Nutrient regeneration in coastal seas by Noctiluca scintillans, a red tide-causing dinoflagellate

Ammonium nitrogen, phosphate, and silicate contents in Noctiluca scintillans cell fluid were estimated at 2470, 183, and 54 pmol per individual, respectively. Ammonium nitrogen and phosphate concentrations at the surface layer of the water column seemed to be related to cell abundance of N. scintillans at the sampling location. Patches of N. scintillans provided 16 and 25 times greater concentrations of ammonium nitrogen and phosphate in the uppermost layer (0-10 cm depth) of the water column than those in the ambient seawater, respectively. Silicate concentration within patches, however, was close to that in surrounding water. The morphology of fecal pellets of N. scintillans fed on the diatom, Thalassiosira sp., was examined with a scanning electron microscope. The fecal pellet was composed entirely of visible structural diatom cells. It could be inferred that N. scintillans excretes silica from the diatom undigested in its fecal pellets and thus no remarkably high silicate concentration within red tide patches was observed.