Combined effects of antifouling biocides on the growth of three marine microalgal species.

The toxicity of the antifouling compounds diuron, irgarol, zinc pyrithione (ZnPT), copper pyrithione (CuPT) and copper was tested on the three marine microalgae Tisochrysis lutea, Skeletonema marinoi and Tetraselmis suecica. Toxicity tests based on the inhibition of growth rate after 96-h exposure were run using microplates. Chemical analyses were performed to validate the exposure concentrations and the stability of the compounds under test conditions. Single chemicals exhibited varying toxicity depending on the species, irgarol being the most toxic chemical and Cu the least toxic. Selected binary mixtures were tested and the resulting interactions were analyzed using two distinct concentration-response surface models: one using the concentration addition (CA) model as reference and two deviating isobole models implemented in R software; the other implementing concentration-response surface models in Excel, using both CA and independent action (IA) models as reference and three deviating models. Most mixtures of chemicals sharing the same mode of action (MoA) were correctly predicted by the CA model. For mixtures of dissimilarly acting chemicals, neither of the reference models provided better predictions than the other. Mixture of ZnPT together with Cu induced a strong synergistic effect on T. suecica while strong antagonism was observed on the two other species. The synergy was due to the transchelation of ZnPT into CuPT in the presence of Cu, CuPT being 14-fold more toxic than ZnPT for this species. The two modelling approaches are compared and the differences observed among the interaction patterns resulting from the mixtures are discussed.