Influence of prey concentration, light intensity, and temperature on the growth and ingestion of the mixotrophic dinoflagellate <i>Pyrophacus horologium</i> , a predator of the harmful species <i>Heterocapsa niei</i>

Pyrophacus horologium belongs to the family Pyrocystaceae and was recently identified as a mixotrophic dinoflagellate capable of feeding on other dinoflagellates. In this study, the growth and feeding responses of P. horologium were investigated under various environmental conditions by providing Heterocapsa niei, its preferred prey. The mixotrophic growth rate was measured in a feed concentration range of approximately 350 to 35,000 cells · mL^-1, reaching saturation at approximately 8343 cells · mL^-1. In the temperature experiment conducted under mixotrophic conditions (10-35°C at 90 μmol photons · m^-2 · s^-1), positive growth was observed between 15 and 32°C, with the maximum mixotrophic growth rate of 0.48 · day^-1 recorded at 30°C (at 90 μmol photons · m^-2 · s^-1). In the light intensity experiment conducted under mixotrophic conditions (0-348 μmol photons · m^-2 · s^-1 at 30°C), positive growth was observed between 24 and 170 μmol photons · m^-2 · s^-1, with the maximum mixotrophic growth rate of 0.38 · day^-1 at 120 μmol photons · m^-2 · s^-1 (at 30°C). The maximum ingestion rates observed were 30.2 cells · predator^-1 · day^-1 (at 28°C and 90 μmol photons · m^-2 · s^-1) and 28.5 cells · predator^-1 · day^-1 (at 30°C and 120 μmol photons · m^-2 · s^-1). The result indicate that growth rates were significantly affected by temperature and light intensity. Collectively, these results provide insights into the environmental adaptability of P. horologium and its potential ecological implications, enhancing our understanding of mixotrophic dinoflagellates that graze on the harmful species H. niei.