Heavy metals removal by the microalgae Chlorococcum dorsiventrale : insights from enzymatic and molecular analysis

Abstract

Heavy metals are one of the main threats to marine life and ecosystems and any remedial action in that regard is urgently required. The aim of this work is to study the bioaccumulation of cadmium, chromium and lead in the microalgae Chlorococcum dorsiventrale isolated from Tunisian coastal waters along with assessing its enzymatic and molecular responses. This strain tolerated Cd and Cr accumulation while resisting that of Pb. Cd and Cr exposures elicited a decrease in chlorophyll, lipid and polysaccharide contents whereas no damages were detected following Pb treatment. For protein content, no significant changes were seen except for Pb which exhibited a slight increase after treatment with 5 mg/L. Stress defense-related genes expression using qRT-PCR revealed that exposure to Pb and Cr induced an up-regulation of catalase, superoxide dismutase and photosystem II protein D1 encoding genes while heat shock protein 70 was slightly overexpressed. As a result, removal efficiencies for Cr and Pb attained 89% and 95% respectively. The mechanisms whereby C. dorsiventrale was able to induce Cr biosorption were intracellular and extracellular adsorption while Pb was predominantly removed by adsorption.