A surprising diversity of unicellular green algae have evolved to live in desert microbiotic crusts of the southwestern U.S., serving as part of the crust community that influences soil erosion, rain infiltration, nitrogen fixation, and seed germination in arid landscapes. Desert green algae are derived from multiple, independent adaptations to land by taxa from aquatic groups, providing a phylogenetically broad system in which to examine traits correlated with the habitat shift to the desert lifestyle.  One dramatic environmental shift associated with terrestrial life is a modification not only in the amount of sunlight but also in the wavelengths of incident radiation, which has important consequences for photosynthetic energy capture.  In terrestrial higher plants, the partitioning of energy between photosystems I and II, which absorb radiation at slightly different wavelengths, is associated with the stacking of thylakoid membranes into grana.

Results/Conclusions

Using a phylogenetically paired set of algal taxa from desert and aquatic environments, we have found that the degree of grana stacking does not appear to be related to the desert lifestyle (McManus et al. 2005), and that species-specific patterns of grana stacking have dramatic impacts on photosynthetic function.  Previous work in higher plants (e.g. Goss et al. 2007) has suggested that grana stacking is also important for the dissipation of excess excitation energy by the xanthophyll cycle (detected as non-photochemical quenching (NPQ) using chlorophyll fluorescence methods).  We find that in all our desert and aquatic species pairs, the xanthophyll cycle pigments are present, and conversion of violaxanthin to zeaxanthin proceeds under high light.  Relaxation kinetics of NPQ during darkness following high light exposure show both rapidly relaxing and slowly (tens of minutes) relaxing components, though many of the algae do not exhibit thylakoid stacking into grana.  For algae examined to date, treatment with dithiothreitol prior to high light exposure, expected to decrease conversion of violaxanthin to zeaxanthin, depressed NPQ under high light, suggesting that the xanthophyll cycle may contribute to energy dissipation under high light despite minimal grana stacking.