Effects of prey of different nutrient quality on elemental nutrient budgets in Noctiluca scintillans

Noctiluca scintillans (Noctiluca) is a cosmopolitan red tide forming heterotrophic dinoflagellate. In this study, we investigated its ingestion, elemental growth yield and excretion when supplied with different quality food (nutrient-balanced, N-limited and P-limited). Total cellular elemental ratios of Noctiluca were nearly homeostatic, but the ratio of its intracellular NH₄⁺ and PO₄^3- was weakly regulated. Noctiluca thus seems able to differentially allocate N and P to organic and inorganic pools to maintain overall homeostasis, and it regulated its internal N more strongly and efficiently than P. The latter was substantiated by its comparatively stable C:N ratio and compensatory feeding on N-limited prey. Using both starvation experiments and mass balance models, it was found that excretion of C, N, and P by Noctiluca is highly affected by prey nutritional quality. However, based on modeling results, nutrients seem efficiently retained in actively feeding Noctiluca for reproduction rather than directly released as was shown experimentally in starved cells. Moreover, actively feeding Noctiluca tend to retain P and preferentially release N, highlighting its susceptible to P-limitation. Recycling of N and P by Noctiluca may supply substantial nutrients for phytoplankton growth, especially following bloom senescence.