Confusing selective feeding with differential digestion in bacterivorous nanoflagellates

Food selectivity and the mechanisms of food selection were analyzed by vide o microscopy for three species (Spumetla. Ochromonas, Cafeteria) of interce ption-feeding heterotrophic nanoflagellates. The fate of individual prey pa rticles, either live bacteria and/or inert particles, was recorded during t he different stages of the particle-flagellate-interaction, which included capture, ingestion, digestion, and egesrion. The experiments revealed speci es-specific differences and new insights into the underlying mechanisms of particle selection by bacterivorous flagellates. When beads and bacteria we re offered simultaneously, both particles were ingested unselectively at si milar rates. However. the chrysomonads Spumella and Ochromonas egested the inert beads after a vacuole passage time of only 2-3 min. which resulted in an increasing proportion of bacteria in the food vacuoles. Vacuole passage time for starved flagellates was significantly longer compared to that of exponential-phase flagellates for Spumella and Ochromonas. The bicosoecid C afeteria stored all ingested particles, beads as well as bacteria, in food vacuoles for more then 30 min. Therefore "selective digestion" is one main mechanism responsible for differential processing of prey particles. This s election mechanism may explain some discrepancies of former experiments usi ng inert particles as bacterial surrogates for measuring bacterivory.