Diverse photoprotection dynamics in desert-dwelling algae and their aquatic relatives

Background/Question/Methods

Microbiotic crusts occupy soil surfaces in deserts throughout the world, and the diverse community of microorganisms present in crusts includes a number of species of free-living, single-celled green algae. These algae represent numerous independent evolutionary transitions to a terrestrial habitat, and thus present a natural study system for the photosynthetic adaptations necessary for life on land. Photosynthesis in a desert environment requires tolerance of desiccation, high temperatures and high light intensities. Tolerance of radiation stress could be accomplished in a variety of ways. High-light-adapted species might have a larger capacity for non-photochemical quenching (NPQ), increasing energy dissipation and reducing photosynthetic yield dramatically in response to light; or conversely, such species might possess alternate photoprotective mechanisms, and be able to withstand relatively high light intensities without reducing photosynthetic yield.

Results/Conclusions

We analyzed photosynthetic efficiency and response to varying light exposure in a suite of 9 algal species isolated from crusts in the desert southwest, and in 10 closely related aquatic species. Photoprotection dynamics varied by lineage; for example, within the class Chlorophyceae, traits associated with a desert habitat varied between three distinct clades containing desert and aquatic members. Two desert species from the Bracteacoccus genus, which has no aquatic members, exhibited higher NPQ levels when exposed to high light than their close aquatic relatives, while desert Scenedesmus and Chlorosarcinopsis species demonstrated lower NPQ development than their aquatic relatives at these same high light levels. In all four desert species, however, some quenching was active in the dark, moreso than in aquatic relatives, suggesting that a quenched state is to some extent the default for these desert taxa.