Bacterivory in seawater samples estimated by a dual radioactive-labelling technique

A dual radioactive-labelled bacteria technique using Vibrio (DRLV), developed for laboratory studies on bacterivory, has been refined for use at the concentrations of prey and predators typicially found at sea. Experiments with estuarine water collected in spring and in autumn showed that bacterivorous nanoflagellates (HNF) (concentration 1.38 +/- 0.35 X 10(3) HNF ml(-1))ingested 2.7 +/- 0.96 DRLV flagellate(-1) h(-1) at concentrations of 0.8-2.2 X 10(6) DRLV ml(-1) in the presence of 2.04 +/- 0.68 x 10(6) natural bacteria ml(-1). The method was also applied to samples collected in October in the Celtic Sea, when on average 1 ml of water from the surface layer contained 1.41 +/- 0.16 x 10(6) natural bacteria, 14.6 x 10(3) cyanobacteria, 530 +/- 170 HNF, 7.3 +/- 3.0 x 10(3) phototrophic nanoflagellates (1.5-4 mu m), 49.0 +/- 26.5 phototrophic dinoflagellates, 36.3 +/- 12.6 heterotrophic dinoflagellates and 21.3 +/- 9.5 Leucocryptos marina. Under these conditions the grazing rate in most samples did not exceed the coefficient of variation of the method (2%), although we estimate the grazing rate was similar to 1.6 DRLV HNF-1 h(-1) and on one occasion a rate of 2.45 was recorded. The gross growth efficiency for protein of similar to 30% displayed by natural HNF means that they could release about similar to 70% of inorganic nutrients derived from their food.