Double-staining epifluorescence technique to assess frequency of dividing cells and bacteriovory in natural populations of heterotrophic microprotozoa.

We have developed a double-staining procedure for use with epifluorescence microscopy which allows the detection both of dividing cells and of ingested bacteria in food vacuoles of heterotrophic microprotozoa. Microprotozoan cells are stained sequentially with the DNA-specific fluorochrome DAPI (4',6-diami-dino-2-phenylindole) and the nonspecific protein stain fluorescein isothiocyanate. During microscopic examination, heterotrophic microprotozoan cells are first located with fluorescein isothiocyanate fluorescence and then epifluorescence filter sets are switched to permit inspection under DAPI fluorescence of the cell nuclei and of the contents of food vacuoles. Among in situ populations of estuarine microprotozoa sampled over a tidal cycle, we found from 2.2 to 5.2% of the heterotrophic cells in a recognizable stage of division (nuclei elongated or double). Batch culture growth experiments were also carried out both with natural populations and with two isolated species of estuarine microprotozoa. In these experiments, the frequency of dividing cells ranged from 1.2 to 3.8% and appeared to be negatively correlated with growth rate. Microprotozoan populations sampled in continental shelf waters off Savannah, Ga., had mean frequencies of dividing cells ranging from 2.0 to 5.0%. A large fraction of cells in heterotrophic microprotozoan populations (an average of 27.4 +/- 1.0% in estuarine water and of 30.1 +/- 4.8% in shelf water) had DAPI-stained inclusions, presumably recently ingested bacteria, in their food vacuoles.