The physiological ecology of diarrhetic shellfish poisoning (DSP) toxin production by the dinoflagellate Exuviaella lima (Ehr.) Butschli

The diarrhetic shellfish poisoning (DSP) toxins okadaic acid (OA) and dinophysistoxin-1 (DTX-1) are potent phosphatase inhibitors produced by the dinoflagellate Exuviaella lima. It is known that the majority of the toxins are stored in an inactive form within E. lima as DTX-4 and can be enzymatically hydrolysed through an intermediate, OA diol-ester, to yield OA following cell lysis. The significance of the ester intermediate and the biological function of the toxins are not known.; The physiological ecology of DSP production by E. lima was examined using quantitative bioassay and measures of toxin production and excretion by E. lima in culture. Liquid chromatography with UV or mass spectrometric detection was used for toxin quantification. A computer-controlled turbidostat was constructed to facilitate the physiological characterization of toxin production by E. lima during steady-state growth. Both OA and DTX-1 were shown to inhibit phytoplankton growth at concentrations of about 1 muM in seawater, but not E. lima, suggesting an allelopathic role for the toxins. However, physiological data did not support this interpretation. The DSP toxins were shown to be constitutive metabolites, positively correlated with growth and not released from the cell in sufficient quantity to inhibit other phytoplankton. Quantitative, structure-activity relationships determined for DTX-4, OA diol-ester and OA using the diatom Thalassiosira weissflogii demonstrated comparable toxicity for both OA and the diol-ester intermediate contradicting the accepted notion that only the free-acids were toxic. Mass-spectral analysis of post assay material found that T. weissflogii was metabolizing the diol-ester. The relative partitioning of OA diol-ester and the metabolites between cells and medium supported the interpretation that OA diol ester, unlike OA, can readily penetrate cell membranes and once within the target cell undergo hydrolysis to release the active toxin. The sequential hydrolysis of the primary DSP toxin DTX-4 through a lipophilic ester intermediate may, therefore, play an important role in the uptake and transfer of OA in the food web and also constitute an interesting mechanism of chemical defence directed at predators.