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Bacteria or microflagellates as a major food source for marine ciliates: possible implications for the microzooplankton

Direct visual observations were made on the vacuole contents of the oligotrichous ciliate Strombidium cf. sulcatum fed 12 different types of food. These ranged in size from 0.6 to 11.9 pm and included one fluorescently labelled bacterium, 2 types of cyanobacteria and 9 species of algae. We determined (i) the turnover time of food vacuoles as a function of prey quality and size, (ii) rates of grazing and filtration, and volume-specific clearance as a functlon of prey quality and size, (iii) the food concentration threshold for effective food intake in S. sulcatum when fed different types of prey, and (iv) an optimal particulate food size for S. sulcatum. Our results indicate the shortest feeding turnover time (13.7 ? 2.2 min.) for 2.5 pm prey (Nannochloris sp.). The average number of prey per vacuole and the feeding rate increased with food size to reach a maximum for this flagellate. Feeding efficiency then decreased to zero for particles w i t h the size range 6.6 to 11.9 pm (Platymonas sp., Hymenomonas elongata). Finally, it 1s shown that for food particle concentrations comparable to those of oligotrophic marine environments (e.g. 0.1 to 0.2 ppm by volume) S. sulcatum depends on food particles measuring around 2.5 pm for growth.