Photoprotective responses in the green alga Chlamydomonas reinhardtii

In this thesis, we focus on two main components of non-photochemical quenching: NPQ-qE and ST. The work was carried out using the green alga C. reinhardtii and four common vascular plants: Arabidopsis thaliana, Nicotiana tabacum (tobacco), Hordeum vulgare (barley) and Zea mays (maize). In Chapter 2, we compared the photosynthetic traits and photoprotective capacity of seven C. reinhardtii wild-type strains (CC-124, CC-125, CC-1009, CC-1690, CC-1691, CC-4532, and CC-4533) during their exposure to high light. Combining functional and biochemical methods, we demonstrated the strain-specific differences in their acclimation to high light. The analysis shows variations in NPQ-qE levels, ST capacity and the composition of the photosynthetic apparatus. Interestingly, no correlation between NPQ and LHCSR3, the key player in NPQ, was observed among the strains, and ST appeared to function independently of light intensity. These findings highlight the critical role of strain-specific traits in shaping photoprotective responses and emphasize the need for careful selection of laboratory strains in photosynthesis research. In Chapter 3, we investigated the role of NPQ-qE and state transitions in C. reinhardtii under a day-night cycle. We could show that LHCSR proteins were consistently present, enabling the immediate activation of NPQ throughout. State transitions remained fully functional and PSBS level was stable suggesting a more significant role in photoprotection than previously recognized. In Chapter 4, we investigated the role of the antenna complex LHCBM1 in NPQ using the npq5 mutant of C. reinhardtii, which lacks this complex The mutant expressed LHCSR3 at the wild-type level, but fails to activate NPQ. Our results demonstrate that LHCSR3 and LHCBM1 must be both present for NPQ, with LHCBM1 likely connecting LHCSR3 to the PSII supercomplex. In Chapter 5, we investigated whether anoxia, a method used to induce state II in C. reinhardtii, can also trigger state transitions in vascular plants. Analyzing the phosphorylation of Lhcb1 and Lhcb2 in several plant species, we found that anoxygenic conditions induced changes in PSII antenna size. Functional measurements revealed that N₂ combined with far-red light (N₂/FR) effectively induced the maximum state transition capacity in Nicotiana tabacum and Arabidopsis thaliana. These findings establish N₂/FR as a viable alternative method for studying state transitions in vascular plants.