Diurnal-Rhythmic Relationships between Physiological Parameters and Photosynthesis- and Antioxidant-Enzyme Genes Expression in the Raphidophyte Complex.

Diurnal rhythms in physiological functions contribute to homeostasis in many organisms. Although relationships between molecular biology and diurnal rhythms have been well studied in model organisms like higher plants, those in harmful algal bloom species are poorly understood. Here we measured several physiological parameters and the expression patterns of photosynthesis-related and antioxidant-enzyme genes in the complex to understand the biological meaning of diurnal rhythm. Under a light-dark cycle, Fv/Fm and expression of , , and showed significant increases in the light and decreases during the dark. These rhythms remained even under continuous dark conditions. DCMU suppressed the induction of , , and expression under both light regimes. Oxidative stress levels and HO scavenging activities were relatively stable, and there was no significant correlation between HO scavenging activities and antioxidant-enzyme gene expression. These results indicate that the complex has developed mechanisms for efficient photosynthetic energy production in the light. Our results showed that this species has a diurnal rhythm and a biological clock. These phenomena are thought to contribute to the efficiency of physiological activities centered on photosynthesis and cell growth related to the diurnal vertical movement of this species.