Photosystems I and II in Ulva lactuca are well protected from high incident sunlight

Abstract We investigated the individual as well as combined performance of photosystem II and photosystem I in the ubiquitous green macroalga Ulva lactuca under diurnally changing light conditions using a dual pulse-amplitude modulated chlorophyll fluorescence technique. Along with rising levels of sunlight, the effective and maximal quantum yields of photosystem II decreased with increasing non-photochemical quenching. With declining irradiances in the afternoon, these photosystem II characteristics recovered gradually, such that they reached the same levels in the evening as in the morning without any signs of photodamage during the day. The values of non-photochemical quenching were higher in the afternoon than during pre-noon at equivalent irradiances, indicating the existence of energy-dissipating cycles that lessened energy transfer from photosystem II to photosystem I. The effective quantum yield of photosystem I decreased only slightly during the daytime and recovered fully in the afternoon, showing that photosystem I was also well protected from photodamage. These results were complemented by analyses of short-term fluorescence induction OKJIP-transients of Kautsky curves, showing 1) that the photosystem II electron donor side was kept fully active throughout the day, 2) that there was no change in the activity of the photosystem II acceptor side, 3) highly efficient electron transport through photosystem I, and 4) an enhanced cyclic electron flow around photosystem I with increasing irradiances. Such non-destructive photoprotective capabilities may in part explain the successful adaptation of the genus Ulva to a wide range of irradiance conditions locally as well as globally.