Abstract
Freshwater algae possess remarkable metabolic flexibility and environmental resilience, enabling them to adapt to polluted habitats and contribute to ecological restoration. This study investigates the physiological and biochemical responses of five green algal taxa: Monoraphidium sp., Scenedesmus sp., Nephrocytium sp., Chlorococcum sp., and Klebsormidium sp. during a 25-day phycoremediation of contaminated water of the Yamuna River, New Delhi, India. The water, characterized by high concentrations of organic matter, nutrients, and heavy metals, induced species-specific metabolic adjustments. A decline in chlorophyll a and b (31.25% ± 2.25% to 67.11% ± 5.37% and 11.49% ± 0.25% to 86.98% ± 3.21%, respectively) indicated stress or damage to the photosynthetic system. This decline can be caused by various abiotic or biotic stress factors, while carotenoid accumulation, particularly in Chlorococcum sp. (307.70% ± 4.32%), suggested photoprotective adaptations. Enhanced biosynthesis of phenolic compounds and flavonoids in Chlorococcum sp. (139.33% ± 4.32% and 81.81% ± 2.72%, respectively) correlated with elevated antioxidant activity across all species (27.67% ± 1.61% to 73.51% ± 2.44% DPPH inhibition). Lipid content shifts were species-dependent, with Monoraphidium sp. showing the highest increase (63.02% ± 2.09%). Elemental CHNS analysis revealed increased carbon content and reduced nitrogen and sulfur levels, indicating altered nutrient dynamics. Principal Component Analysis (PCA) elucidated distinct clusters reflecting interspecific differences in stress-responsive metabolic traits. This study demonstrates the metabolic plasticity and stress tolerance of green algae under complex pollutant loads, advancing our understanding of algal adaptation mechanisms. It shows that phycoremediation not only enhances interspecific biochemical divergence but also alters algal elemental stoichiometry. By integrating multivariate biochemical analysis with CHNS profiling, we identify nitrogen as the primary driver of post-treatment differentiation. These findings highlight both the ecological and biotechnological relevance of algae in integrated water treatment and sustainable biomass utilization, while offering a novel framework for selecting candidate species in environmental remediation and biotechnological applications.