Metal accumulation and metallothionein induction in Oreochromis niloticus grown in wastewater fed fishponds

Pisciculture by using composite effluent of Kolkata city is a common indigenous practice in East Calcutta Wetlands (ECW), a Ramsar site in India. Apprehension over the consequences of heavy metal pollution through this practice led to the present study to examine metal accumulation related histological changes in gill and liver tissues of Tilapia (Oreochromis niloticus) cultivated in the wastewater fed fishponds of ECW if any. Further, induction of metallothionein, in liver tissues of the same fish was also studied. Concentration of metals (chromium(Cr), manganese(Mn), Iron(Fe), Copper(Cu), Zinc(Zn) and lead(Pb)) were evaluated and found higher in both water and bottom sediment of the wastewater fed fishponds. In fish, liver was the major site for accumulation of metals with a metal profile of Fe > Cu > Zn > Mn > Cr > Pb. Microscopic studies including optical, scanning electron microscopic (SEM), transmission electron microscopic (TEM) and atomic force microscopic (AFM) studies showed considerable changes. Higher expression of metallothionein was found in the liver of contaminated fish hepatocytes through flow cytometry, immunohistochemistry (IHC) and immuno-gold labelling (IEM). It is assumed that chronic exposure of different metal complexes lead to tissue-specific changes in organisms which may also reflect their potential to adapt survival strategies.