Modelling growth and toxin production in Ostreopsis cf. ovata

The dinoflagellate Ostreopsis cf. ovata, by producing large amounts of toxins, represents a serious threat for human health. In this work, we used a combined experimental and modelling approach to investigate the mechanisms underpinning growth and toxin production in this alga. Using a batch culture experiment, we estimated the internal nutrient to carbon ratios of O. cf. ovata obtaining a significant inverse relationship between cellular toxin content and cellular nutrient to carbon ratios. A new model, describing cellular toxin production and fate, was then developed and incorporated in a simplified version of the European Regional Seas Marine Ecosystem Model (ERSEM) to test the hypothesis that toxin production is stimulated by nutrient deficiency. More specifically, toxin production was assumed to be composed by two distinct additive terms: the first is constant and is assumed to take place under any physiological conditions while the second is assumed to be dependent on cellular nutrient deficiency. With these assumptions, the model is able to reproduce the temporal evolution of biomass, chlorophyll, nutrient and toxins observed in the cultures and this allows us to support the starting hypothesis. Our results also suggest that the cellular content of chlorophyll in O. cf.ovata is tightly regulated by internal nutrient quota while the release of organic carbon is minimal even under severe nutrient limitation. We suggest that high production of toxic compounds along with the cellular carbon accumulation with respect to nutrient and chlorophyll are both mechanisms aimed to reduce the palatability of the alga and therefore to reduce the grazing pressure.