Modelling changes in paralytic shellfish toxin content of dinoflagellates in response to nitrogen and phosphorus supply

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 pro- moted 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 simulta- neously 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 conse- quences 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 cap- abilities within the genus Alexandrium.