Trophic significance of solitary cells of the prymnesiophyte Phaeocystis globosa depends on cell type

With the use of five different isolates of Phaeocystis globosa solitary cells from the North Sea, we conducted experiments to reveal whether grazing and development of the nauplii of the calanoid copepod Temora longicornis varies in response to the cell type. Two P. globosa strains representing nonflagellated cells were ingested at intermediate to high rates and resulted in high survival and development, comparable to the Rhodomonas sp. control. In contrast, the response to three mesoflagellate strains was highly variable. Feeding on two of these strains was avoided, whereas the third strain was ingested; however, the mesoflagellates induced poor survival and development regardless of the feeding response. These observations differ from previous results, which generally demonstrate microzooplankton feeding on Phaeocystis. The morphological characterization of strains, together with mixture experiments, revealed that neither the production of transparent exopolymer particles and chitinous threads nor toxicity can explain the observed response. The cohesion of the threads into pentagonal stars was observed only in the avoided mesoflagellate and might cause a mechanical hindrance for the ingestion of mesoflagellates. Our results suggest that grazing loss and trophic transfer efficiency might be overestimated when solitary cells are treated as a single functional group with regard to their trophic position.