Algal monitoring programs for Diarrhoetic Shellfish Poisoning (DSP) are generally based on cell counting; however, the quantification of diarrhoetic toxins (DT) in phytoplankton could strongly improve their efficacy. Therefore, we modified a very sensitive PP2A inhibition assay, previously set up to determine DT in mussels (Tubaro et al. 1996), in order to quantify okadaic acid (OA) and its derivatives directly in algal samples. The phytoplankton was collected by vertical net hauls from bottom to surface to concentrate the phytoplankton present in the water column. After centrifugation, the phytoplankton samples were extracted with 80 % methanol, washed with hexane and dried under vacuum. To be submitted to the PP2A inhibition assay, the dried extracts were resuspended with an appropriate amount of methanol. In our conditions the assay was sensitive to 0.08 ng of OA equivalents per L of sample (0.008 ppb). The reproducibility of the assay was good as well as its accuracy: the CV ranged from 1% to 5% and the recovery of OA in "spiked" phytoplankton samples ranged from 102% to 105%. Natural phytoplankton samples, containing different concentrations of Dinophisis ssp. (mostly D.fortii and D.caudata), were analyzed using the modified PP2A inhibition assay. A significant correlation was found between the DT concentration and the D.fortii cell number in the sample (r=0.9978; n=4), allowing to calculate an OA equivalent content of about 50 pg /cell. On the contrary, no correlation was found between the toxins content and the D.caudata cell number or the total Dinophysis cell number. These data confirm the main role of D.fortii in DSP contamination in the Gulf of Trieste, Northern Adriatic Sea.

Tubaro A., Florio C., Luxich E., Sosa S., Della Loggia R., Yasumoto T. - Toxicon (1996) 34:743-752.