Feeding of the heterotrophic freshwater dinoflagellate Peridiniopsis berolinense on cryptophytes: analysis by flow cytometry and electronic particle counting

We measured feeding rates and food selectivity of the heterotrophic freshwater dinoflagellate Peridiniopsis berolinense by analytic flow cytometry (AFC), electronic particle counting and sizing (EPCS), and conventional microscopy. Several differently sized strains of the common cryptophytes Rhodomonas minuta and Cryptomonas sp. were offered in short-term (1 to 4 h) laboratory experiments as food in concentrations ranging from 3.0 x 10 super(4) to 1.0 x 10 super(5) cells/ml. P. berolinense is a raptorial species that attacks its prey by peduncle-feeding. Since P. berolinense sucks out its prey but does not take up whole cells, grazing rates cannot be quantified by established techniques such as fluorescently labeled algae. We measured dinoflagellate ingestion rates as loss rates of algal prey cells, by the increase of autofluorescence within food vacuoles of P. berolinense, and by the direct formation of food vacuoles. Biovolumes of prey and predator were measured by EPCS. The grazing response was variable and depended on the nutritional status of P. berolinense. Heavily starved cells were small, started feeding immediately and were less selective than moderately starved specimens. We show how nutritional status can be assessed by AFC and EPCS prior to the beginning of an experiment.