Warming Fluctuations Strengthen the Photo-Phagotrophic Coupling in Mixoplanktonic Protists

Mixoplankton, a major trophic group in aquatic ecosystems, are being affected by global warming. However, most studies on temperature effects use constant mean conditions, overlooking how short-term thermal fluctuations could deviate from climate projections and impact this group. We experimentally quantified how increasing amplitudes of warming fluctuation (± 1, 3, and 5 °C) alter carbon-specific electron transport (ETR^c), net photosynthesis (P^c), respiration (R^c), phagotrophy (Ph^c), carbon use efficiency (CUE), and growth (µ) in four protist species (three mixoplanktonic and one strict phototroph). We observed a consistent positive link between photosynthetic efficiency (P^c:ETR^c ratio) and Ph^c, and a shift towards a strengthening of the phagotrophy (P^c:ETR^c / Ph^c ratio) with greater thermal fluctuation. A potential explanation is a selective behavior aimed to increase phagotrophy to obtain inorganic nutrients through ingested prey internal re-cycling rather than relying on the environment, to support an enhanced photosynthetic efficiency and growth. An enhanced, coupled photo-phagotrophy activity could boost mixoplankton competitiveness compared to phytoplankton. Our findings underscore the need to incorporate trophic flexibility and its interaction with environmental variability into trait-based models to better predict community dynamics, biogeochemical cycling, and food web structure in aquatic ecosystems.