Toxicological impacts of fluoxetine on the freshwater microalga Raphidocelis subcapitata: a multi-endpoint assessment

Introduction: Pharmaceuticals are increasingly recognised as active stressors in aquatic systems. Among them, the antidepressant fluoxetine (FLX) is frequently detected in freshwater systems. Despite its widespread occurrence, FLX mode of action, at the cellular level, remains poorly understood. This study aimed to investigate how FLX interferes with the physiology of the freshwater microalga Raphidocelis subcapitata at environmentally relevant concentrations.

Methods: Algal cultures were exposed to FLX (15−30 µg L⁻¹) in OECD medium for 72 h. Growth, metabolic activity, mitochondrial function, photosynthetic pigments content, photosynthetic performance, oxidative stress (via reactive oxygen species, ROS, accumulation and lipid peroxidation, LPO), and membrane integrity were evaluated.

Results: Exposure to 20−30 µg L⁻¹ FLX resulted in a pronounced inhibition of algal growth, likely associated with disruption of nuclear division. At 15−30 µg L⁻¹, FLX induced significant metabolic alterations, mitochondrial dysfunction (characterized by inner membrane hyperpolarization), and reduction in chlorophyll a (Chla), and carotenoids (CAR) content. Photosynthetic performance was severely impaired by decreases in maximum photochemical quantum yield (F_v/F_m), effective quantum yield (ɸPSII), and electron transport rate (ETR) of photosystem II. At 30 µg L⁻¹, oxidative stress was evident through ROS accumulation and increased LPO, while membrane integrity showed only minor disruption.

Conclusions: FLX acts as a multi-target disruptor in R. subcapitata, affecting cell division, metabolism, and photosynthesis in an integrated way. These findings highlight the ecological relevance of pharmaceutical contamination and emphasize the need to consider sublethal mechanistic effects when assessing risks in freshwater environments.