Considerations on the toxigenic nature and prey sources of Phalacroma rotundatum

Abstract

The heterotrophic dinoflagellate Phalacroma rotundatum (Claparède & Lachman) Kofoid & Michener is considered a toxic species, but there is controversy about its toxigenic nature. In the present study, about one-third of the toxin analyses done with liquid chromatography-mass spectrometry (LC-MS) of P. rotundatum specimens picked from field populations in Galicia (NW Spain) between 2003 and 2005 revealed traces of lipophilic toxins-okadaic acid (OA) and/or dinophysistoxin- 2 (DTX2) and/or pectenotoxin-2 (PTX2) (if any)-that mimicked the toxin profile of cooccurring toxigenic mixotrophic species of Dinophysis (D. acuminata, D. acuta, and D. caudata). Thus, during the period of study, P. rotundatum was never a relevant contributor to diarrhetic shellfish poisoning (DSP) toxins contaminating shellfish resources in Galicia. Observations of phycoerythrin-like autofluorescence in P. rotundatum and in its co-occurring potential ciliate prey-Tiarina cf. fusus- led to the suspicion that P. rotundatum had taken up toxins by feeding on this ciliate prey that had previously fed on Dinophysis spp. Nevertheless, toxins in P. rotundatum specimens with orange auto - fluorescence were under detection levels, and the source of these orange pigments may be a prey different from Dinophysis spp. (e.g. Myrionecta spp.). New results here add evidence to suggest that P. rotundatum does not produce toxins de novo, but acts as a vector from toxin-containing prey to shellfish, and that M. rubra may be one of its potential ciliate prey. Conclusive testing of these hypotheses is now under investigation with laboratory cultures of Dinophysis and Phalacroma spp. and M. rubra.