We investigated the survival strategies and the competitive strength of pigmented and colourless bacterivorous flagellates using 2 model organisms, the bacterivorous chrysophytes Poterioochromonas malhamensis and Spumella sp., fed small ultramicrobacteria (0.044 mu m(3)) and large bacteria (0.38 mu m(3)). The numerical responses indicated that small bacteria are not such a good food source as large bacteria, even when bacterial cell volume was taken into account. The growth rates of the heterotrophic Spumella sp. exceeded those of the mixotrophic P malhamensis at high bacterial concentrations of both sizes of bacteria. At very low prey abundances, P. malhamensis grew better than Spumella sp., mainly because the mortality rate of the latter was very high. At bacterial biovolumes encountered in oligotrophic/mesotrophic lakes, the growth curves of the 2 flagellates intersected. Although P. malhamensis possesses a chloroplast, growth of cultures incubated in the fight and the dark did not differ even at very low prey concentrations, indicating that photosynthesis plays a minor role for this chrysophyte. Experiments carried out with mixed flagellate cultures showed that P. malhamensis ingested Spumella sp. cells. Applying our data to natural abundances of small and large bacteria as well as pico/nanoplankton we could demonstrate that P. malhamensis is a K-strategist. Large bacteria made up the major portion of its carbon uptake, but small bacteria, pico-nanoplankton and (to a minor extent) photosynthesis also contributed to its gross carbon uptake, indicating a broad use of different resources by this species. Spumella sp. is more of an r-strategist and relies mainly on large bacteria for growth although small bacteria can contribute significantly to its diet.