Effects of diet nutritional quality on the growth and grazing of Noctiluca scintillans

Noctiluca scintillans is a cosmopolitan red tide-forming heterotrophic dinoflagellate which can feed on a variety of algae. In this study, we examined the effects of diet nutritional quality on its ingestion and reproduction. Functional and numerical response experiments were conducted using 3 types of algae:a diatom (Thalassiosira weissflogii), a chlorophyte (Platymonas helgolandica) and a dinoflagellate (Prorocentrum dentatum) that were grown under nitrogen- (N-) and phosphorus- (P-) replete, N-depleted and P-depleted conditions. Ingestion and growth rates of N. scintillans were fitted using Type II and modified Type II models, respectively. N. scintillans generally exhibited higher maximum ingestion rate under nutrient-deficient conditions than when fed on N- and P-sufficient prey, presumably in order to maximize its nutrient pool and meet growth requirements. All phytoplankton cultures, except P-deficient T. weissflogii, supported the growth of N. scintillans. However, nutrient deficiency, especially P-deficient prey, yielded lower growth rates of N. scintillans than their nutrient-sufficient counterparts. No optimum curve was obtained for P-deficient T. weissflogii, which may become toxic under P limitation. Based on the hyperbolic regression models simulated for N. scintillans growth rate using different resources' nutritional contents as variables, P limitation appears to be the major constraint affecting N. scintillans reproduction and survival under nutrient deficiency. Polyunsaturated fatty acids, e.g. α-linolenic acid (18:3ω3, ALA) and eicosapentaneoic acid (20:5ω3, EPA), are also important in determining food quality for N. scintillans based on their high correlation with N. scintillans growth rate.