Serotonin Application Decreases Fluoxetine-Induced Stress in Lemna minor and Spirodela polyrhiza

The aim of this study was to evaluate the impact of fluoxetine, a widely used selective serotonin reuptake inhibitor, on two aquatic plants: <i>Lemna minor</i> and <i>Spirodela polyrhiza</i>. Additionally, the effect of exogenous serotonin on the level of fluoxetine-induced stress in duckweed will be studied. Increasing presence of antidepressants in surface waters poses ecological risks, and the duckweed species are ideal model organisms for ecotoxicological studies due to their rapid growth and ability to accumulate pollutants. For 14 days, plants were exposed to fluoxetine (0.001-150 mg L^-1), followed by a recovery phase in a drug-free medium or a medium supplemented with exogenous serotonin. We analysed morphological/physiological parameters (frond length and area, fresh and dry mass, hydration, stomatal size), the activity of antioxidant enzymes (catalase, ascorbate peroxidase, superoxide dismutase), cell viability, and the level of heat-shock proteins. The plants' ability to remove fluoxetine from the medium was also assessed. High fluoxetine concentrations (50-150 mg L^-1) significantly reduced fresh mass (by 63-98% in <i>L. minor</i> and 56-97% in <i>S. polyrhiza</i>), frond area (by 21-48% in <i>L. minor</i> and 11-25% in <i>S. polyrhiza</i>), and cell viability (by 36-94% in <i>L. minor</i> and 49-94% in <i>S. polyrhiza</i>), and induced oxidative stress. Despite this, both species showed high regeneration potential after the stressor's removal. Serotonin supplementation did not affect morphology but increased antioxidant enzyme activity, improved cell viability, and elevated heat-shock proteins levels. Crucially, serotonin significantly increased the efficiency of fluoxetine removal. The data can provide a basis for predicting fluoxetine removal efficiency in plants with different levels of endogenous serotonin. <i>L. minor</i> and <i>S. polyrhiza</i> exhibit substantial tolerance to fluoxetine, and antioxidative enzymes are sensitive markers of this stress.