Dias Machado, ManuelaSoares, EduardoBarata, Carlos2026-02-262026-02-262025-10-10Machado, M. D., & Soares, E. V. (2025). The Unhappy Effects of the Antidepressant Fluoxetine on the Freshwater Microalga Raphidocelis subcapitata. Toxics, 13(10), 876. https://doi.org/10.3390/toxics131008762305-6304http://hdl.handle.net/10400.22/31918Pharmaceuticals can be found in the aquatic environment and cause unwanted effects on organisms. The present work aimed to characterize the toxic mode of action of the antidepressant fluoxetine (FLX) on the freshwater microalga Raphidocelis subcapitata. With this aim, the microalga was exposed to low levels (µg/L) of FLX for 72 h. Exposure to 20–30 µg/L FLX arrested algal growth, which can be explained by the blockage of algal nuclear division. In addition, FLX (15–30 µg/L) deeply altered the alga’s metabolism, which was reflected by an increase in esterase activity, mitochondrial dysfunction (hyperpolarization of inner mitochondrial membrane), and reduction in the content of photosynthetic pigments: chlorophyll a (chla) and carotenoids (car). A sharp decline in photosynthetic performance, revealed by the reduction in maximum photochemical quantum yield (Fv/Fm), effective photochemical quantum yield (ΦPSII), and photosynthetic electron transport rate (ETR) of photosystem II (PSII), was also observed. FLX, at 30 µg/L, induced the intracellular accumulation of reactive oxygen species (ROS) and lipid peroxidation, with a marginal loss (1%) of cell membrane integrity. The results presented here contribute to the elucidation of the toxic mode of action of FLX on the microalgae R. subcapitata and, simultaneously, warn of the negative impact of the presence of pharmaceutical compounds in freshwater aquatic environments.engintracellular ROS accumulationlipid peroxidationmitochondrial dysfunctionoxidative stress biomarkersphotosynthetic activityPseudokirchneriella subcapitata (=Selenastrum capricornutum)toxic mode of actionresearch article10.3390/toxics13100876