MEPS 225:147-160 (2002)  -  doi:10.3354/meps225147

ABSTRACT: A dynamic mathematical model is presented for the growth and Paralytic Shellfish Poison (PSP) content of Alexandrium fundyense. The model includes cellular nitrogen-cycling to enable the synthesis of toxins in the absence of an external nitrogen-source. PSP synthesis is promoted by phosphorus stress but depressed by nitrogen stress, with the model containing sigmoidal functions relating nitrogen and phosphorus status to toxin synthesis. The model was tuned simultaneously to 4 sets of experimental data for ammonium- and nitrate-grown cultures of A. fundyense that were P replete or P-stressed. The good fit of the model, with a single set of control constants, to almost all data (cellular N:C and P:C, cell size, chlorophyll a:carbon, toxin content, external nutrients) demonstrates the capabilities of mechanistic models under dynamic situations. The potential consequences of recycling toxin-N, or of not making toxins at all, were considered using the model. Loss of toxin from cells could be by turnover or by leakage/excretion; model output and experimental data sets suggest that turnover may be the fate in P-stressed nitrate-grown cells but not in ammonium-grown cells. From simulations, there appears to be no significant disadvantage in expending nitrogen on toxin synthesis, thus there need not be a specific evolutionary advantage in the process. If PSP synthesis were selection-neutral this could explain the significant diversity in PSP synthesis capabilities within the genus Alexandrium.

KEY WORDS: Alexandrium fundeyense · Dinoflagellate · PSP toxins · Mechanistic model · Nutrient status