Toxicological Effects of Combined Exposure of Cadmium and Enrofloxacin on Zebrafish

The combined pollution of cadmium (Cd) and enrofloxacin (ENR) in aquatic environments represents a critical issue in environmental toxicology. Using zebrafish as model organisms, we systematically investigated the combined toxicity of Cd and ENR through both acute (96-h) and chronic (20-d) exposure experiments. Our results demonstrated significant synergistic effects: co-exposure reduced the 96-h LC50 values from 89.12 mg/L (Cd alone) and 190.11 mg/L (ENR alone) to 46.35 mg/L and 99.39 mg/L, respectively (combined effect index = 0.96). Chronic exposure revealed that ENR enhanced Cd accumulation in the liver, intestine, and muscle tissues by 1.11-2.33-fold compared to single Cd exposure. Oxidative stress markers showed dynamic temporal changes, with superoxide dismutase (SOD), glutathione peroxidase (GSH-Px), and catalase (CAT) activities initially increasing by 1.34-7.06-fold, 0.98-3.28-fold, and 1.53-3.65-fold at 8 d, respectively, followed by 9.9-48.98% reductions after 20 d of exposure. Malondialdehyde (MDA) levels progressively accumulated, reaching up to 4.06-fold higher than controls. Notably, co-exposure elevated oxidative stress by 11.24-34.48% relative to single exposures. The 16S rDNA sequencing analysis indicated that Cd exposure significantly reduced the α-diversity of zebrafish gut microbiota (57-63% decrease in Shannon index), characterized by a 16-20% reduction in beneficial <i>Cetobacterium</i> and a 44-114% increase in pathogenic <i>Aeromonas</i> abundance. The combined exposure further exacerbated these gut microbiota dysbiosis patterns. These findings provide crucial evidence for ecological risk assessment, suggesting that current environmental standards based on single-pollutant evaluations may substantially underestimate the actual risks of heavy metal-antibiotic co-contamination in aquatic ecosystems.