Simultaneous Cd2+, Zn2+, and Pb2+ uptake and accumulation by photosynthetic Euglena gracilis

The ability of Euglena gracilis to simultaneously remove and accumulate Zn2+, Cd2+, and Pb2+ from culture up- to media was evaluated. E. gracilis was able to remove up to 80% of the Cd2+ present in the medium when cultured with 20 or 50 microM CdCl2. Higher external Cd2+ concentrations increased Cd2+ accumulation per cell but decreased cell growth, thus decreasing the capacity of the cell culture to remove Cd2+. E. gracilis removed 70% to 80% of the Zn2+ present in the medium when cultured with 5 to 50 microM ZnSO4. Zn2+ did not affect Cd2+ removal capacity. E. gracilis was much less efficient in removing Pb2+ (<15%) when cultured with 100 or 200 microM Pb(NO3)2. Moreover, Pb2+ decreased the efficiency to remove Cd2+, but it did not affect Zn2+ removal. Cd2+ induced a generalized increase in the cellular thiol compounds, including phytochelatins, and Pb2+ had an additive effect only at 200 microM. Zn2+ did not stimulate phytochelatin synthesis. Cd2+ and Pb2+ colocated in the same cytosolic high-molecular-weight fraction. Because Pb2+ is a weak phytochelatin inducer, competition between Pb2+ and Cd2+ for transportation across the plasma membrane and binding to phytochelatins and other thiol compounds is proposed to explain the detrimental effects of Pb2+ on the Cd2+ removal capacity of E. gracilis.