Europium Oxide Disrupts Metabolic Homeostasis and Antioxidant Defenses in African Catfish ( Clarias gariepinus ): Integrated Physiological and Molecular Docking Approaches

ABSTRACT Chemical pollution from rare earth elements (REEs) has emerged as a significant threat to the stability of freshwater ecosystems. Among these elements, europium oxide (Eu 2 O 3 ) is widely used across various industrial sectors; however, its specific impacts on fish physiology, health, and immune function remain poorly understood. This study aimed to evaluate the effects of Eu 2 O 3 on growth, hepatic markers, intestinal enzyme, systemic immune response, and cytokine expression in African catfish ( Clarias gariepinus ). Fish were divided into four experimental groups and exposed to Eu 2 O 3 concentrations of 0 (Eu0), 5 (Eu5), 10 (Eu10), and 50 (Eu50) μg/L of water for 28 days. The results revealed a lower doses (Eu5) appeared to stimulate growth, whereas higher doses (Eu10 and Eu50) significantly impaired growth performance, digestive enzymes, and hepatic damage ( p < 0.01). High doses of europium (Eu10 and Eu50) significantly suppressed immune function, as shown by lower IgM, IgG, lysozyme activity, and C3 levels (linear effect, p < 0.05). High concentration exposure induced severe oxidative stress levels (ACE, myeloperoxidase, cortisol, glucose, MDA, and nitric oxide), along with a depletion of antioxidant status (TAC, GSH, and SOD) (linear effect, p < 0.05). Compared to the control, the Eu10 and Eu50 groups showed significantly higher levels of proinflammatory cytokines (IL‐1β, IL‐6, and TNF‐α) and significantly lower activities of mitochondrial metabolic enzymes (citrate synthase and malate dehydrogenase) (linear effect, p < 0.05). Histological examination revealed severe tissue damage in the kidneys and intestines of catfish exposed to high concentrations of Eu 2 O 3 . Molecular docking analyses suggested that Eu 2 O 3 potentially binds to ATP, cytochrome P450 aromatase, and HSP70, proposing that it potential triggers mitochondrial dysfunction and disrupts metabolic homeostasis. Overall, exposure to elevated concentrations of Eu 2 O 3 causes marked immune dysfunction, impairs antioxidant defenses, damages mitochondrial pathways, and provokes systemic inflammation, ultimately compromising the physiological integrity of C. gariepinus .