First toxic effects of all tested heavy metals on Dunaliella were studied. Investigating the resistance of Dunaliella to each of the metals, live cells were cultured in solutions containing each metal ion. Growth curve of Dunaliella alga in the presence of various concentrations of Pb is represented in Figure 1. Results have demonstrated that the inhibitory effects of Pb increased as the concentration of this element was elevated in the culture medium. As stated in Figure 1, 40 mg/l Pb caused death in many of the algal cells on the first day. Dunaliella alga growth curve, in the presence of different concentrations of Cd is represented in Figure 2. The inhibitory effects of Cd on the algal cell growth increased with the elevation of Cd concentration. As shown in Figure 2, 20 and 40 mg/l Cd resulted in a significant amount of growth inhibition as compared to the control group. The effect of Cd, especially on the second day, was much less toxic on the algal cell growth as compared to that of Pb. Figure 3 represents the growth curve of Dunaliella alga in presence of different concentrations of Hg. Results have demonstrated that as the Hg concentration increases, the inhibitory effect of this element increases too. At 20 and
40 mg/l Hg, the growth inhibition could be observed in comparison to the control groups. Toxic effects of Hg, upon this alga, are lesser than those of Pb in contrast to Cd (Figure 3). Results of toxic effects of heavy metals