Effects of protozoan grazing on colony formation in Phaeocystis globosa (Prymnesiophyceae) and the potential costs and benefits

Abstract

Colony formation by Phaeocystis globosa was enhanced in terms of colony size when solitary P. globosa cells were grazed by the heterotrophic dinoflagellates Noctiluca scintillans and Gyrodinium dominans. Enhancement of colony size in the grazing treatment was evident after 5 to 8 d. Grazing by N. scintillans increased the mean colony size by up to 50% relative to the controls, whereas grazing by G. dominans enhanced the mean colony size by up to 3-fold. Microscopic obser- vations confirmed that N. scintillans was also able to ingest small colonies. In contrast, G. dominans apparently did not ingest colonies, and in most cases the abundance of colonies was also increased in the grazing treatment. Up to 92% of P. globosa cells were in colonial form when G. dominans was present, in contrast to <31% in the controls. Enhanced colony formation provided refuge for P. glo- bosa cells such that G. dominans starved and declined after the initial depletion of solitary P. globosa cells. Slow recovery of starved grazers subsequently allowed solitary P. globosa cells to resume expo- nential growth. The apparent specific growth rate of colonial P. globosa cells was not different from that of solitary cells; thus, the costs of colony formation must be sought elsewhere.