To assess the mechanisms of photoprotection in T. lutea, two clonal strains with different basal pigments composition were studied. One synthesized echinenone, while the other did not but showed a high amount of diadinoxanthin and diatoxanthin.

We investigated the photosynthetic response of these two clonal strains in turbidostat, at different growth culture irradiances from 50 to 550 μmol photons m2 s−1. To this end, variable chlorophyll a fluorescence, pigment composition and transcription level of specific genes were monitored. In addition to the genes coding for the Fucoxanthin Chlorophyll a, c binding Protein (FCP), we followed the expression of several putative genes coding for the diadinoxanthin de-epoxidase, the violaxanthin de-epoxidase and the zeaxanthin epoxidase enzymes. It was the first time that these genes were characterized in T. lutea.

Both clonal strains decreased their photosynthetic pigments with increasing irradiance. Nevertheless, the two clonal strains had different photoprotection strategies illustrated with the extent of the dissipation of excess light energy. It was accompanied by the synthesis of photoprotective pigments to different extents: T-5cl6 increased its pool of diadinoxanthin-diatoxanthin with increasing irradiance, while T 4cl3 preferentially synthesized echinenone above a certain level of irradiance. These diverging phenotypes were correlated with variations in the expression of lhcx, lhcr, and of the putative genes of the enzymes involved in the xanthophyll pigment cycles.