Enzymatic biomarkers in the crab Carcinus maenas from the Minho River estuary (NW Portugal) exposed to zinc and mercury

Zinc and mercury are common contaminants of estuaries and coastal ecosystems where they may induce adverse effects on the biota. Carcinus maenas is a key-species in several European estuaries, living in close association with the sediment where a considerable number of environmental contaminants, including zinc and mercury may accumulate. In the present study, the acute effects of zinc and mercury on C. maenas from the Minho River Estuary (NW Portugal) were investigated by using the activity of the enzymes cholinesterase (ChE), lactate dehydrogenase (LDH) and glutathione S-transferases (GST) as effects’ criteria. Crabs were exposed for 96 h to several concentrations of Zn2+ (0, 1.84, 3.71, 7.39 and 14.79 mg/l) or Hg2+ (0, 0.09, 0.19, 0.37 and 0.74 mg/l). The choice of Zn2+ test concentrations was based on the LC50 value (14.86 mg/l) determined in a first part of the study, while the choice of Hg2+ concentrations was based on values from the literature. At the end of the bioassays, eye, muscle and hepatopancreas tissues were isolated and used for ChE, LDH and GST determinations, respectively. Zinc significantly inhibited ChE activity (p < 0.05, EC50 = 14.68 mg/l), caused significant alterations of GST activity (p < 0.05) and induced LDH activity (p < 0.05) at the exposure of 14.79 mg/l. Mercury significantly inhibited ChE activity (p < 0.001, LOEC = 0.09 mg/l, EC50 = 0.235 mg/l) and increased both GST (p < 0.05, LOEC = 0.774 mg/l) and LDH activities (p < 0.05, LOEC = 0.119 mg/l). These results suggest that both metals interfere with cholinergic neurotransmission in C. maenas by inhibiting ChE activity. In addition, they also interfere with GST activity and this may reduce the capacity of detoxification of some chemicals and/or to increase the probability of oxidative stress to occur. Furthermore, both metals increase LDH activity, suggesting an interference with energy production pathways. Therefore, the presence of zinc and mercury in estuaries at concentrations in the mg/l or μg/l range, respectively, may have a negative impact on C. maenas.