Mixotrophic Protists among Marine Ciliates and Dinoflagellates: Distribution, Physiology and Ecology

The ciliate Mesodinium pulex (Class Litostomatea) ingested all 5 species of cryptophytes and the autotrophic dinoflagellate Heterocapsa rotundata offered as prey. Despite this, it could only grow on the cryptophytes Teleaulax sp. and Guinardia theta and the dinoflagellate H. rotundata, because ingestion rates of the other prey cells, even at very high prey concentrations, were too low to support growth. The numerical and functional responses of Mesodinium pulex fed Heterocapsa rotundata were investigated in the laboratory in the light (100 μmol photons m s) and in the dark. In the light the growth rate was significantly higher than in the dark at all prey concentrations. Active photosynthesis was measured in M. pulex and the measured rates could not explain the increased growth rates in the light. Instead, the increased growth rates could mainly be explained by elevated ingestion rates in the light. We also studied the starvation response at different irradiances and in the dark (100, 50 μmol photons m s and dark). M. pulex survived for up to 2 weeks without food, but mortality rates of in the light were larger than in the dark.