Tolerance of Pseudochlorella pringsheimii to Cd and Pb stress: Role of antioxidants and biochemical contents in metal detoxification.

The tolerance and antioxidant response of the green alga P. pringsheimii to cadmium (Cd) and lead (Pb) was investigated. The algal biomass was constant at the relatively lower metal concentrations of Cd and Pb (5, 12 µM, and 2.5-200 µM, respectively), whereas higher concentrations severely inhibited the algal biomass yield. The pigment content of P. pringsheimii decreased due to the investigated metals, especially with Cd concentrations. However, the Pb concentrations of 2.5-200 µM enhanced the pigment content. The carotenoids content was highly repressed by the Cd concentrations. Nevertheless, Pb concentrations highly stimulated the carotenoids content, with the exception of 400 and 500 µM Pb. The biochemical contents of P. pringsheimii including phenolic, total soluble protein and carbohydrate contents responded variably to the investigated metals. The concentrations of Cd were found to be harmful to total soluble protein and carbohydrates, but not the phenolic contents. However, all biochemical contents were stimulated under relatively lower Pb concentrations. Markedly for Pb, the radical scavenging, reducing power, and chelating activities improved under the metals exposure excluding higher concentrations. The activities of the antioxidant enzymes (SOD, CAT, and POD) were highly stimulated with all treatments (except for CAT activities at the highest Cd and Pb concentrations, 300 and 500 µM, respectively). Remarkably, Cd treatments have higher antioxidant enzyme activities compared to that of Pb treatments. The antioxidants augmentation of P. pringsheimii under the metal stress may be exploited for future application in several fields.