Contrasting detoxification mechanisms of Chlamydomonas reinhardtii under Cd and Pb stress

Chlamydomonas reinhardtii has been frequently investigated for its resistance to metals; however, few studies have systematically compared the intracellular and extracellular processes involved in the detoxification of Cd and Pb by this microalga. We found that C. reinhardtii was more tolerant to Pb (concentration for 50% of the maximal effect; EC50: 29.48 ± 8.83 mg L−1) than to Cd (EC50: 12.48 ± 1.30 mg L−1) after 96 h of exposure. Extracellular polymeric substances (EPS), intracellular starch granules, lipid droplets, and glutathione were significantly increased under Cd and Pb treatments. Lead-containing particles were formed outside of the cells exposed to 30 mg L−1 of Pb, whereas no minerals were present when Cd was added. Various EPS functional groups, including –COOH, C–O–C (polysaccharides), and amide I and II (proteins), were involved in the interactions with Cd and Pb. The Pb removal rate (60.46–78.27%) by C. reinhardtii was higher than that of Cd (50.61–59.38%), and the microalgal cells with intact EPS bound more metals than those without EPS. Adsorption accounted for 79.62% of the total Cd accumulation in the low-Cd treatment, whereas absorption dominated the Pb accumulation at low Pb concentrations. The distributions of Cd and Pb in and out of the microalgal cells were reversed when the concentrations of the two metals increased. The detoxification strategies of C. reinhardtii for Cd and Pb were completely different, and these findings may assist in the phycoremediation of metal pollution in aquatic environments. [Display omitted] •Chlamydomonas reinhardtii was more tolerant to Pb than to Cd.•Generation of EPS, starch granules, lipid droplets, and GSH were induced by the two metals.•EPS enhanced the accumulation of Cd and Pb by C. reinhardtii.•Chlamydomonas reinhardtii had different detoxification strategies for Cd and Pb.