Effects of dietary phytol supplementation on growth performance, immunological parameters, antioxidant and stress responses to ammonia exposure in common carp, Cyprinus carpio (Linnaeus, 1758)

The present study aimed to investigate the effects of dietary phytol on growth performance, humoral immune parameters, antioxidant and biochemical responses of common carp, Cyprinus carpio, to ammonia toxicity. Diets containing four concentrations of phytol (mg kg⁻¹): 0 (CTRL), 125 (PH125), 250 (PH250), and 500 (PH500) were fed to the fish (average weight: 26 ± 0.5 g) for 78 days; then the fish were exposed to 0.5 mg L⁻¹ unionized ammonia nitrogen. According to the results, growth performance and survival were similar among the treatments. Dietary phytol supplementation significantly increased plasma total immunoglobulin (Ig) and complement (ACH50); the highest ACH50 levels were observed in PH250 and PH500 treatments, as the highest globulin and Ig levels were observed in PH250 treatment. In addition, PH250 and PH500 treatments presented significantly higher plasma lysozyme activity compared to CTRL, with the highest activity in PH250 treatment. Dietary phytol induced no changes in plasma cortisol, alanine aminotransferase and aspartate amino transferase, and hepatic malondialdehyde levels before ammonia exposure; whereas, it increased hepatic superoxide dismutase (PH125, PH250, PH500), catalase (PH250, PH500), glutathione peroxidase (PH125), plasma total antioxidant capacity (PH125, PH250, PH500) and decreased plasma glucose (PH125, PH250), and malondialdehyde (PH125, PH250). Ammonia exposure led to elevations in plasma cortisol, glucose, alanine aminotransferase, aspartate amino transferase, malondialdehyde and hepatic antioxidant enzymes and malondialdehyde levels; but decreased plasma total antioxidant capacity. Dietary phytol mitigated the changes in plasma cortisol, glucose, malondialdehyde, alanine aminotransferase, aspartate aminotransferase, total antioxidant capacity, and hepatic malondialdehyde levels, particularly at 125 and 250 mg kg⁻¹ levels. On the other hand, the phytol-treated fish exhibited higher elevation in hepatic antioxidant enzymes after ammonia exposure, compared to CTRL. In conclusion, the present results indicate phytol is an immunostimulant in common carp and can stimulate antioxidant system to protect the fish against oxidative stress caused by ammonia exposure.