Simultaneous Cd 2+ , Zn 2+ , and Pb 2+ Uptake and Accumulation by Photosynthetic Euglena gracilis D. G. Mendoza-Cózatl, E. Rangel-Gonzµlez, R. Moreno-Sµnchez Departamento de Bioquímica, Instituto Nacional de Cardiología, Juan Badiano 1, Sección XVI Tlalpan, 14080 MØxico, D.F. MØxico Received: 12 August 2005 /Accepted: 31 March 2006 Abstract. The ability of Euglena gracilis to simultaneously remove and accumulate Zn 2+ , Cd 2+ , and Pb 2+ from culture up- to media was evaluated. E. gracilis was able to remove up to 80% of the Cd 2+ present in the medium when cultured with 20 or 50 lM CdCl 2 . Higher external Cd 2+ concentrations in- creased Cd 2+ accumulation per cell but decreased cell growth, thus decreasing the capacity of the cell culture to remove Cd 2+ . E. gracilis removed 70% to 80% of the Zn 2+ present in the medium when cultured with 5 to 50 lM ZnSO 4 . Zn 2+ did not affect Cd 2+ removal capacity. E. gracilis was much less effi- cient in removing Pb 2+ (<15%) when cultured with 100 or 200 lM Pb(NO 3 ) 2 . Moreover, Pb 2+ decreased the efficiency to remove Cd 2+ , but it did not affect Zn 2+ removal. Cd 2+ induced a generalized increase in the cellular thiol compounds, including phytochelatins, and Pb 2+ had an additive effect only at 200 lM. Zn 2+ did not stimulate phytochelatin synthesis. Cd 2+ and Pb 2+ colocated in the same cytosolic high-molecular- weight fraction. Because Pb 2+ is a weak phytochelatin inducer, competition between Pb 2+ and Cd 2+ for transportation across the plasma membrane and binding to phytochelatins and other thiol compounds is proposed to explain the detrimental effects of Pb 2+ on the Cd 2+ removal capacity of E. gracilis. Industrial activity and the inappropriate disposal of residues have turned heavy-metal pollution into a serious environ- mental problem. Removal of heavy metals from polluted waters by the use of plants and microorganisms, a process called ‘‘bioremediation,’’ is an expanding technology with several advantages over physical remediation methods (Salt et al. 1995; Dhankher et al. 2002). Thus, several works have studied the ability of bacteria, plants, yeast, and microalgae and macroalgae to tolerate, bind, and accumulate heavy metals (Salt et al. 1995; Trevors et al. 1996; Hamdy 2000; Nedelkoska and Doran, 2000; Dhankher et al. 2002). Euglena gracilis is a photosynthetic, free-living, unicellular protist that has been widely used for the study of the bio- chemical mechanisms involved in the resistance to heavy metals such as Hg 2+ , Cd 2+ , Cu 2+ , Cr 6+ , and Pb 2+ (Bariaud et al. 1985; Coppellotti 1989; Navarro et al. 1997; Devars et al. 1998, 2000; Cervantes et al. 2001; Einicker-Lamas et al. 2003). For instance, decreased Cd 2+ uptake accounts for the differential Cd 2+ sensitivity in resistant and sensitive strains of photosyn- thetic E. gracilis (Bariaud et al. 1985). Also, enhanced content of thiol compounds (cysteine, glutathione, and phytochelatins), together with active intracellular compartmentalization into chloroplasts and mitochondria, has been described for Cd 2+ resistance in E. gracilis (Mendoza-Cózatl et al. 2002; AvilØs et al. 2003). Cu 2+ accumulation in intracellular vesicles has also been suggested (Einicker-Lamas et al. 2002), whereas volatilization and increased levels of glutathione were reported for Hg 2+ resistance (Devars et al. 2000; see also Mendoza- Cózatl et al. 2005 and Perales-Vela et al. 2006 for reviews on heavy-metal detoxification in Euglena). Cd 2+ resistance in Euglena has been evaluated under dif- ferent conditions. For instance, photosynthetic cells grown under light-to-dark cycles have a remarkable Cd 2+ resistance and accumulation capacity (inhibitory concentration of 50% (IC 50 ) = 100 lM; Devars et al. 1998), whereas cells cul- tured under constant illumination show a lower resistance (IC 50 = 10.6 lM; Einicker-Lamas et al 1996). E. gracilis may develop increased Cd 2+ resistance when pre-exposed to low Cd 2+ concentrations, although this resistance is mainly asso- ciated with a decreased Cd 2+ uptake capacity (Devars et al. 1998). E. gracilis, grown under heterotrophic conditions in the dark and pretreated with low Hg 2+ concentrations, also exhibit enhanced Cd 2+ accumulation capacity: Cells cultured in the presence of 200 lM CdCl 2 for 96 hours accumulated 11.2 g Cd 2+ /Kg dry weight (1.1%) (AvilØs et al. 2005). Therefore, E. gracilis may be a suitable model for bio- remediation of heavy-metal–polluted water bodies (Devars et al. 1998; Mendoza-Cózatl et al. 2002). However, few works, if any, have determined the ability of E. gracilis to remove heavy metals from aqueous solutions, either synthetic or natural wetlands. Furthermore, most of the experiments related to heavy metal removal using free-living organisms have been performed with only one heavy metal (Kaplan et al. 1995; Carr et al. 1998; Matsunaga et al. 1999). Although Correspondence to: D. G. Mendoza-Cózatl; email: dgmcozatl@ yahoo.com.mx Arch. Environ. Contam. Toxicol. 51, 521–528 (2006) DOI: 10.1007/s00244-005-0207-4