Indirectly fluorescently labelled flagellates (IFLF): A tool to estimate the predation on free-living heterotrophic flagellates

A procedure was developed to estimate the direct grazing impact on free-living heterotrophic nanoflagellates (HNF). Cultured flagellates were labelled by feeding on brightly fluorescing bacteria (FLB) and then offered as indirectly fluorescently labelled flagellates (IFLF) to potential predators of HNF. The number of FLB in the predators' food vacuoles could be converted into IFLF uptake and consumption of HNF. This new technique was used to study the HNF-ciliate relationship in the pelagic zone of Lake Constance. Three groups of ciliates were detected as HNF grazers: small representatives of the genus Strobilidium, a small Halteria-like ciliate (probably Halteria grandinella) and a Codonella sp./Tintinnidium sp. group. The ingestion of HNF by these groups of ciliates ranged between 3 and 15, 3 and 39, and 3 and 7 HNF ciliate(-1) h(-1); respectively. The IFLF method allows the direct determination of ingested flagellate prey in the food vacuoles of their predators. Because indigenous living prey organisms were used, tracer discrimination can be reduced.