Light versus food supply as factors modulating niche partitioning in two pelagic mixotrophic ciliates

We studied the vertical distribution of two mixotrophic ciliates, Ophrydium naumanni and Stentor araucanus in two contrasting summer seasons (strong vs. mild windy years) to determine whether differences in vertical mixing affect their success by changes in light availability for their endosymbiotic algae. Field experiments were done to evaluate the effect of light climate on the photosynthetic efficiency. To test for a potential overlap in food niche we studied prokaryote abundance and examined food vacuole contents of the ciliates, using the catalyzed reported deposition-fluorescence in situ hybridization (CARD-FISH). S. araucanus, a species resistant to ultraviolet radiation, was present in the epilimnion. In contrast, O. naumanni preferred the metalimnetic layers and was more abundant in years with deeper thermoclines. Variation in the diffuse extinction coefficient was significantly correlated with S. araucanus abundance, suggesting a shading effect of this dark ciliate. The expected extinction coefficient due to Stentor, measured by spectrophotometric analyses, did not differ from that observed in nature. O. naumanni was found to be photosynthetically efficient at low light intensities and susceptible to photoinhibition at epilimnetic light irradiances. Conversely, S. araucanus needed a high light supply to maintain endosymbiotic algal photosynthesis and was favored during years of relatively shallow thermocline and high epilimnetic mean irradiance. CARD-FISH analysis revealed that O. naumanni fed on Archea, Eubacteria, and picocyanobacteria. In contrast, S. araucanus did not feed on prokaryotes. Based on these results, we suggest that light climate, created by temporal or spatial variations in thermocline depth, is a key factor modulating niche partitioning for mixotrophic ciliate species.