Is the photo-acclimatory response of Rhodophyta conditioned by the species carotenoid profile?

The success of Rhodophyta in coastal areas is largely determined by their capacity to withstand high-light conditions. To understand the photo-acclimatory response of Rhodophyta, we compared differences in pigmentation, light absorption, and changes in the chlorophyll a fluorescence signal between apical and basal thalli of Eucheuma isiforme, a lutein-predominant species, and Gracilaria damaecornis, a zeaxanthin-predominant species. Variation in the fluorescence signal was characterized after exposure of basal and apical segments of each species to five saturating light levels, and monitoring of the subsequent recovery in the absence and presence of chloramphenicol, an inhibitor of the photosystem II (PSII) repair mechanism. Our results confirm that Rhodophyta is a highly diverse algal group with respect to antenna composition and photoprotective mechanisms. The permanent activation of an energy quencher in the zeaxanthin-predominant species requires switching to antheraxanthin in the low-light response and higher dependence on PSII repair. No such quencher was detected in the lutein-predominant species, which showed no change in relative carotenoid composition, slight differences between apical and basal thalli in PSII pressure, and less dependence on PSII repair. It remains unclear if the combined presence of lutein-zeaxanthin allows effective photoprotection or if other mechanisms (i.e., phycobilisome quenching) may explain the effective photoprotective response shown by E. isiforme. The differences highlighted in this comparison may contribute to explain the distribution and abundance of Rhodophyta in marine coastal areas through understanding differences within the group in their ability to protect the photosynthetic apparatus from absorbing light in excess, and in the metabolic costs of PSII repair.