Effects of cadmium on growth and superoxide dismutase activity of the marine microalga Tetraselmis gracilis (Prasinophyceae)

Marine planktonic algae are frequently exposed to metallic contaminants. Because heavy metals can be assimilated and accumulated by algal cells, they can then be transferred to higher trophic levels of food chains. We studied the effects of cadmium on protein production and the growth of the marine prasinophyte Tetraselmis gracilis (Kylin) Butcher. By means of toxicological assays, we estimated the LC(50) of cadmium as 3.2 ppm and 1.8 ppm after 48 h and 96 h of exposure to this heavy metal, respectively. The growth curves and survival percentages of cell cultures in the presence of cadmium were determined, and a proportional reduction of both parameters with increasing metal concentrations was found. When chronically exposed to sublethal concentrations of cadmium, T. gracilis contained high levels of superoxide dismutase (SOD) activity, one of the main enzymes of the cell's antioxidant defense mechanism. Under these growth conditions, total SOD activity in crude extracts was increased by 41% (at 1.5 ppm) and 107% (at 3.0 ppm). Assays of SOD activity in nondenaturing polyacrylamine gels also showed a similar induction by cadmium. These results show that cadmium has potentially toxic properties since it significantly inhibited the growth of T. gracilis at low concentrations and promoted the induction of SOD activity, suggestive of an oxidative stress state. Besides being the first report of SOD in T. gracilis, this work describes experimental evidence of SOD induction by cadmium in this species.