BALSEIRO esteban Gabriel
Light vs food supply as factors modulating niche partitioning in two pelagic mixotrophic ciliates.
MODENUTTI, BE; ESTEBAN GABRIEL BALSEIRO; CALLIERI, C.; BERTONI, R.
LIMNOLOGY AND OCEANOGRAPHY
AMER SOC LIMNOLOGY OCEANOGRAPHY
Año: 2008 vol. 53 p. 446 - 455
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 if 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 prokaryotes and examined food vacuoles, using the catalyzed reported depositionfluorescence in situ hybridization (CARD-FISH). S. araucanus, an ultraviolet radiation resistant species, was present in the epilimnion and attained higher abundances when the thermocline was shallower and the mean irradiance higher. In contrast, O. naumanni preferred the metalimnetic layers and was more abundant in years with deeper thermoclines. Variation in 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 resulted 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 favoured during years of relatively shallow thermocline. CARD-FISH analysis revealed that O. naumanni fed on Archea, Eubacteria, and picocyanobacteria. In contrast, S. araucanus did not feed on prokaryotes. We indicate light climate created by temporal or spatial variations in thermocline depth as the key factor modulating niche partitioning for mixotrophic ciliate species.