The influence of different bacterial phenotypic traits on the feeding selectivity of a bacterivorous nanoflagellate was investigated in laboratory experiments. Twelve bacterial isolates from freshwater habitats were characterized in terms of cell size, morphology, capsule formation, surface hydrophobicity and charge, and swimming behavior. Mechanisms of differential flagellate feeding on these isolates were studied in short-term grazing experiments by high-resolution video microscopy using the

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... anoflagellate Spumella sp. as a model interception-feeding predator. The differentiation into distinct stages of the feeding process (contact, capture, ingestion) revealed a complex selection behavior illustrated by the relatively small proportion of ingested bacteria over cell contacts. We found that bacterial swimming speed increased contact rates but also decreased flagellate capture efficiency, which had a compensating effect on overall ingestion rates. Bacterial cell size revealed no correlation with flagellate contact rates but an effective inhibition of ingestion rates when exceeding a critical size limit. Correlation analysis presented no bacterial property to account for the residual variability of flagellate ingestion efficiencies. Experiments with specifically coated artificial particles provided evidence for the relative importance of the biochemical surface composition in flagellate food selection compared to physicochemical interaction forces. Only extreme charges beyond -45 mV reduced flagellate ingestion rates. Our results reinforce the idea that bacterial cell size strongly affects the feeding success of bacterivorous flagellates, and further implicates size-independent bacterial traits such as swimming speed and biochemical surface composition in this feeding success. KEY WORDS: Selective flagellate feeding · Bacterial properties · Size · Swimming speed · Cell surface · Video microscopy Resale or republication not permitted without written consent of the publisher