Protective role of probiotics or Spirulina platensis supplementation against mercury-induced hematological, histopathological, and antioxidant-immune gene dysregulation in Nile Tilapia (Oreochromis niloticus).

Abstract

Mercury (Hg) contamination in the aquatic environment poses serious ecological and food safety concern due to its non-degradability, persistence, and bioaccumulation in fish tissues, particularly in Nile tilapia (Oreochromis niloticus), a widely farmed species. This study evaluated the efficacy of probiotics (Bacillus subtilis, Lactobacillus plantarum, and L. buchneri) and dietary Spirulina platensis supplementation on Hg-induced toxicity and associated damage in Nile tilapia, particularly regarding hematological alterations, gill histopathology and antioxidant-immune gene regulation. A total of 250 monosex tilapia (10.74 ± 1.23 g; 8.93 ± 0.43 cm) were allocated into four groups: control (T1), Hg-exposed (0.03 mg L⁻¹; T2), Hg + probiotics (0.03 mg L⁻¹ + 1.0 mL L⁻¹; T3), and Hg + S. platensis (0.03 mg L⁻¹ + 50 g kg⁻¹ diet; T4) each maintained in triplicate tanks (n = 20 per tank) for 42 days. Hg exposure (T2) significantly altered hematological indices and increased the frequency of erythrocytic cellular (twin, elongated, spindle) and nuclear (notched nuclei, nuclear bridge, nuclear bud) abnormalities. Additionally, gill of Hg-exposed fish exhibited severe degeneration, hypertrophy, and hyperplasia in both primary and secondary lamellae. At the molecular level, the T2 group showed a significant upregulation in the mRNA expression levels of antioxidant-related genes (SOD and CAT) (p < 0.05), alongside a significant downregulation in the mRNA levels of immune-related cytokine genes (IL-1β, TNF-α, and IFN-γ), compared with the control group. Supplementation with water additives probiotics and dietary S. platensis remarkably restored hematobiochemical parameters (glucose and hemoglobin), reduced erythrocytic anomalies, alleviated gill tissue deformities, and restored antioxidant and immune gene expression toward control levels. Collectively, this study demonstrates that probiotics or S. platensis effectively counteract Hg-induced hematological, histopathological, and molecular stress in Nile tilapia, underscoring their potential as sustainable dietary interventions to enhance fish health and aquaculture productivity under heavy metal exposure.