BACTERIVORY BY HETEROTROPHIC NANOFLAGELLATES IN MARINE-SEDIMENTS MEASURED BY UPTAKE OF FLUORESCENTLY LABELED BACTERIA

In situ grazing of heterotrophic nanoflagellates on bacterid in marine sediments was investigated. We adapted a method to study the intake of food particles, i.e. fluorescently labeled bacteria (FLB), to use with flagellates in sediments. Effects of different techniques to disperse the added FLB in sediments were studied. The variables measured were flagellate density, grazing rate, percentage of flagellates with flagellum and percentage of flagellates containing FLB. In order to relate grazing rates to flagellate biovolume, flagellate cell dimensions were measured. To correct for cell shrinkage due to fixation we measured shrinkage in 4 cultured benthic flagellate species. In sandy sediments nanoflagellate grazing rates varied between 0 and 104 bacteria flagellate-1 h-1 and increased with increasing flagellate biovolume. Cell biovolume shrinkage ranged widely depending on species, fixation and staining (range 21 to 57 %). No differences were found between the effects of the various mixing methods on any of the measured parameters. Percentage of total flagellates in the community ingesting FLB varied between 5 and 27. The occurrence in our sediment samples of flagellates without ingested FLB is discussed.