This study examined the effect that silica content in diatom cells has on the behavior of protists. The diatoms Thalassiosira weissflogii and T. pseudonana were cultured in high or low light conditions to achieve low and high silica contents, respectively. These cells were then fed to a heterotrophic dinoflagellate Noctiluca scintillans and a ciliate Euplotes sp. in single and mixed diet experiments. Our results showed that in general, N. scintillans and Euplotes sp. both preferentially ingested the diatoms with a low silica content rather than those with a high silica content. However, Euplotes sp. seemed to be less influenced by the silica content than was N. scintillans. In the latter case, the clearance and ingestion rate of the low silica diatoms were significantly higher, both in the short (6-h) and long (1-d) duration grazing experiments. Our results also showed that N. scintillans required more time to digest the high silica-containing cells. As the high silica diatoms are harder to digest, this might explain why N. scintillans exhibits a strong preference for the low silica prey. Thus, the presence of high silica diatoms might limit the ability of the dinoflagellate to feed. Our findings suggest that the silica content of diatoms affects their palatability and digestibility and, consequently, the grazing activity and selectivity of protozoan grazers.

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.

Growth and feeding experiments using monospecific and mixed diets were conducted to investigate Noctiluca scintillans' food preference, and the influence of prey composition on its prey consumption and growth rates. In the feeding experiments on 14 monospecific diets of autotrophic and heterotrophic plankton, growth and feeding of N. scintillans showed no significant relationship with the cell size or swimming motility of the prey, but it generally grew better on diatoms and chlorophytes than other plankton. On the mixed diets composed of the diatom Thalassiosira weissflogii and a nutritionally superior (Platymonas helgolandica, chlorophyte), inferior (Prorocentrum dentatum, autotrophic dinoflagellate) or poor (Lepidodinium sp., heterotrophic dinoflagellate) prey (in terms of the ability to support N. scintillans growth), N. scintillans' feeding preference seemed to depend on the prey abundance in short (6 h) incubations. However, as incubation time increased (2 d), N. scintillans generally preferentially consumed the superior prey in food mixtures, regardless of prey abundance. Growth of N. scintillans on mixed diatom−dinoflagellate diets benefited from the increase of diatoms in the diets, but rates were lower than that achieved on T. weissflogii alone. When fed with diatom−chlorophyte prey pairs, growth of N. scintillans was slightly higher on the mixtures than on the single diatom diet, but there was no significant difference among them. Therefore, the dietary choice of N. scintillans may be a trade-off between maximizing food and nutrient intake and minimizing the energy cost of handling different prey. Such feeding strategies of N. scintillans should have great ecological significance on plankton bloom formation and succession.