Sterols used in chemical defence pf the freshwater heterotrich ciliate Stentor polymorphus

Many species of ciliates belonging to the order Heterotrichida, are able to release toxic secondary metabolites for chemical defence against unicellular or multicellular predators. The chemical diversity of these toxic compounds is quite relevant and, sometimes, ciliate species on the same genus discharge very different kinds of molecules. In this study we analysed the chemical defence strategy of the freshwater heterotrich ciliate Stentor polymorphus against the predatory ciliate Coleps hirtus, and the microturbellarian flatworm Stenostomum sphagnetorum. The results showed that S. polymorphus defends itself discharging a mix of four sterols, ergosterol, 7-dehydroporiferasterol and their two peroxidised analogues. Sterols were isolated in vivo and characterized by liquid chromatography-mass spectrometry and nuclear magnetic resonance. Toxicity tests against other predators and competitors, indicated that at least, ergosterol and ergosterol peroxide, are essential for the efficiency of chemical defence in S. polymorphus. To the best of our knowledge, this is the first report about the use of sterols in predator-prey interaction among ciliated protozoa, highlighting the huge chemical diversity in the ciliate secondary metabolites.