Seasonal variation in antioxidative responses and acetylcholinesterase activity in Perna viridis in eastern oceanic and western estuarine waters of Hong Kong

A year-round study was conducted to assess the seasonal variations and potential influence of the riverine discharge from the Pearl River on biomarker responses in Hong Kong waters. A suite of biomarkers including antioxidant enzymes superoxide dismutase (SOD), catalase (CAT), the lipid peroxidation product malondialdehyde (MDA), a Phase II detoxification enzyme glutathione-S-transferase (GST) and the neural transmitter enzyme acetylcholinesterase (AChE) in the green mussel, Perna viridis, were monitored from three coastal sites, Port Shelter, Tung Chung and Tai O, stretching from the east to the west of Hong Kong. Despite of the seasonal variations, the total protein profiles suggested that mussels from the three sites had a growth cycle that was in phase with each other. This implied that intrinsic variation between sites due to a different phase of growth was minimal. Seasonal variations of the biomarker responses in the mussels were found to be significant (Tukey multiple comparison test, p<0.05) with a summer minimum and winter maximum. On top of seasonal variations, the western site, Tai O, was further subjected to the reduced salinity effect of the Pearl River discharge in the summer wet season. This was demonstrated by the significant July minimum in all the biomarker responses at Tai O in relation to the extreme low salinity of 8‰. Mussels from the western site also revealed a higher oxidative stress than those from the eastern side throughout the year (Tukey multiple comparison test, p<0.05), which could be caused by chemical pollutants from the Pearl River discharge. ANOVAs of the year-round dataset suggested that size was a minor factor in affecting the biomarker responses. Gill tissues of the mussels were more advantageous for biomarker studies or monitoring because their protein levels were less sensitive to seasonal variations and they yielded a higher protein normalized biomarker response than the whole body tissues. This increases their discrimination ability for site comparisons. The feasibility of biomarker monitoring in areas of sub-lethal or low chemical contamination will also be discussed.