Chemical Defence by Sterols in the Freshwater Ciliate Stentor polymorphus

Simple Summary Predator-prey interactions are widely observed in nature and involve not only animals but also unicellular organisms. In this context, the basic techniques for offence or defence adopted by predators and prey can be mediated by mechanical/morphological and/or chemical strategies. Among unicellular organisms, ciliated protozoa represent a paradigmatic model for the investigation of both mechanisms. Therefore, in this study, we choose to investigate the defensive functions adopted by the ciliate Stentor polymorphus against predators that share the same aquatic ecosystem. On the basis of the experimental observations, we found that the defensive strategy evolved by this ciliate is essentially chemical and mediated by a mix of sterols. The defensive function of these molecules appears as a novelty, at least among the ciliated protozoa. Overall, our study represents an advance in the knowledge of the ethology and ecology of ciliates, with particular regard to the natural compounds that this group of organisms has selected in the last 1.8 billion years. Heterotrich ciliates typically retain toxic substances in specialized ejectable organelles, called extrusomes, which are used in predator-prey interactions. 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 is able to defend itself against these two predators by deploying a mix of bioactive sterols contained in its extrusomes. Sterols were isolated in vivo and characterized by liquid chromatography-mass spectrometry (LC-MS), and nuclear magnetic resonance (NMR), as ergosterol, 7-dehydroporiferasterol, and their two peroxidized analogues. The assessment of the toxicity of ergosterol and ergosterol peroxide against various organisms, indicated that these sterols are essential for the effectiveness of the chemical defence in S. polymorphus.