Numerical and functional responses of the mixotrophic dinoflagellate Alexandrium pohangense to various water temperatures and irradiances

The growth and survival of phototrophic dinoflagellates are known to be affected by temperature and irradiance. The maximum growth rate of a dinoflagellate is achieved at a certain temperature and irradiance. However, cells of the dinoflagellate are dead below certain low temperature or irradiance and also above certain high temperature or irradiance. A little change in temperature or irradiance near these critical temperatures or irradiances can affect its survival and in turn the distributions. Subsequently, dominant species in phytoplankton assemblages or a causative species of red tides may change due to this little change. Global warming is expected to elevate water temperature and photosynthetically active radiance. Thus, to predict effects of global warming on communities of phytoplankton and the grazers, the growth and survival of phototrophic dinoflagellates as a function of temperature and irradiance should be explored. The newly described phototrophic dinoflagellate Alexandrium pohangense feeds exclusively on the ichthyotoxic dinoflagellate Margalefidinium polykrikoides. We investigated the growth and ingestion rates of A. pohangense with and without prey as a function of temperature (10-35 °C) and light intensity (0-346 µE m^-2s^-1). Both the autotrophic and mixotrophic growth rates were significantly affected by temperature or light intensity. A. pohangense grew at 15-30 °C, but became dead at 10 or ≥ 32 °C. Thus, elevation of water temperature to ≥ 32 °C due to global warming may cause death of A. pohangense cells. With increasing light intensity, both autotrophic and mixotrophic growth rates rapidly increased at 0-58 µE m^-2s^-1, but became saturated at the higher light intensities. Thus, an increase in light intensity is likely to increase growth rates of A. pohangense. Therefore, changes in water temperature and / or irradiance driven by global warming may change growth and survival of A. pohangense and the population dynamics of related organisms.