Influence of inorganic nutrients, irradiance, and time of day on food uptake by the mixotrophic dinoflagellate Neoceratium furca

While phagotrophy is reported for a number of phytoflagellates, the factors that influence feeding are known for only a few species. We examined the effects of nutrients, irradiance, and time of day on ingestion rate by field populations of the mixotrophic dinoflagellate Neoceratium furca. In nutrient addition assays, N. furca consistently showed a significant reduction in feeding when inorganic nitrogen (N) and phosphorus (P) were added together. Responses to separate N or P addition varied, with ingestion rates decreasing, increasing, or remaining unaltered depending on which nutrient was potentially limiting, as indicated by inorganic nutrient concentrations. Feeding was also influenced by light and was often highest at or below irradiance levels comparable to those experienced by cells at time of collection. Darkness and/or high irradiance sometimes led to a decrease in feeding, possibly due to reduced nutrient demand or photoinhibition. Ingestion rates of N. furca over the course of a day showed distinct but conflicting diel feeding patterns. While feeding increased during the day in one experiment, ingestion rates were higher at night in the other. These patterns were likely a direct response to the changing light environment over the course of the day, as they agreed well with findings for the same N. furca populations during the corresponding light level experiments. Results from both nutrient addition and light level experiments confirm that N. furca employed phagotrophy primarily as an alternative means of obtaining limiting nutrients rather than carbon.