الفهرس 
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Abstract
The present study was designed to evaluate the toxic effect of sublethal concentration of mercuric chloride (0.3 mg/L HgCl2) on hemato-immunological parameters and oxidative and antioxidants responses in liver. Also, bioaccumulation and histopathological alterations in different organs of Nile tilapia (Oreochromis niloticus) and the protective effect of microalgae, Nannochloropsis oculata against induced toxicity for 3 weeks. Fish were randomly assigned to 4 groups, group1: control (basal diet), group 2 (Hg/ exposed to HgCl2 at a dose of 0.3 mg/L (1/4 of LC50), and fed basal diet), group 3: (Hg+N5, similar to group2, but fed diet supplemented with N. oculata 5% and group 4 (Hg+N10, similar to group2, but fed diet supplemented with N. oculata 10 %. Our results revealed significant decrease in the values of total Red blood cells (RBCs) count, hemoglobin (Hb), packed cell volume (PCV), White blood cells (WBCs) count and lymphocytic count and significant increase in the values of mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH), monocyte and neutrophil counts in Hg exposed group while supplementation with N. oculata restored these values within the control. Glutathione (GSH) and Malondialdehyde (MDA) levels were significantly increased while glutathione peroxidase (GPx) activity was significantly decreased at all time points. Catalase (CAT) activity was significantly increased only at day 7 in Hg exposed group while N. oculata supplementation restored their values to the control. Lysozyme activity and immunoglobulin M (IgM) were significantly decreased in Hg exposed group while supplementation with N. oculata elevated their values. Mercury was significantly accumulated in all examined organs with bioaccumulation pattern of liver>gills>muscle. Histopathological lesions included Vacuolar degeneration, Congestion, hemorrhage, focal necrosis and leukocytic infiltration in the liver. kidney showed vacuolated tubular epithelium, congestion, hemosiderosis and hemorrhage with hyaline droplet degeneration and necrosis while the gill lesions included hyperplasia of the gill epithelium with severe loss of primary and secondary lamellae. supplementation with N. oculata at both concentrations reduced these lesions to great extent.