Ingestion and clearance rates of the red Noctiluca scintillans fed on the toxic dinoflagellate Alexandrium minutum (Halim)

In this study, we evaluated the ingestion and clearance rates of the heterotrophic dinoflagellate Noctiluca scintillans (Macartney) fed on the toxic microalgae Alexandrium minutum Halim in laboratory cultures. Specimens of the red form of N. scintillans were collected in the Ría de Vigo and kept in culture. The results showed that N. scintillans actively fed on A. minutum, with maximum ingestion rates of 0.37 μg C ind−1 day−1, showing that there was non-satiated feeding by N. scintillans on A. minutum. The mean cellular toxin content of A. minutum in the stock culture ranged from 2.57 to 3.44 fmol cell−1. The HPLC analyses of the extracts from N. scintillans fed with 4 different abundances of A. minutum revealed no detectable amounts of toxin. This suggested that although N. scintillans was able to ingest a toxic dinoflagellate such as A. minutum into its vacuoles, the ingested toxin was bioconverted or excreted by Noctiluca. Our findings support the theory that Noctiluca may inflict grazing pressure on the growth of Paralytic Shellfish Toxin (PST) species in the field, and could therefore play an important role as a regulator against PST-producing phytoplankton. This is the first report about grazing rates of N. scintillans on Alexandrium cells.

Research highlights▶ The ingestion and clearance rates of red Noctiluca scintillans were evaluated. ▶ Noctiluca fed on the toxic PSP producer Alexandrium minutum. ▶ No detectable amounts of toxin were measured in Noctiluca. ▶ The toxin was bioconverted or excreted by Noctiluca.