Members of the lichen photobiont genus <i>Trebouxia</i> show species-specific photophysiological and transcriptome-level responses to high light

Members of the common lichen photobiont genus Trebouxia occur from the arctic to tropical terrestrial habitats however, the mechanisms of environmental stress tolerance in Trebouxia are little understood. Here we studied six species, which belong to the S-clade and A-clade, and were isolated from the lichen-forming fungi Umbilicaria pustulata and U. phaea. These species have demonstrated extensive genomic divergence, particularly in genome regions associated with photosynthesis. Therefore, we hypothesized that they will exhibit differential performance under varying light conditions. We assessed their physiological and transcriptomic responses to short and prolonged exposure to high light (HL). Average levels of Fv/Fm and NPQ showed significant reduction following HL exposure, but this varied among species. Further, only a few differentially expressed genes (DEGs) were found for specific species following exposure to 1 h of HL. On the other hand, there are more DEGs found for those exposed to prolonged HL, particularly photoprotection-associated genes related to NPQ, photosystem II repair, oxygen evolving assembly and biosynthesis of photoprotective pigments. Overall, our findings show that in Trebouxia, the capacity to withstand high light conditions is highly species-specific, and not driven by phylogenetic relatedness or climatic niche preference.