Defending photosynthesis from photoinhibition and reactive oxygen species (ROS); how does a desert alga thrive at extremely high-light intensities?

Light drives most life on earth as photosynthetic organisms convert it to chemical energy. However, too much light causes photodamage that may lead to photoinhibition (PI). Photodamage happens when photosynthetic electrons/energy are transferred from the excited chlorophyll molecule to oxygen. Those excited oxygen molecules are called ROS and they tend to damage the photosynthetic complexes and particularly the D1 of photosystem II. Living in the desert, the microalgae Chlorella ohadii exhibits exceptional resistance to extreme high light intensity. Here, we analyzed the ROS formation in C. ohadii grown at a high light intensity where other photosynthetic organisms cannot survive. Examining the O₂^- and H₂O₂ formation revealed that O₂^- formation was about 50% in HL-adapted cells in comparison to low light (LL) grown cells, while H₂O₂ displayed no difference. Similar differences were obtained with thylakoids of HL and LL-grown cells. Next, purified PSI, PSII, and LHCII photosynthetic complexes of HL and LL-grown cells were analyzed for ROS accumulations. The results revealed 80% less accumulation of O₂^- in HL-PSII. H₂O₂ accumulated 50% in PSI and LHCII of HL compared to isolated complexes from LL cells. These results implied that ROS formation is significantly reduced when grown in HL, a condition where massive amounts of protective carotenoids accumulate in the thylakoids. Together with additional PI protection mechanisms that evolved in C. ohadii (Levin et al Plant J. 2021) the enormous quenching of ROS accumulation enables this alga to thrive at light intensities where others cannot survive