The effect of short-term temperature exposure on vital physiological processes of mixoplankton and protozooplankton.

Sudden environmental changes like marine heatwaves will become more intense and frequent in the future. Understanding the physiological responses of mixoplankton and protozooplankton, key members of marine food webs, to temperature is crucial. Here, we studied two dinoflagellates (one protozoo- and one mixoplanktonic), two ciliates (one protozoo- and one mixoplanktonic), and two cryptophytes. We report the acute (24 h) responses on growth and grazing to a range of temperatures (5-34 °C). We also determined respiration and photosynthetic rates for the four grazers within 6 °C of warming. The thermal performance curves showed that, in general, ciliates have higher optimal temperatures than dinoflagellates and that protozooplankton is better adapted to warming than mixoplankton. Our results confirmed that warmer temperatures decrease the cellular volumes of all species. Q coefficients suggest that grazing is the rate that increases the most in response to temperature in protozooplankton. Yet, in mixoplankton, grazing decreased in warmer temperatures, whereas photosynthesis increased. Therefore, we suggest that the Metabolic Theory of Ecology should reassess mixoplankton's position for the correct parameterisation of future climate change models. Future studies should also address the multigenerational response to temperature changes, to confirm whether mixoplankton become more phototrophic than phagotrophic in a warming scenario after adaptation.