Growth and grazing of a large heterotrophic dinoflagellate, Noctiluca scintillans, in laboratory cultures

Carbon (C) and nitrogen (N) content and growth/grazing characteristics of the heterotrophic dinoflagellate Noctiluca scintillans were examined in laboratory cultures at 24 degrees C. The C and N contents of N.scintillans per unit cell volume were 2.3 x 10(-3) pg C mu m(-3) and 4.1 x 10(-4) pg N mu m(-3), respectively. These values are almost two orders of magnitude lower than those reported for other protozoans such as ciliates and 'typical' heterotrophic dinoflagellates due to an extensive vacuole in the cell of N.scintillans. Among five autotrophic flagellates used as prey, species with an equivalent spherical diameter (ESD) of >10 mu m served as good prey for N.scintillans, but species with an ESD of <5 mu m did not support growth. Growth rates of N.scintillans fed Chattonella antiqua (ESD = 27 mu m; Raphidophyceae) or Heterosigma akashiwo (ESD = 11 mu m; Raphidophyceae) increased linearly with prey C concentration (PC), with lower growth thresholds of similar to 100 mu g C l(-1). Grazing rates of N.scintillans fed C.antiqua or H.akashiwo increased linearly with PC without an obvious threshold. Carbon budget calculations for grazing and growth indicated that N.scintillans converts 'excess' C (= total prey C ingested - C necessary for the maintenance of basic metabolic activities) to body C with efficiencies of 0.31 and 0.17 for C.antiqua and H.akashiwo, respectively.