Temperature- and nutrient-mediated phagotrophic and phototrophic responses in the polar-adapted Dinobryon faculiferum

Rising temperature, variation of light levels, and changes in nutrient availability are among the top environmental factors known to have profound effects on growth, grazing, and photosynthesis in mixotrophic species. Polar species, in particular, are facing rapid changes in their environment. In the present study, phagotrophic and phototrophic responses of an Arctic chrysophyte, Dinobryon faculiferum , were investigated at different temperatures (2, 4, and 6°C), light levels (0-200 μmol photons m -2 s -1 ), and nutrient concentrations (12.5 and 50% f/2 + Si nutrient media in 32 PSU artificial seawater). Bacterivory by D. faculiferum , estimated by ingestion of fluorescent proxies, increased with rising temperature, while photosynthetic rates, determined by 14 C uptake, were low under all experimental regimes; phagotrophy appeared to contribute more to this species’ carbon budget than phototrophy. Furthermore, D. faculiferum survived long periods of darkness, implying it may not be an obligate phototroph, as found for some other species in the genus. In experiments limiting nitrogen and/or phosphorus, there was no effect of limiting phosphorus on either phototrophy or phagotrophy, but both were reduced in treatments with limited nitrogen, presumably impeding metabolic processes when strongly limited. Our results demonstrate the influences of abiotic environmental factors on phagotrophic and phototrophic responses, providing further understanding of how mixotrophic protists in polar systems may respond to a changing climate.