Intact Bryopsis chloroplasts were exposed to light without oxygen, leading to a decrease in chlorophyll fluorescence and damage to Photosystems I and II. The activity of Photosystem I decreased as active P700 disappeared, while the reaction center of Photosystem II was destroyed similarly to a specific chemical reaction. The initial step in photoinhibition is the destruction of Photosystems I and II, turning them into quenchers of chlorophyll fluorescence. Inhibitors of electron transfer and an electron donor to Photosystem I affected the photoinhibition process, showing a connection to Photosystem I activity.