Diurnal vertical migration rhythms and their photoresponse in four phytoflagellates causing harmful algal blooms

We observed diurnal vertical migrations (DVMs) of four noxious phytoflagellates: Chattonella antiqua, Heterosigma akashiwo (raphidophyte), Karenia mikimotoi, and Heterocapsa circularisquama (dinoflagellate) under various light conditions such as fluence rate, day length, light regime, and wavelength in the laboratory. For all flagellates cyclic DVMs were observed at an irradiance of 30 μmol quanta m−2 s−1 on a 12 h : 12 h light : dark (L : D) cycle. Start times of upward and downward migrations under the light condition differed among species, indicating that each flagellate has its own original DVM cycle. The ascent period of C. antiqua during the photoperiod was much longer than the other flagellates, whereas K. mikimotoi remained submerged longer. No flagellate displayed cyclic DVM in the dark, and only K. mikimotoi did even under weak light of 1 μmol quanta m−2 s−1. C. antiqua and K. mikimotoi extended their surface-accumulation periods at longer photoperiods, but the other species scarcely changed theirs. For all the flagellates, a DVM rhythm was entrained by shifting the L : D cycle; this occurred under white or blue light, but not under red light. These results indicate that all flagellates require (blue) light for maintenance and adjustment of DVM cycle but responses to change in light intensity and day length differ among species. The present study suggests that the species-specific DVM cycle and its photoresponse property can change time staying in surface or bottom layers in the natural environment, and therefore, may affect photosynthesis and nutrient uptake for survival and growth in four phytoflagellates.