Mixotrophic abilities of dinof lagellate <i>Gonyaulax</i> species: compensation for low growth rates under low-light conditions

Several species within the genus <i>Gonyaulax</i> produce toxins and form blooms in diverse regions worldwide, necessitating further research on their eco-physiological characteristics to better understand their population dynamics. This study examined whether prey availability compensates for low-light limitation in <i>Gonyaulax</i> spp. by evaluating photosynthetic efficiency (<i>F</i><sub>v</sub>/<i>F</i><sub>m</sub>) and specific growth rate (<i>μ</i>) under two light levels and three prey concentrations. <i>G. kunsanensis</i> was the only species to ingest bacterium-sized particles, as evidenced by the uptake of fluorescein isothiocyanate-labeled fluorescent beads. <i>G. kunsanensis</i> growth rates were significantly increased under phagomixotrophic and low-light conditions. Under the same light condition without phago-mixotrophy, <i>G. kunsanensis</i> exhibited a low growth rate. Conversely, <i>G. whaseongensis</i> showed enhanced growth under low-light relative to highlight conditions. A light-shift experiment from high to low light showed that <i>G. kunsanensis</i> compensated for reduced light through phago-mixotrophy. These findings indicate that prey presence enhances growth in <i>G. kunsanensis</i> under low light, whereas changes in <i>F</i><sub>v</sub>/<i>F</i><sub>m</sub> were species-specific and did not consistently track growth responses.