Light spectrum regulates cell accumulation during daytime in the raphidophyte Chattonella antiqua causing noxious red tides

•Blue light induced negative phototaxis in a raphidophyte causing noxious red tides.•Ultraviolet-A/blue and red lights induced descending of cells.•Mixing of green/orange light diminished the effects of blue light.

Most marine raphidophyte species cause noxious red tides in temperate coastal areas around the world. It is known that swimming abilities enable raphidophytes to accumulation of cells and to actively acquire light at surface layers and nutrients over a wide depth range. However, it remains unclear how the swimming behavior is affected by environmental conditions, especially light condition. In the present study, we observed the accumulation of the harmful red-tide raphidophyte Chattonella antiqua under various light conditions during the daytime in the laboratory. When exposed to ultraviolet-A/blue light (320–480 nm) or red light (640–680 nm) from above, cells moved downward. In the case of blue light (455 nm), cells started to swim downward after 5–15 min of irradiation at a photon flux density ≥ 10 μmol m− 2 s− 1. When exposed to monochromatic lights (400–680 nm) from the side, cells moved away from the blue light source and then descended, but just moved downward under red light. However, mixing of green/orange light (520–630 nm) diminished the effects of blue light. When exposed to a mixture of 30 μmol m− 2 s− 1 of blue light (440 nm) and ≥ 6 μmol m− 2 s− 1 of yellow light (560 nm) from above, cells did not move downward. These results indicate that blue light induces negative phototaxis and ultraviolet-A/blue and red lights induce descending, and green/orange light cancels out their effects in C. antiqua.