Effects of light on diurnal vertical migration in the red-tide flagellates

Abstract

Most noxious red-tide flagellates display diurnal vertical migration (DVM), in which they swim upward during the daytime and downward during the night. It has been suggested that the DVM cycle is produced by phototaxis and geotaxis controlled primarily by an endogenous clock. A DVM rhythm is entrained by shifting the light : dark cycle in most flagellates, but start times of upward and downward migrations under the light condition, photon flux density required for maintenance of the DVM cycle, and effects of day length on DVM cycle differ among the species. In natural environment, some dinoflagellates such as Alexandrium tamarense accumulate at subsurface layers during daytime, whereas a raphidophyte Chattonella antiqua frequently accumulates at surface. According to a recent laboratory work, C. antiqua cells display negative phototaxis to blue light, but mixing of slight green/orange light diminishes the effect of blue light.