Feeding by heterotrophic dinoflagellates on the common marine heterotrophic nanoflagellate Cafeteria sp.

To investigate interactions between heterotrophic dinoflagellates (HTDs) and heterotrophic nanoflagellates (HNFs), we tested whether or not 5 HTDs (Oxyrrhis marina, Gyrodinium cf. guttula, Pflesteria piscicida, Luciella masanensis, and Stoeckeria algicida) feed on the common HNF Cafeteria sp. and measured the growth and ingestion rates of O. marina, G. cf. guttula, and P. piscicida when feeding on Cafeteria sp. (equivalent spherical diameter = ca. 3.5 mu m). We calculated grazing coefficients by combining field data on abundances of O. marina and G. cf. guttula and co-occurring HNFs with laboratory data on ingestion rates obtained in the present study. O. marina, G, cf. guttula, R piscicida, and L. masanensis were able to ingest Cafeteria sp., while S. algicida did not feed on it. Cafeteria sp. supported positive growth in O. marina and G. cf. guttula, but not in P. piscicida. Specific growth rates of O. marina and G. cf. guttula on Cafeteria sp. increased with increasing mean prey concentration before becoming saturated at the mean prey concentration of ca. 100 ng C ml(-1) (20 000 cells ml(-1)). The maximum growth rates of O. marina and G. cf. guttula on Cafeteria sp. were 0.19 and 0.05 d(-1), respectively. With increasing mean prey concentration, the ingestion rates of O. marina, G. cf. guttula, and P. piscicida on Cafeteria sp. rapidly increased at mean prey concentrations < 100 ng ml(-1) (ca. 20 000 cells ml(-1)), but slowly increased at higher mean prey concentrations. The maximum ingestion rate of O. marina on Cafeteria sp. (0.294 ng C predator(-1) d(-1)) was much higher than that of G. cf. guttula (0.034 ng C predator(-1) d(-1)) or F piscicida (0,028 ng C predator(-1) d(-1)). The maximum clearance rates were 0.18 to 0.54 mu l predator(-1) h(-1). Mean grazing coefficients for 0. marina and G. cf. guttula on co-occurring HNFs calculated in the present study were 2.88 and 0.42 d-1, respectively. The results of the present study suggest that O. marina and G. cf. guttula sometimes have considerable grazing impacts on populations of HNFs.