Diurnal Regulation of Flagellar Length and Swimming Speed in the Red-Tide Raphidophyte Chattonella marina

The raphidophyte Chattonella marina is a harmful algal bloom (HAB) species known for its distinct diurnal vertical migration (DVM), a behavior important for its survival and bloom formation. However, the single-cell mechanisms governing this migration remain unclear. In this study, we investigated the swimming characteristics of individual C. marina cells during day (light) and night (dark) periods. We observed a strong positive correlation between the length of the propulsive anterior flagellum and the cell's swimming speed. We discovered that the length distribution of the anterior flagellum is different during the day and at night. We also found that the beat frequency of the anterior flagellum was significantly higher during the day compared to the night. This resulted in faster mean swimming speeds during the light period. To investigate the mechanism of length regulation, we tested the role of intraflagellar transport (IFT) using the IFT dynein inhibitor, ciliobrevin D. Treatment with ciliobrevin D induced a time- and concentration-dependent shortening of the anterior flagellum. This is the first pharmacological evidence to suggest that an IFT-like mechanism may actively control motile flagellar length in C. marina. These findings suggest that C. marina modulates its swimming speed through diurnal changes in both flagellar length and beat frequency, likely as an energy-saving strategy coupled to its DVM.