High-resolution video-microscopy was used to investigate the feeding, digestion and selection behavior of different heterotrophic protists (single-cell eukaryotic organisms). Behavioral observations in protists are complicated for two reasons: their small size (5-100 microns) and the lack of comparative data sets. A high resolution video-microscope system was used for observation (Figure 1). This allows the study of interactions between bacteria and protists. However, the method is limited when studying fast moving organisms. As no expedient comparable data sets are available, the feeding process was divided into feeding phases based on typical behavioral elements (e.g. flagella movements, formation of food vacuoles, etc.; Figure 2).

Figure 1. Experimental set-up and observation chamber.

Figure 2. Feeding steps of a flagellate.

The mechanisms of food selection were the focus of the study. Food selectivity of heterotrophic nanoflagellates can be subdivided into: (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 actively select food during food uptake). These experiments revealed a high variability between species, and also high intraspecific variability. Advantages and disadvantages of the technique will be discussed. These include photochemical effects, experimental artifacts and the general suitability of the method for investigating behavioral patterns in microbial populations.

Paper presented at Measuring Behavior 2002 , 4^th International Conference on Methods and Techniques in Behavioral Research, 27-30 August 2002, Amsterdam, The Netherlands

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