Lipid peroxidation vs. antioxidant modulation in the bivalve Scrobicularia plana in response to environmental mercury--organ specificities and age effect

This study aimed at the assessment of mercury burden and its association to damage vs. antioxidant protection in the bivalve Scrobicularia plana environmentally exposed to mercury. Inter-age and organ-specific approaches were applied by using different annual size classes (2+, 3+, 4+ and 5+ year old) and assessing specific organs (gills, digestive gland), respectively. Bivalves were collected from moderately and highly contaminated sites at Laranjo basin - Ria de Aveiro (Portugal), where a mercury gradient was identified, and compared with those from a reference site. Besides total and organic mercury accumulation, endpoints combining lipid peroxidation (LPO), as a damage sign, and antioxidant protection (catalase, CAT; glutathione peroxidase, GPX; glutathione S-transferase, GST, activities) were determined. The whole-body accumulation as total mercury changed according to the environmental gradient for all age classes, while in terms of organic mercury, only 5+-year-old animals showed increased accumulation with increased environmental mercury level. Mercury induced peroxidative damage, showing that antioxidative mechanisms were insufficient. The adaptive capacity to pro-oxidant challenge, expressed as antioxidant induction and lesser vulnerability to enzyme inhibition, increased with age. The specific analyses of gills and digestive gland revealed that both organs were able to mirror external levels of exposure in the accumulation of total and organic mercury. Nevertheless, gills displayed higher potential to accumulate organic forms. The organ specificity was evident for antioxidant response with a clear pattern of overall increase in gills and decrease in digestive gland. In addition, only gills displayed statistical correlations between oxidative stress responses and mercury accumulation. Besides the contribution to understand mercury toxicodynamics, specific organs approach is strongly recommended in order to avoid misinterpretations. The use of whole-body analyses can be particularly compromising when oxidative stress responses (rather than mercury accumulation) are addressed.