Feeding in the mixotrophic dinoflagellate Ceratium furca is influenced by intracellular nutrient concentrations

Abstract

In field populations of the mixotrophic dinoflagellate Ceratium furca, feeding on micro- zooplankton is commonly observed and is influenced by inorganic nutrient concentrations and ratios. Using batch and 'semi-continuous' laboratory cultures, we examined in more detail the nutrient con- ditions that trigger feeding in C. furca. Cultures were maintained at a range of nitrogen (N) and phos- phorus (P) concentrations for 31 to 36 d. Several parameters were determined at regular intervals, including inorganic nutrient concentrations of the medium, C. furca abundance and size, cellular chlorophyll a (chl a), C, N, and P contents and densities, photosynthetic rates, and ingestion of ciliate prey. We detected significant differences between treatments in all cellular parameters measured. Most notably, feeding only occurred in cultures that had been growing under N- or P-deplete condi- tions for 11 to 16 d, whereas nutrient-replete cells did not ingest prey. Feeding increased markedly as cellular C:N:P ratios deviated farther from those found under optimum growth conditions. Specifi- cally, feeding in P-deplete cultures increased at C:P ratios >130 and at N:P ratios >19, whereas N-deplete cultures required a C:N ratio of ~10 and an N:P ratio <7 to commence feeding. Growth and photosynthetic rates were reduced compared to those of nutrient-replete cells regardless of the limiting nutrient. In N-limited cultures, cell size and chl a density decreased compared to nutrient- replete cells, whereas the size of P-limited cells significantly increased. This change in average cell size was caused by the development of a bimodal size distribution under N- and P-reduced condi- tions, raising the possibility of sexual reproduction in C. furca. Changes in cellular parameters were reversible upon nutrient addition, and feeding decreased or ceased within hours to days. The find- ings presented here indicate that cellular nutrient quotas and ratios are more critical than absolute inorganic nutrient concentrations in regulating feeding behavior of C. furca.