- Marine primary production can be modeled and estimated using remotely-observable physiological signatures such as chlorophyll and carbon. Current models are based on strict physiological relationships based on photoautotrophic phytoplankton, and discrepancies between modeled and in situ data may stem from unaccounted-for physiological deviations from photoautotrophy. Mixotrophic phytoplankton can obtain energy from photosynthesis and phagocytosis of prey, and so their expressions of chlorophyll and carbon may deviate significantly from these parameters previously described in photoautotrophs. The physiology of Amphidinium carterae, a well-studied mixotroph, was characterized under a range of different growth irradiances and availabilities of dissolved organic carbon and prey. We found that A. carterae upregulated its photosynthetic machinery (i.e., chlorophyll a and carbon fixation) when growing with dissolved organic carbon and bacterial prey compared to growth on only CO2. Additionally, the relationship between the chlorophyll a to carbon ratio (Chl:C) vs. growth irradiance differed substantially from the modeled Chl:C vs. growth irradiance relationship (Westberry et al. 2008). These results provide evidence that the current approach to modeling primary production fails to capture the heterogeneity of phytoplankton physiology.
- Key Words: phytoplankton, mixotrophy, primary production, chlorophyll, carbon