Gill Oxidative Stress and Histopathological Biomarkers of Pollution Impacts in Nile Tilapia from Lake Mariut and Lake Edku, Egypt

Various oxidative stress and histopathological biomarkers in gill tissues of Nile tilapia Oreochromis niloticus were investigated. Fish were collected from four sites that differ in their extent of pollution load, including heavy metals: the southeast basin (SEB), main basin (MB), and northwest basin (NWB) of Lake Mariut; and Boughaz El-Maadiya, a channel in Lake Edku. The oxidative stress biomarkers that were analyzed included lipid peroxidation (LPO), superoxide dismutase (SOD), catalase (CAT), and glutathione redox cycle enzymes (glutathione peroxidase [GPx] and glutathione reductase [GR]). Levels of reduced glutathione (GSH) were also evaluated. Gill morphology was analyzed by light microscopy and scanning electron microscopy (SEM). Gill LPO was significantly higher in gill tissues of fish collected from the more heavily contaminated MB (40.0%) and NWB (51.4%) sites than in gill samples from the less-contaminated (reference) site, the SEB. Gill LPO in fish from Lake Edku was intermediate but was not significantly higher (17.1%) than the reference. The activities of antioxidant enzymes and the redox-sensitive thiol compound GSH were significantly lower in gill samples from the disturbed sites than in samples from the reference site. Specifically, SOD in MB, NWB, and Lake Edku samples; CAT and GPx in NWB samples; and GR activity and GSH content in MB and NWB samples were lower than those in SEB samples. In most cases, gill tissues from Lake Edku fish had intermediate levels of antioxidants. The main histopathological alterations observed in gills were epithelial lifting, hyperplasia and hypertrophy of the respiratory epithelium, lamellar fusion, and aneurysms. In addition, SEM results demonstrated transformation of the surface structure of epithelial pavement cells. Pathological reactions in the gills of Nile tilapia were most severe at the MB and NWB sites. Our findings suggest that Nile tilapia responded differently according to the environmental stress index in each sampling area. This study is the first to report gill oxidative stress and histopathologies in Nile tilapia from Egyptian aquatic environments.