MEPS prepress abstract   -  DOI: https://doi.org/10.3354/meps14322

ABSTRACT: Constitutive mixotrophs (CM), combining phagotrophy with inherent phototrophy, are critical primary producers and consumers in marine pelagic food web. However, how mixotrophs, especially obligate CM, balance phototrophy and phagotrophy in response to changing environmental conditions remains equivocal. Here, we examined the growth performance, photosynthesis traits, and phagotrophic activities of a mixotrophic green alga, Picochlorum sp. GLMF1, over a wide range of temperature-irradiance combinations. The growth of Picochlorum sp. exhibited a unimodal thermal response under all the experimental light intensities. The optimal growth temperature decreased dramatically when the irradiance exceeded 80 μmol m^-2 s^-1, whereas the maximum growth rates kept a relatively constant and high value (around 1.1 d^-1). This might be attributed to the trade-off between photosynthesis and ingestion under various light and temperature conditions. We found that photosynthetic electron transport rates (ETR) and chlorophyll-a specific inorganic carbon uptake rates increased with increasing irradiance. The maximum ETR (p_max) decreased from 22.26 ± 1.93 μmol e m^-2 s^-1 at 22°C to 8.96 ± 0.42 μmol e m^-2 s^-1 at 31°C, suggesting a significant constraint of photosynthesis at high temperature. By contrast, the ingestion increased with increasing temperature in most cases, and was significantly low under high irradiance conditions. Combining the contributions of phototrophic and phagotrophic processes to total carbon acquisition, Picochlorum sp. showed a flexible nutritional strategy in response to temperature and light. Increasing light favors photosynthesis, whereas warming stimulates phagotrophy. Such complementation renders them flourishing in a wide range of temperature and irradiance. Our study adds knowledge to the nutritional strategy of mixotrophic green algae, with significant implications on their ecological roles in the marine food web under the projected climate change.