Attachment of picophytoplankton to the cell surface of the toxic dinoflagellates Dinophysis acuminata and D. fortii

Coccoid picophytoplankton cells (1-2 μm in diameter) were observed to attach to the cell surface of the toxic dinoflagellates Dinophysis acuminata and D. fortii in water samples collected in Maizuru Bay, Japan, in spring of 1998. A relatively large number of picophytoplankton cells were noticed in the cingulum near the flagellar-pore region in D. fortii. These picophytoplankton cells were not cyanobacteria. Food vacuolate cells of D. fortii were observed. In view of the existence of a tongue-like structure (food ingestion organelle), the size of the aperture of the flagellar pore (cytostome) of about 1.5 μm, and the finding of picophytoplankton cells attached to the Dinophysis cell surface, we suggest that the food microorganisms for mixotrophic D. acuminata and D. fortii are picophytoplankton. The food picophytoplankton may be composed of different species, and some of them may contain diarrhetic shellfish poisoning toxins and/or their precursors. We suggest that Dinophysis species such as D. acuminata and D. fortii may originally be nontoxic and may only become toxic secondarily, through the ingestion of toxic picophytoplankton. This working hypothesis could reasonably explain the marked fluctuations observed in the toxicity of D. acuminata and D. fortii.