Heavy metals phycoremediation using tolerant green microalgae: Enzymatic and non-enzymatic antioxidant systems for the management of oxidative stress

Heavy metal pollution is one of the most serious problems facing the world today. Given their toxicity and persistence in the environment, the treatment of these pollutants is of great concern. The use of microalgae is a promising solution for the remediation of metal-contaminated wastewater. In the present work, eight microalgae strains: Chlorella vulgaris, C. ellipsoidea, C. sorokiniana, C. pyrenoidosa, Scenedesmus dimorphus, S. obliquus, Chlamydomonas reinhardtii, and Aphanothece sp. were screened for their tolerance against heavy metals: copper (Cu (II)), chromium (Cr(VI)), lead (Pb(II)) and cadmium (Cd(II)). The outcomes revealed that S. obliquus was the most tolerant strain to Pb(II), with an EC50 value of 141 ppm. The phycoremediation potential of Pb(II) by this strain was carried out in scaling-up culture conditions. The Pb(II)-removal efficiency reached over fifty percent after 18 days of culture. The measurement of oxidative stress markers in S. obliquus cells after Pb(II) phycoremediation, showed a significant increase in the level of hydrogen peroxide (H2O2) and lipid peroxidation (MDA). Interestingly, non-enzymatic antioxidant systems, including alpha,alpha-diphenyl-beta-picrylhydrazyl (DPPH) radical scavenging activity, proline, ascorbate, and polyphenol production. Besides, the ROS-scavenging enzymes including superoxide dismutase, catalase, guaiacol peroxidase, ascorbate peroxidase and glutathione reductase activities were significantly higher in the biomass from Pb(II)-medium compared to the control medium. This indicates that the oxidative stress caused by Pb(II) ions in S. obliquus cells could be attenuated via these non-enzymatic and enzymatic pathways. Our study highlights S. obliquus as a promising strain for the phycoremediation of polluted effluents on a large-scale.