Assessing lead toxicity in the clam Ruditapes philippinarum: Bioaccumulation and biochemical responses

Lead (Pb) is a non-essential metal. Its occurrence in the environment is related principally to anthropogenic contamination. Pb is toxic to aquatic organisms and can provoke damage to membranes and inhibit the activity of essential enzymes. The filter-feeding, Manila clam Ruditapes philippinarum is widely used as a biomonitor organism to assess metal toxicity. Among biomarkers related to the Pb toxicity, the enzymatic activity of δ-aminolevulinic acid dehydratase (δ-ALAD) has been adopted as a specific tool. Metallothionein (MT), lipid peroxidation (LPO) and antioxidant enzymes activities, such as catalase (CAT), glutathione peroxidase (GPx), glutathione reductase (GR) and glutathione S-transferase (GST) and superoxidase dismutase (SOD) have also been employed to assess the toxic effect of metals. Two target tissues, the gills and the digestive gland, were selected to examine biomarker responses. In order to assess the effects of Pb accumulation and the mechanisms involved in the recovery from it, clams were exposed at two Pb levels (10 and 100 µg/L) for 7 days and were later maintained in clean water for 7 days as a depuration period. Pb accumulation was dependent on the exposure concentration and higher Pb levels were observed in the gills compared to the digestive gland. Inhibition of δ-ALAD, GST and SOD and the induction of MT and LPO over the exposure period were observed in the gills and the digestive gland of R. philippinarum. The depuration period showed a continuous inhibition of the δ-ALAD activity and induction of MT and LPO in both tissues. These results demonstrate that lead induced an exposure effect and the 7 days of depuration were not sufficient to recover the basal health status of the clams.