Mixotrophy is increasingly recognized as an important and widespread nutritional strategy in various taxonomic groups ranging from protists to higher plants. We hypothesize that the availability of alternative carbon and energy sources during mixotrophy allows a switch to photoheterotrophic growth, where the photosynthetic apparatus mainly provides energy but not fixed carbon. Because such a change in the function of the photosynthetic machinery is probably reflected in its composition, we compared the photosynthetic machinery in Ochromonas danica during autotrophic and mixotrophic growth. Compared with autotrophic growth, the total pigmentation of O. danica was reduced during mixotrophic growth. Furthermore, the photosystem I (PSI):PSII ratio increased, and the cellular content of Rubisco decreased not only absolutely, but also relative to the content of PSII. The changing composition of the photosynthetic apparatus indicates a shift in its function from providing both carbon and energy during photoautotrophy to mainly providing energy during mixotrophy. This preference for photoheterotrophic growth has interesting implications for the contribution of mixotrophic species to carbon cycling in diverse ecosystems.
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