Temperature-light shapes the nutritional strategy of a mixotrophic green alga, Picochlorum sp. GLMF1 (Trebouxiophyceae)

Constitutive mixotrophs (CM), combining phagotrophy with inherent phototrophy, are critical primary producers and consumers in marine pelagic food webs. 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 mu mol m-2 s-1, whereas the maximum growth rate 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 the photosynthetic electron transport rate (ETR) and chlorophyll a specific inorganic carbon uptake rate increased with increasing irradiance. The maximum ETR decreased from 22.26 +/- 1.93 mu mol e m-2 s-1 at 22 degrees C to 8.96 +/- 0.42 mu mol e m-2 s-1 at 31 degrees C, suggesting a significant constraint of photosynthesis at high temperature. By contrast, ingestion increased with increasing temperature in most cases, and was significantly lower 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 allows them to flourish under a wide range of temperature and irradiance. Our study adds knowledge to the nutritional strategy of mixotrophic green algae, with significant implications for their ecological role in marine food webs under projected climate change.