THE EFFECT OF LONG-TERM LOW BACTERIAL DENSITY ON THE GROWTH-KINETICS OF 3 MARINE HETEROTROPHIC NANOFLAGELLATES

Three marine heterotrophic nanoflagellates were each maintained under two cultural conditions for one month. They were either subcultured every 5 days in batch culture, to familiarise the cells to high bacterial density, or they were maintained in continuous culture in the presence of low bacterial density to familiarise the flagellate cells to this biotic state. The kinetic behaviour of each of the species and each preconditioned state were compared in batch culture grazing experiments. The maximum specific growth rate (mu(max)) and half-saturation constant (K(s)) were the parameters used for comparison. After low prey preconditioning, Paraphysomonas imperforata was found to have a mu(max) value less than half that obtained if the culture was maintained with high prey density and this species did not increase its affinity for the bacterium under chemostat conditions. Bodo designis however, increased its affinity for the bacterium but the mu(max) value was severely reduced. Stephanoeca diplocostata increased its affinity for the bacterium and the mu(max) value after low prey density conditions. It appeared that the maintenance of the cultures at <mu(max) in the chemostats and the absence of bacterial aggregates may have reduced the performance of both P. imperforata, because it prefers being attached whilst feeding on bacteria in suspension, and B. designis, because it prefers to feed on attached bacteria. S. diplocostata on the other hand, appeared not to rely on aggregate formation for good performance and could therefore be a representative of pelagic flagellates known to occur as unattached cells in relatively oligotrophic waters.