Conditions favoring phagotrophy can lead to larger cell sizes in some freshwater mixoplankton.

Cell size is a critical regulator of many metabolic processes in protists. We explored whether body size and abundances vary consistently in phytoplankton capable of both autotrophy and heterotrophy (mixoplankton) by manipulating environmental stoichiometric conditions in a mesocosm experiment. We applied two allochthonous subsidy treatments: high C: nutrient ratios (leaves) should favour bacterivory through phagotrophy, while low ratios (insects) should favour autotrophy. We identified three focal mixoplankton taxa, common in our study system and that represented facultative (Cryptomonas sp. and Plagioselmis sp) and more obligate phagotrophs (Ochromonas sp.). Ochromonas was largest in the leaf treatment, which were also associated with larger sizes in Cryptomonas (but not the other cryptophyte). The obligately mixotrophic Ochromonas responded more significantly to conditions favouring phagotrophy than did the facultative phagotrophic cryptophytes. All mixoplankton taxa densities declined with insect subsidies that favour autotrophy. Future research should examine a wider range of mixoplankton under varying ecological conditions.