The increasing use of antidepressants, especially fluoxetine (FLX), has resulted in their presence in aquatic environments due to wastewater discharges from municipal, industrial, and hospital sources. Simultaneously, microplastics (MPs) have been extensively studied in short-term (acute) exposures, showing adverse effects such as oxidative stress, behavioral alterations and neurotoxicity. However, the embryotoxic and teratogenic effects of these compounds, as well as their impacts on the survival, development, morphology, behavior, and reproduction of fish embryos in aquatic ecosystems, remain limited. This study evaluated the toxic effects of FLX (5, 40 ng/L), MPs (25, 100 particles/L) and their mixtures (FLX-MPs) at environmentally relevant concentrations. Microscopic characterization of fluorescent MPs was performed, and their presence was evaluated in Danio rerio embryos at 24 and 96 hpf, observing their localization in the chorion. In embryotoxicity and teratogenesis tests, FLX (100 ng/L) increased the rate of malformations, including deformations of the spine (CD), tail (TM) and hypopigmentation (H)
MPs induced scoliosis (S), tail deformation (TM), incomplete hatching (IE) and a dose-dependent increase in malformations. FLX-MPs mixtures caused pericardial edema (PE), tail and spine deformation, delayed hatching (HR) and increased mortality. Oxidative damage analysis showed that FLX (40 ng/L) dose-dependently increased SOD and CAT activities, with an increase in cellular oxidation biomarkers (LPX, POX, HPX). Besides, MPs (100 particles/L) showed similar effects, with increased SOD, CAT, POX and HPX activities. The FLX-MPs mixture showed the most pronounced response. In gene expression, FLX (5, 40 ng/L) modulated genes such as bax, blc2 and casp3. MPs (25, 100 particles/L) induced the expression of bax, blc2, p53 and casp3. FLX-MPs mixtures (25 particles/L-40 ng/L, 100 particles/L-40 ng/L) expressed nfr1, p53, nfe2l2a and casp3. Histological damage revealed abnormal muscle fibers (AMF) and yolk sac edema (YSE) at 40 ng/L FLX, and lamellar fusion (LF) and scoliosis (S) in MPs (100 particles/L). Brain swelling (IBT) and neuromast loss (NL) were detected in FLX-MPs mixtures. In conclusion, both FLX and MPs and their combinations affect the embryonic development and physiological state of Danio rerio.