Chronic fluoride exposure induces tissue damage, oxidative stress, and immune dysfunction in juvenile crucian carp (Carassius auratus).

Fluoride pollution in aquatic ecosystems poses substantial risks to aquatic organisms. However, most previous studies have focused on acute toxicity and skeletal fluorosis, whereas the chronic toxic effects of fluoride on non-skeletal tissues in fish remain poorly understood. In particular, the mechanisms underlying fluoride-induced toxicity, especially those associated with immune dysfunction and intestinal microbiota disruption, have yet to be fully elucidated. In the present study, juvenile crucian carp (Carassius auratus) were used as an experimental model to investigate the chronic toxic effects of fluoride on aquatic organisms. Fish were divided into four groups: a control group, an environmentally relevant NaF group (5 mg/L), and two sub-lethal NaF exposure groups (50 and 95 mg/L). Samples were collected at 45 and 90 days post-exposure. The results showed that exposure to fluoride resulted in significant pathological damage to gill, liver, intestine, and kidney tissues in a dose- and time-dependent manner. After 90 days of fluoride exposure, the activities of antioxidant enzymes (CAT, SOD) and T-AOC in the gill, liver, intestine, and kidney were inhibited, while MDA levels were markedly increased, indicating enhanced oxidative stress. Meanwhile, the expression of immune-related genes, including NF-κB, IL-1, TNF-α, IL-8, C3, and LYZ-C, was significantly downregulated in the 5 and 50 mg/L groups but markedly upregulated in the 95 mg/L group, indicating a dose-dependent immune response. In addition, 16S rRNA sequencing showed that fluoride exposure induced gut microbiota dysbiosis, characterized by an increased abundance of potential pathogens Alsobacter and a reduced abundance of beneficial bacteria Gemmobacter and Dinghuibacter. Such dysbiosis may impair intestinal barrier function and disturb local immune homeostasis, thereby contributing to the altered immune responses. Overall, this study provides new insights into the molecular mechanisms underlying the chronic toxicity of fluoride in fish, particularly in non-skeletal tissues and the immune system, and highlights the ecological risks associated with long-term fluoride exposure in aquatic environments.