Toxicity effects of copper on the marine diatom, Chaetoceros calcitrans

Chaetoceros calcitrans is widely present as a dominant marine diatom in estuarine and coastal waters and therefore a potential target to the toxicity effects exerted by trace metals. Copper induced changes in growth and chlorophyll content in C. calcitrans were analysed. At all the concentrations above 320 μg/L, the pro-oxidant copper caused a significant decrease in growth rate and chlorophyll concentration. The 96 h IC50 value of copper based on percentage growth inhibition of C. calcitrans was 450 μg/L. Copper concentration in the ambient water ranged from 2.38 to 608 μg/L. The kinetics of copper accumulation in intracellular and extracellular compartments of the cells were determined and correlated with the corresponding antioxidant enzyme activity (catalase, superoxide dismutase, ascorbate peroxidase, glutathione reductase) on exposure to 50, 180 and 450 μg Cu/L for 168 h. Copper adsorption to binding sites on the cell surface is relatively fast but further uptake into the cell is slow and considered as the rate limiting step. At sublethal concentrations as 50 and 180 μg/L, intracellular accumulation of copper increased linearly over 168 h. The intracellular accumulation of copper showed significant positive correlation (p<0.05) with catalase (CAT) and superoxide dismutase (SOD) activity when exposed to 50, 180 and 450 μg/L. The reduction in the CAT and SOD activity on prolonged exposure to 450 μg/L could be due to inactivation of enzyme as a result of higher reactive oxygen species (ROS) production. The extended period of exposure as in the present study resulted in sufficient accumulation of copper to activate CAT and SOD activity. The transient reduction in GR activity at 450 μg/L could be due to the inactivation of enzyme by copper. The results suggest that slower uptake together with an efficient antioxidant system is to some extend contributes to the tolerance of the species to copper.