The Mutual Effect of Microparticles and Antidepressants on the Protozoan Spirostomum ambiguum (Müller, 1786) Ehrenberg, 1835

Antidepressants, especially selective serotonin re-uptake inhibitors, which are among the most commonly used pharmaceuticals, are ubiquitous in effluents and freshwaters. Microparticles, including microplastics, show sorption properties to different compounds, thus becoming a potential vector of toxic substances. This study aimed to evaluate the effects of four antidepressants on the protozoan Spirostomum ambiguum in the presence of four types of microplastics and baker’s yeast. The Spirotox, measuring the acute toxicity, and food uptake inhibition assay were applied. The microparticles did not influence the toxicity of the tested antidepressants in the acute toxicity assay. Moreover, they did not adsorb the drugs during a seven-day incubation in dark. However, sublethal levels of sertraline and duloxetine decreased the number of food vacuoles formed by the protozoa. The highest effect was observed in the case of the suspension of edible particles of baker’s yeast, where a significant decrease in the number of food vacuoles was observed in the sertraline concentration as low as 0.025 mg L−1. A lower but statistically significant effect was observed when wettable microparticles of phenolic resin were used as the artificial food source. These results indicate that serotonin re-uptake inhibitors can interfere with the feeding processes of ciliates.