Thermal performance of the heterotrophic dinoflagellate <i>Oxyrrhis marina</i> under long-term warming and acute heat stress

We investigated the thermal tolerance and physiological responses of the heterotrophic dinoflagellate <i>Oxyrrhis marina</i> reared for more than 34 months at 16, 19 or 22°C, using <i>Rhodomonas salina</i> as prey. First, we constructed the thermal performance curves for the growth rate of the three thermal lines of <i>O. marina</i> and found that the maximum growth rates and the optimum temperature increased with higher rearing temperature. Next, we examined the functional responses at each rearing temperature and under acute heat stress (+3°C). Maximum ingestion rates (in cells and carbon) showed limited temperature sensitivity, regardless of chronic or acute heat exposure. Under heat stress, the 16 and 19°C lines saturated at lower prey abundances and showed enhanced clearance rates, whereas the 22°C line experienced negative effects. Growth rates increased with temperature, particularly under acute stress, while growth efficiencies remained stable under long-term thermal exposure and increased only when subjected to acute heat stress. Overall, these findings suggest <i>O. marina</i> follows a "Hotter is partially better" model, with notable differences between the 16 and 22°C lines. Our results highlight the importance of considering both gradual and abrupt temperature changes when predicting climate change impacts on marine food webs and biogeochemical processes.