Phytochelatin induction, cadmium accumulation, and algal sensitivity to free cadmium ion in Scenedesmus vacuolatus

Phytochelatins are small, intracellular, metal-binding polypeptides produced by algae on exposure to increased metal concentration in their environment. The aim of the present study was to examine the relationship between phytochelatin concentration, bioaccumulated metal, and sensitivity of algal growth on cadmium exposure. For that purpose, intracellular cadmium concentration and thiol (glutathione, gamma-glutamylcysteine [gammaGluCys], and phytochelatins [PCn]) content were determined in the freshwater green alga Scenedesmus vacuolatus exposed to growth-inhibitory and noninhibitory concentrations of free Cd2+ in the range from 10(-14) to 10(-7) M. The algal growth rate was optimal up to a free Cd2+ concentration of 10(-9) M and then decreased by 40% at higher concentrations. The intracellular cadmium content increased sharply from 0.22 to 746 amol/cell over this free Cd2+ range. At the lowest Cd2+ concentration (control), glutathione was the only detectable thiol (127 amol/cell). With increasing Cd2+, formation of gammaGluCys and phytochelatins from PC2 to PC6 were observed. The predominant oligomer was PC3, with 42 amol/cell at the highest Cd2+ concentration (10(-7) M). The ratio of the concentration of total thiol groups to intracellular cadmium was important for maintaining optimal growth. In contrast, thiol groups from phytochelatins were never measured in excess to intracellular cadmium content.