Effect of photoperiod on photosynthetic characteristics of two marine diatoms

Abstract

Photoadaptation to changes in photoperiod was studied in the marine diatoms Skeletonema costatum (Grev.) Cleve and Phaeodactylum tricomutum Bohlin. The algae were grown under cycling light of 2 h : 2 h and 12 h : 12 h light-dark periods. The daily C assimilation · cell −1 was enhanced by a factor 1.5 in 2 : 2 light regime. In cells grown under 12 : 12 h light regime, the pigment content, the number of PSI, PSUO 2 size, and to a lesser extent, the photosystem II-antenna size were modulated by the light-dark alternations, whereas these characteristics were almost constant over the circadian cycle in 2 : 2 cells. The modifications in the cellular content of photosynthetic pigments were correlated with no change in the relative pigment composition of the photosynthetic units. In S. costatum, the changes in pigment content corresponded to an increase of the ratio PSI : PSII which is emphasized at least by a factor 2.12 and to modifications of the photosystem-antenna sizes (increase of 28% for PSII and decrease of 40% for PSI) when the cells are transferred from 2 : 2 to 12 : 12 light regime (light period). The responses of diatoms to a change of the photoperiod is different according to the species. Indeed, P. tricomutum did not show the same variations in cellular contents of pigments and PSI; moreover, the photosystem-antenna sizes were weakly affected by a change in the photoperiod. The modifications observed in S. costatum, inducing variations of PSI : PSII ratio, were related to the existence of an active cyclic photophosphorylation around PSI in diatoms which modulate the ATP : NADPH 2 ratio in response to the requirements of the metabolic processes.