An Individual- Based Model for the Toxic Algae Species Pseudo-nitzschia multiseries

In 1987 an outbreak of a previously unobserved disease occurred in Canada and was traced back to the toxin domoic acid produced by the diatom Pseudo-nitzschia multiseries. Since then, fisheries closures due to domoic acid have occurred worldwide. Pseudo-nitzschia species produce domoic acid under nutrient stress, including low silicon or phosphorus under high nitrogen conditions. However, it is still unclear what conditions cause the dangerously high levels that have sometimes been observed. We present an individual-based algae model detailing the physiology of an algal cell with a focus on nutrient and energy flows to delineate the causes of domoic acid production. The model has been adapted to the specific problem of Pseudo-nitzschia multiseries by including silicon dynamics, a frustule component, domoic acid production, and sexual reproduction. The individual model is incorporated into a population model using a McKendrick-von Foerster partial differential equation. The model is compared to experimental data from chemostat and batch experiments on two separate strains of Pseudo-nitzschia multiseries. The differences in parameter values required to fit each experiment reveal differences in the physiology of the two strains, specifically in nutrient uptake, photosynthetic rate and the level of toxin production possible. Simulations using the calibrated model show that silicon limitation must be concurrent with an abundance of nitrogen for domoic acid production to be high.