Harnessing Chlorella vulgaris for the phycoremediation of azo dye: A comprehensive analysis of metabolic responses and antioxidant system

Abstract

This study investigates the phycoremediation potential of microalgae Chlorella vulgaris in the degradation of Direct green 6 (DG6), a synthetic azo dye commonly used in the textile industry, which poses significant environmental and health risks due to its toxic and carcinogenic properties. Over 50 days of treatment, we analyzed the effects of varying concentrations of DG6 on the biodegradation capabilities of C. vulgaris with the characterization of growth and antioxidant parameters. The findings demonstrate that C. vulgaris reduced DG6 levels significantly (p < 0.05) at higher temperatures (40 °C) compared to other environmental ambient temperatures. Within the acidic range pH < 7 progressive removal efficiency was observed within 25 days in consistency with the enhanced growth indices of biomass concentration (X_m), productivity (P_x), specific growth rate (μ_m), and doubling time (t_d) at the higher concentration of 60 mg L⁻¹. Induction of both enzymatic and non-enzymatic antioxidant activities were quantified for superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), ascorbate peroxidase (APX), azoreductase, and laccase, as well as changes in the total ascorbate pool (AsA + DHA) and ferric reducing antioxidant power (FRAP). Furthermore, the underlying biodegradation mechanisms were elucidated by identifying the reductive cleavage of azo bonds by azoreductase and breakdown by peroxidases and laccase. These molecular by-products were identified using gas chromatography–mass spectrometry (GC–MS), which shed light on the metabolic pathways involved in DG6 biodegradation. This study underscores the effectiveness of C. vulgaris as a sustainable, low-cost solution for the bioremediation of azo-dye-polluted water. This study lays the groundwork for further exploration into the genetic and metabolic adaptations of algae under complex organic pollutant stress, with potential implications for ecophysiology, biotic interactions, and evolutionary biology.