded land. Traits they share may be essential for surviving on land, or they might simply be traits inherited from the common ancestor. To tease apart these two possibilities, other green, photosynthetic, terrestrial organisms must be studied that are not descended from that common green algal ancestor. Such organisms are found among diverse, free-living, microscopic green algae inhabiting microbiotic crusts of the desert Southwestern U.S. Desert green algae survive desiccation much better than their aquatic algal relatives, and desert algae recover from desiccation very rapidly (within minutes) when rehydrated. Thus, they make an ideal system to study basic mechanisms of desiccation tolerance. The traits land plants developed to protect themselves from intense sunlight during drought, high heat, and cold are particularly important as agriculture expands into marginal lands while climate also shifts. Desert green algae share many characteristics with larger agriculturally and ecologically important green plants, and also with the green algae increasingly used for biofuel production. Research integration with education and training will include development of 6-8th grade Lesson Plans, available to teachers leading excursions at Cape Cod National Seashore focusing on "Surprising Microbes of the Dunes". Research opportunities for undergraduates will be focused on projects studying desiccation stress and photobiology in green plants and microbiotic crusts.The overall research objective is to determine the process(es) underlying (and phylogenetic distribution of) rapidly-reversible mechanism(s) protecting the photosynthetic apparatus during desiccation and rehydration, in paired desert and aquatic green algae within four algal classes. Broad phylogenetic distribution of photoprotective or desiccation-tolerance mechanisms across desert members of multiple green algal classes will suggest (through parsimony) that these mechanisms are likely ancestral traits in the larger green plant clade. Comparison of traits in desert and closely-related aquatic relatives will identify lineage-specific characteristics. Research will establish whether the capacity for desiccation tolerance and desiccation-induced photoprotection has a persistent genetic component (even when algae are grown hydrated), and whether influences of light and speed of desiccation on the efficiency of photoprotection are similar to their effects in lichens and mosses. Mechanisms that have been recently proposed to underlie desiccation-induced photoprotection (PSII to PSI spillover, P700+ oxidation, and chlorophyll-chlorophyll charge transfer states) will be tested in the lichen Parmelia sulcata and desiccation-tolerant green algae using fluorescence decay kinetics and modeling, EPR, and linear and circular dichroism spectroscopy. Whole transcriptome shotgun sequencing will be used to test whether, as in bryophytes, the transcriptome in the representative desert-derived green alga Scenedesmus rotundus possesses constititive cellular protection against desiccation.