Actavia 0 MATTILA, H. HAVURINNE, V. ANTAL, T. TYYSTJÄRVI, E. Evaluation of visible-light wavelengths that reduce or oxidize the plastoquinone pool in green algae with the activated F<sub>0</sub> rise method 2022 2022-12-21 529-538 We recently developed a chlorophyll a fluorescence method (activated F<sub>0</sub> rise) for estimating if a light wavelength preferably excites PSI or PSII in plants. Here, the method was tested in green microalgae: Scenedesmus quadricauda, Scenedesmus ecornis, Scenedesmus fuscus, Chlamydomonas reinhardtii, Chlorella sorokiniana, and Ettlia oleoabundans. The Scenedesmus species displayed a plant-like action spectra of F<sub>0</sub> rise, suggesting that PSII/PSI absorption ratio is conserved from higher plants to green algae. F<sub>0</sub> rise was weak in a strain of C. reinhardtii, C. sorokiniana, and E. oleoabundans. Interestingly, another C. reinhardtii strain exhibited a strong F<sub>0</sub> rise. The result indicates that the same illumination can lead to different redox states of the plastoquinone pool in different algae. Flavodiiron activity enhanced the F<sub>0</sub> rise, presumably by oxidizing the plastoquinone pool during pre-illumination. The activity of plastid terminal oxidase, in turn, diminished the F<sub>0</sub> rise, but to a small degree. 4 60