Bacterivory by microheterotrophic flagellates in seawater samples

Changes in the population sizes of bacteria and of heterotrophic microflagellates in seawater during the first 30–60 h after sampling indicate that these protozoa control bacterial numbers in situ. The observations allow crude estimates of in situ grazing rates and of the minimum bacterial concentration which sustains protozoan growth. In the water samples studied, an average flagellate will clear 1–2 × 10 −5 ml h −1 (15°C). If this result is extrapolated to other areas, typical concentrations of microflagellates in the sea suggest that between 5 and 250% of the water column is cleared of bacteria per day. Bacterial numbers above about 10 6 ml −1 will sustain flagellate growth.