Section of the gill of the reference fish showing the normal structure, secondary lamellae ( SL ), pillar cell ( PC ), normal epithelium (NE), and erythrocytes ( E ) in the lamellae 

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... Table 4 summarizes the data of LPO, GSH, SOD, CAT, and GST in gills of C. punctatus inhabiting in metal- contaminated canal. In the gills of exposed fish, there was an increase in the LPO, SOD, CAT, and GST levels; however, GSH level declined. In the present study, the heavy metals which were detected in the aquatic environment were Cu, Ni, Fe, Co, Mn, Cr, and Zn, which are redox active metals. Their accumulation in the gills was in the decreasing order of Zn>Fe>Mn>Cu>Cr>Co>Ni. All of them belong to first transition series of metals and are highly redox active. Hence, they generate reactive oxygen species (ROS) and free radicals which are responsible for damage to lipids, proteins, and DNA. The elevation or decline in these parameters could be due to carbonyla- tion or peroxidation by free radicals/ROS generated as a result of metal toxicity (Tabrez and Ahmad 2011c). Lipid peroxidation is a free radical-induced oxidative degeneration of lipids. It is initiated when the double bonds in unsaturated fatty acids of membrane lipids are attacked by oxygen-derived free radicals. ROS are also produced as intermediates in these reactions. The primary damaging effect of lipid peroxidation is exerted by the interactions with proteins and DNA which is revealed at the subcellular (cellular organelles), cellular, and organ levels (Wilhelm 1990). Fe plays the major role in the chain reaction of the generation of ROS and free radicals, leading to the production of lipid peroxides which interferes with the regulation of several metabolic pathways. In the present study, a rise in LPO level in the gills could be attributed to the accumulation of heavy metals, and the observed LPO levels showed that the membrane damage was presumably due to the generation of ROS since biomembranes are made up of a lipid bilayer containing various polyunsaturated lipids which are more susceptible to peroxidation. Similar observations were also reported in C. gariepinus inhabiting the Ogun River, Nigeria (Farombi et al. 2007); Oreochromis mossambicus exposed to H 2 S (Sreejai and Jaya 2010), and Chanos chanos inhabiting the Kaattuppalli Island polluted by petrochemicals, fertilizers, and oil refinery waste (Sivakumar et al. 2013). In the present study, all antioxidant enzymes and nonenzymatic parameters were reflecting the heavy metal- induced toxicity in the inhabiting fishes. The activity of antioxidant enzymes may be elevated or inhibited de- pending upon the type and concentration of the stressor. SOD lowers the cellular level of the superoxide radical anion (O 2 ) by converting it to H 2 O 2 ; therefore, its activity in the gills was elevated as compared to the reference. Similarly, CAT destroys H 2 O 2 which can penetrate through all the biomembranes and inactivate several enzymes (Vutukuru et al. 2006). In the present case, an increase in CAT activity was found in the gills. Similar findings have also been reported in O. mossambicus (Sreejai and Jaya 2010) and C. chanos (Sivakumar et al. 2013). SOD-CAT forms the first line of defense against xenobiotics (McCord 1996). In the present study, elevation in GST activity was also found (Table 4). Sivakumar et al. (2013) reported the elevation in GST activity in C. chanos . Be- sides the machinery of antioxidant enzymes, nonenzymatic defense system is also present for scavenging of ROS/free radicals in which GSH is one of them. In the present case, GSH level declined in the gills. A similar finding has been reported in C. gariepinus (Farombi et al. 2007). However, other workers reported an elevation in GSH levels along with an elevation in LPO, SOD, CAT, and GST (Sreejai and Jaya 2010; Sivakumar et al. 2013). GSH is an extremely important cell protec- tant since it directly quenches the reactive hydroxyl radicals. Therefore, maintenance of high levels of antioxidant enzymes and nonenzyme scavengers is essential to prevent ROS-mediated oxidative damage (Tabrez and Ahmad 2009). The histological alterations observed in the gills of C. punctatus are shown in Fig. 2 (reference) and Fig. 3 (exposed). The structure of the gill of exposed fish varied from that of the reference. The reference has a normal arrangement of secondary lamellae on the fila- ment with large spaces between them for fast diffusion of ions. However, the gill of exposed fish exhibited several histological alterations like complete fusion of lamellae coupled with hyperplasia, hypertrophy and epithelial lifting, necrotic and shrinked/curved lamellae, gill syneching and bridging, and infiltration of lymphocytes. In M. armatus , similar histopathological lesions have been reported except gill bridging, syneching, and infiltration of diffused lymphocyte (Javed and Usmani 2013c). Similar results have been reported in fishes such as L. rohita exposed to Cr (VI) (Sesha Srinivas and Rao 1998); Tilapia mossambica to Cu, Ni, and Cr (Ravanaiah and Narasimha 2010); C. carpio to Cr (Parvathi et al. 2011); and C. gariepinus to sewage and domestic wastewater containing Cu, Fe, Pb, Cd, and Zn (Authman et al. 2013). The gill epithelium is the site of exchange of gases, regulation of ionic and acid – base balance, and nitrogenous waste excretion by fishes (Evans 1987). However, in the present study, due to heavy metal exposure, the epithelium gets degenerated and separates from the lamellar tissue. Therefore, its osmoregulation function gets disturbed due to which fish may become hypoxic. It can, therefore, be argued that the gill epithelium was the principal entry point of contamination which, on exposure to heavy metals, would multiply, thereby causing hyperplasia. Moreover, these structural damages in the gill reflect that the fish was under chronic stress due to heavy metal accumulations as these are responsible for oxidative stress. So, to get rid of these xenobiotics, the fish ’ s immune response gets activated as the presence of diffused lymphocytes was recorded in the exposed gill (Fig. 3c). Moreover, other observed lesions in fish gill could also be interpreted as defense responses of the fish as these alterations increase the distance across which the dissolved heavy metals must diffuse to reach the bloodstream. The effluents released from the power plant contain heavy metals; consequently, in water, Fe and Ni were beyond the limits, while Cr was equal to the USA standards set for ecosystem and human health. These heavy metals, besides affecting the quality of water, is also influencing the major protein source in the form of fish since high levels of accumulations were observed in the gills of inhabiting fish, C. punctatus . Therefore, C. punctatus can suitably be used as a bioindicator for monitoring the water quality of the canal. Moreover, these metals also induce oxidative stress as these are redox active metals and are capable of generating ROS/ free radicals. ROS/free radicals consequently induce peroxidation in biomolecules like lipid and protein, which are the major constituents of membrane. This has been confirmed by an alteration in the antioxidant defense system of the fish. Despite the antioxidant defense system, severe pathology occurred in the gills, affecting the respiratory and osmoregulatory function of the fish. It is hence suggested that the fish gill can be used as a model system for the study of the adverse effects of toxicants in general and heavy metals in particular. It is especially because gills always remain in direct contact with the ambient environment, coun- teract first to the xenobiotics, and are metabolically active organ. Thus, LPO, SOD, CAT, and GST serve as suitable biomarkers of fish health. These power plants no doubt are useful to mankind, but on the other hand, they have adverse impacts on aquatic habitats, their diversity, and the livelihood of humans. Therefore, conservation of aquatic habitats is essential for main- taining the right balance in ...