Members of the lichen photobiont genus Trebouxia show species-specific photo-physiological and transcriptome-level responses to high light.

Members of the common lichen photobiont Trebouxia occur in all terrestrial habitats, from the arctic to the tropics, however, the mechanisms of environmental stress tolerance in Trebouxia are little understood. Currently, lineages belonging to this genus are grouped into clades A, C, I, S, D. Here we study six species, which belong to the S-Clade and A-Clade of Trebouxia, and were isolated from the lichen-forming fungi Umbilicaria pustulata and U. phaea. Three of the Trebouxia species have a climatic niche preference at low elevation (Mediterranean climate), two at high elevation (cold temperate climate), and one is found in both climate zones. These species have demonstrated extensive genomic divergence, particularly in genome regions associated with photosynthesis. Therefore, we hypothesize that they will exhibit differential performance under varying light conditions. We assessed physiological and transcriptomic responses to high light (HL) (control: 60 µmol photons/m2/s; HL:150 µmol photons/m2/s) using a controlled environmental chamber. We examined the cultures´ responses after 1 hour and 3 days (12 hours per day) of HL exposure. We measured photo-physiological parameters including maximum quantum yield (Fv/Fm), non-photochemical quenching (NPQ) and chlorophyll a (chl a) concentration in combination with differential gene expression analysis via RNASeq. Average levels of Fv/Fm and NPQ showed significant reduction following HL exposure, however, this varied among species. Species from high elevation (i.e. Trebouxia S12 C0006 and A10 C0009) exhibited relatively high NPQ capacity throughout the experiment. There was no significant change in average chl a concentration. Further, only a few differentially expressed genes (DEGs) were found for specific species following exposure to 1 hour HL, including those associated with chloroplast thylakoid membrane, transposon TX1 and photorespiration. On the other hand, there are more DEGs found for all Trebouxia species exposed to prolonged HL, which involved genes associated to DNA biosynthetic process, cell cycle and cell wall organization. Photoprotection-associated genes related to NPQ, photosystem II repair, oxygen evolving assembly and biosynthesis of photoprotective pigments (carotenoid and chl) also showed differential expression due to prolonged HL. 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. We do not observe parallel patterns in species preferring similar climatic niches, with the exception of species from high elevation (Trebouxia S12 C0006 and A10 C0009), which exhibit generally high NPQ capacity.