Mixotrophy in Fragilidium subglobosum (Dinophyceae): Growth and grazing responses as functions of light intensity

Growth and ingestion rates of the marine mixotrophic dinoflagellate Fragilidium sub-globosum were studied as functions of light intensity with and without the presence of particulate food (Ceratium spp.). F. subglobosum was shown to be a facultative mixotroph; it was capable of phototrophic growth in monocultures as well as of heterotrophic (phagotrophic) growth in the dark. When F. subglobosum was growing in light in food-satiated cultures, growth rates were higher than pure phototrophic growth rates at corresponding light intensities. At high light intensities, ingestion was inversely related to light intensity, but this did not apply to lower light intensities. Phagotrophy thus accounted for 34 to 100% of total growth in food-satiated cultures according to light intensity. Chlorophyll a content per cell of F. subglobosum decreased with increasing light intensity and ingestion of prey also resulted in a decreased chlorophyll a content. Phototrophic monocultures of F. subglobosum needed approximately 24 h following the addition of prey to fully adapt to phagotrophic metabolism.