Pulse Feeding and Pulse Growth: A Highly Adaptive Strategy of Heterotrophic Dinoflagellates Oxyrrhis marina

Abstract

Pulse feeding and growth of the Black Sea strain of dinoflagellates Oxyrrhis marina (Dujardin, 1841) (OXY–IBSS), equivalent spherical diameter (ESD) (23.5 ± 3.1 μm) have been studied under experimental conditions simulating phytoplankton blooms. Microalgae Phaeodactylum tricornutum (P, ESD 3.4 ± 0.3 μm), Isochrysis galbana (I, ESD 3.7 ± 0.4 μm), Tetraselmis suecica (T, ESD 6.1 ± 0.9 μm), and Rhodomonas salina (R, 7.4 ± 0.7 µm) were used as food objects in a one-component and three-component suspensions. Microalgae concentrations (∼10⁶ cells/mL for T and R; up to ∼4 × 10⁶ cells/mL for P and I) were chosen to ensure their equal total carbon biomass ∼0.02 μg C/μL in the food mixtures at the beginning of the experiment. Under ad libitum conditions, the maximum clearance rates of the OXY–IBSS reached 0.1–0.5 μL ind./day, and the grazing rate of microalgae was 34–44 cells/(ind h) for P and I, and 2–11 cells/(ind. h) for R and T, respectively. The grazing rate of microalgae in carbon units was significantly higher when feeding on I (3.9 ng C/(ind. day)) and significantly less when fed with a mixture of microalgae TRP (1.5 ng C/(ind. day)). Maximum abundance of OXY–IBSS, achieved within 3 or 4 days (by the time the microalgae concentration decreased below threshold), varied from 19 × 10³ ind./mL (P) to 43 × 10³ ind./mL (I). In the absence of food, dinoflagellates O. marina turned to cannibalism, and within 4 days the oscillating fluctuations (within 50%) in their number took place. Although the specific population growth rate (μ, day^-1) of OXY–IBSS was higher when feeding on small cells (∼2 days^–1 on I), the gross growth efficiency (GGE) of OXY–IBSS was significantly higher when fed on large (T and R) microalgae (26–29% vs. 14–15%). At lower daily rations (DRs) when fed with the mixture TRP, the GGE of OXY–IBSS was significantly higher (41%) when compared to other nutritional conditions. The feeding strategy of opportunistic predator O. marina on diverse (in terms of size and chemotaxonomic characteristics) mixtures of prey lay in a flexible choice between high specific population growth rate, or high gross growth efficiency, that obviously gives the populations of this species the advantages over other protists under conditions of the pulsed phytoplankton blooms.