ABSTRACT: ABSTRACT:The effect of irradiance on photosynthesis of the green form of the dinoflagellate Noctiluca scintillans was studied. Photosynthesis, measured in cells collected from the field and without the addition of prey, increased withirradiance to ca. 200 ng C cell^-1 d^-1 at an irradiance of ~250 to 300 µmol photons m^-2 s^-1. N. scintillans cells were observed to prey and grow actively on a number of different algae, including thedinoflagellate Pyrodinium bahamense var. compressum which produces paralytic shellfish toxin. However, in all cases, N. scintillans lost its endosymbionts when grown in the laboratory for more than 3 wk and became colourless,irrespective of food item, concentration and irradiance. A factor necessary for endosymbiont growth was apparently missing, which was not provided to them by N. scintillans when fed the selected prey types. Thus, further experiments were carriedout with freshly collected organisms. The growth rate of N. scintillans when grown without prey was 0.058 and 0.14 d^-1 at irradiances of 45 and 150 µmol photons m^-2 s^-1 at a light:dark cycle of 12:12 h and atemperature of 26°C. When supplied with P. bahamense as food, N. scintillans increased its growth rate to 0.09 and 0.24 d^-1, at irradiances of 45 and 150 µmol photons m^-2 s^-1 and prey concentrations of 1610and 2740 µg C l^-1, respectively. Ingestion rates were only measured at 150 µmol photons m^-2 s^-1. At this irradiance, the ingestion rate increased linearly with prey concentration and showed no signs of satiation at a preyconcentration of ~2700 µg C l^-1. A comparison of the contribution of photosynthesis and phagotrophy to the carbon metabolism revealed that phagotrophy only contributed significantly (30%) to the direct growth of the green N. scintillansat an irradiance of 150 µmol photons m^-2 s^-1, when the prey concentration was very high. Clearance decreased with prey concentration, from ~0.06 ml N. scintillans^-1 d^-1 at 25 µg C l^-1 to ~0.03ml N. scintillans^-1 d^-1 at a prey concentration of 2740 µg C l^-1. This indicates that N. scintillans, when it occurs at bloom concentrations in nature (1 to 10 cells ml^-1), may have a significantimpact on the bloom dynamics of P. bahamense var. compressum.