Oxidative stress in rainbow trout (Oncorhynchus mykiss) exposed to sewage treatment plant effluent.

Effluents from sewage treatment plants (STPs) can be regarded as "hot spots" of discharge releasing large amounts of chemicals into the aquatic environment. Many of these compounds are toxic to organisms due to their ability to form reactive oxygen species (ROS) and cause oxidative stress. In order to investigate if STP effluents contain compounds that may cause oxidative stress, rainbow trout (Oncorhynchus mykiss) were exposed to effluent from a Swedish STP at different dilutions in a flow-through system. Antioxidant enzymes analyzed were glutathione reductase (GR), catalase (CAT) and DT-diaphorase (DTD). Catalytic activities of CYP1A (EROD) and the conjugating enzyme glutathione-S transferase (GST) were also analyzed. Results indicate that the effluent contains prooxidants since the activities of the antioxidant enzymes GR, CAT, and DTD were all elevated after 5 days of exposure. A prolonged exposure resulted in an inhibition of DT diaphorse activity, suggesting a depleted cellular ROS defence. EROD activities increased in a dose- and time-dependent manner, which suggests the presence of aryl hydrocarbon receptor (AHR) ligands such as polycyclic aromatic hydrocarbons (PAHs) in the effluent. These results indicate that STPs do not have the capacity to biodegrade harmful chemicals sufficiently to protect the aquatic environment. However, STPs are designed to remove nutrients and not persistent pollutants from the sewage and effort should be made to diminish the amount of chemicals entering the sewage in the first place.