Even though bacterivorous flagellates have been recognized to be able to structure the bacterial community the behavioral basis for food selection is hardly known. High resolution video-microscopy was used to observe grazing patterns of the heterotrophic nanoflagellates. Food selectivity and the mechanisms of food selection were analyzed for several species. The fate of individual prey particles, either live bacteria and/or inert particles, was recorded during the different stages of the particle-flagellate-interaction, which included capture, ingestion, digestion, and egestion.

The experiments revealed species-specific differences and new insights into the underlying mechanisms of particle selection by bacterivorous flagellates. Food selectivity of heterotrophic nanoflagellates can be subdivided in
1) passive food selection (contact probability and morphological properties of the feeding structures are responsible for a particle-specific response) and
2) active food selection (flagellates may select for food during food uptake).

In addition, some bacterial strains are completely digested, others are excreted in different stages of digestion. When beads and bacteria were offered simultaneously, both particles were ingested unselectively at similar 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. The bicosoecid Cafeteria stored all ingested particles, beads as well as bacteria, in food vacuoles for more then 30 min.

There are significant species-specific differences in the processing of food particles which explain the coexistence of various bacterivorous nanoflagellates in the size range of 3-5 microm and indicate the existence of specific predation pressure on different bacteria. Thus, digestibility is species-specific and depends on the bacteria as well as on the flagellate species under view. “Selective digestion” is one main mechanism responsible for differential processing of prey particles. This selection mechanism may explain some discrepancies of former experiments using inert particles as bacterial surrogates for measuring bacterivory. The significance of the different selection mechanisms depends on the bacterial concentration as well as on the satiation of the flagellates.