Grazing on phytoplankton prey by the heterotrophic microflagellate Paraphysomonas vestita in nonbalanced growth conditions

Summary The predatory behaviour of a phagotrophic microflagellate, Paraphysomonas vestita, was studied in the transient conditions of a laboratory batch culture. Two phytoplankton species of the same size (Isochrysis galbana and Pavlova lutheri) were used as alternative prey items. The rate of ingestion of phytoplankton and the P. vestita cell yield were similar in the presence or absence of bacteria, indicating that phytoplankton were selected as prey in preference to bacteria by P. vestita. In terms of cell numbers, P. vestita ingested I. galbana more rapidly than P. lutheri although both species are prymnesiophytes of similar size and were available at a similar carbon:nitrogen ratio. The difference in ingestion rate was not the result of P. lutheri being a poor prey species but could be related to the higher biomass content (in terms of C and N) of P. lutheri compared to I. galbana. Maximum ingestion rates in terms of dry weight biomass were similar for the two prey species. Maximum rate of ingestion of prey and maximum rate of microflagellate division were related to the inoculum prey concentration in a hyperbolic manner for each prey species. As prey density decreased, the ingestion rate at a particular time was not correlated with prey density at that time. Rather, non prey saturated ingestion rate was found to be better represented by a hyperbolic function of the prey/predator ratio. A threshold prey concentration was observed in all experiments and was higher when P. lutheri was the prey species. The threshold prey concentration increased with increasing inoculum prey density. However, in terms of prey/predator ratio a single threshold value was observed for both prey species.