Phycoremediation of methylene blue mediated by algal coculture: insights into morpho-physiological and metabolic responses.

Synthetic dyes are persistent environmental pollutants that pose significant ecological and toxicological risks. This study investigates the remediation potential of co-cultivated microalgae Chlorella vulgaris and Chlamydomonas reinhardtii for methylene blue (MB) removal. Optimal decolourization (100%) was achieved at 1-10 mg/L dye concentration and pH 7, with efficiency increasing over time. Morphological assessments revealed that C. vulgaris remained structurally stable with a slight change in the cell size, while C. reinhardtii exhibited reversible stress responses including palmella formation, cell aggregation, flagellar loss and cell wall thickening. FT-IR analyses showed the involvement of hydroxyl, amine, carboxylate and polysaccharide groups in the dye removal. Furthermore, the tentatively identified transformation products detected through LC-MS support the apparent attenuation of the MB dye. Biochemical analyses indicated inhibited pigment synthesis with elevated carbohydrate and protein levels at lower dye concentrations. Antioxidant enzyme profiling showed a biphasic oxidative stress response with enhanced catalase (CAT) and guaiacol peroxidase (POD) activity. In contrast, superoxide dismutase (SOD) and ascorbate peroxidase (APX) declined with increasing dye concentration, but were stimulated to a higher level in relation to the control. Additionally, phenolic and flavonoid content increased in a concentration dependent manner alongside an overall enhancement in total antioxidant activity across the treated concentrations. Furthermore, MTT assay results showed that the increased cell viability by approximately 14-17% following exposure to the algal coculture-treated dye solution highlights the efficacy of algal co-cultivation in dye remediation and stress mitigation.