Variation in the photosynthetic activities of the dinoflagellate Akashiwo sanguinea during formation of resting cysts

Abstract

Resting cysts represent the dormant life stage produced by sexual reproduction of dinoflagellates; it generally maintain a limited level of metabolic activities. In the present study, changes in the photosynthetic apparatus of Akashiwo sanguinea during the encystment process in different nitrogen (N) conditions were characterized, and the transcriptional responses of nine photosynthesis genes were further evaluated. The photochemical efficiency decreased with elevation of N deprivation and growth of algae, and the resting cysts maintained limited photosynthetic activity. Meanwhile, the transcription of photosystem II genes (psbA and psbD) were down-regulated, especially the levels of the psbA gene, suggesting the inactivation of photosystem II under N limitation. Moreover, genes involved in cytochrome f (petA) and ATP synthase (atpA, atpB) were also transcribed at low levels in N-deprivation cultures. In contrast, the transcription of the RubisCO large subunit gene (rbcL) was up-regulated, indicating the maintenance of carbon fixation in N-limited conditions. The transcription levels of photosystem I genes (psaA and psaB) showed no significant correlation with the N condition. Further, most of the photosynthesis genes were down-regulated in the resting cysts, which was coincided with the inactive physiological status of the dormant stage. The results demonstrated that photosynthesis genes are differentially regulated in response to varying N conditions and life cycles. The findings will provide fundamental knowledge for better understanding the physiological characteristics and molecular regulation mechanisms in the developmental life cycle of marine dinoflagellates.