SIZE-SELECTIVE FEEDING BY CYCLIDIUM SP ON BACTERIOPLANKTON AND VARIOUS SIZES OF CULTURED BACTERIA

Size-selective grazing by Cyclidium sp., isolated as a dominant ciliat e bacterivore from the Rimov Reservoir (South Bohemia), was examined u sing fluorescent labelled bacteria (FLB) produced from natural bacteri oplankton or pure bacterial cultures. Sizes of ingested bacteria in fo od vacuoles were measured directly. Three experimental arrangements we re used: (1) Ciliates were grown on the pure culture of Alcaligenes xy losoxidans and fed with various proportions of 'large' and 'small' FLB (mean biovolume, 0.377 and 0.202 mu m(3), respectively) prepared from the same bacterial species. Results clearly showed significant select ion of larger bacteria. (2) Ciliates were grown on natural bacteriopla nkton from the reservoir and subsequently fed on FLB prepared from the reservoir bacterioplankton (mean biovolume, 0.065 mu m(3)). Independe nt of either prey or predator abundance, larger FLB (> 0.100 mu m(3), and especially those > 0.200 mu m(3)) were ingested with much higher f requency than their occurrence in the natural assemblage. (3) Ciliates were grown on the reservoir bacterioplankton and fed by FLB prepared from the culture of Pseudomonas sp. In contrast with previous results, no size selection of the ciliate was found when FLB were different fr om the bacterial food used to grow the ciliate. Ecological impacts of size-selective bacterivory are suggested.