Effects of light and temperature on the growth of Takayama helix (Dinophyceae): mixotrophy as a survival strategy against photoinhibition.

Takayama helix is a mixotrophic dinoflagellate that can feed on diverse algal prey. We explored the effects of light intensity and water temperature, two important physical factors, on its autotrophic and mixotrophic growth rates when fed on Alexandrium minutum CCMP1888. Both the autotrophic and mixotrophic growth rates and ingestion rates of T. helix on A. minutum were significantly affected by photon flux density. Positive growth rates of T. helix at 6-58 μmol photons · m-2  · s-1 were observed in both the autotrophic (maximum rate = 0.2 · d-1 ) and mixotrophic modes (0.4 · d-1 ). Of course, it did not grow both autotrophically and mixotrophically in complete darkness. At ≥247 μmol photons · m-2  · s-1 , the autotrophic growth rates were negative (i.e., photoinhibition), but mixotrophy turned these negative rates to positive. Both autotrophic and mixotrophic growth and ingestion rates were significantly affected by water temperature. Under both autotrophic and mixotrophic conditions, it grew at 15-28°C, but not at ≤10 or 30°C. Therefore, both light intensity and temperature are critical factors affecting the survival and growth of T. helix.