Effects of toxic and non-toxic Microcystis aeruginosa in different mixtures with Scenedesmus obliquus on growth of Brachionus calyciflorus

This study seeks to determine whether microcystins of unicellular Microcystis are the sole explanation for its adverse effects on rotifer growth. Brachionus calyciflorus were exposed to toxic and non-toxic strains of Microcystis aeruginosa separately and together with an edible alga (Scenedesmus obliquus) at different algal ratios and carbon compositions. Complete mortality was observed on B. calyciflorus cultured on toxic M. aeruginosa at all food concentrations (50, 100, and 400 μg C·L^-1) and non-toxic M. aeruginosa at 400 μg C·L^-1. Rotifers fed the pure non-toxic strain exhibited reduced viability at lower food concentrations. Overall, growth rate decreased with increased proportions of M. aeruginosa. In mixed-diet experiments, rotifers showed greater tolerance to the non-toxic M. aeruginosa. Diets composed of both toxic and non-toxic Microcystis combined with Scenedesmus slightly supported rotifer growth at 100 μg C·L ^-1. Due to the mixtures with sufficient S. obliquus at 400 μg C·L^-1, nutritional deficiency could be excluded. Results are in agreement with the fact that unicellular M. aeruginosa cultured in the laboratory is a poor food for aquatic herbivores and might contain other toxic inhibiting substances to B. calyciflorus.