We recently developed a chlorophyll a fluorescence method (activated F₀ 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 F₀ rise, suggesting that PSII/PSI absorption ratio is conserved from higher plants to green algae. F₀ rise was weak in a strain of C. reinhardtii, C. sorokiniana, and E. oleoabundans. Interestingly, another C. reinhardtii strain exhibited a strong F₀ 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 F₀ rise, presumably by oxidizing the plastoquinone pool during pre-illumination. The activity of plastid terminal oxidase, in turn, diminished the F₀ rise, but to a small degree