Species-Specific Photosynthetic Inhibition and Lipid Remodeling in Freshwater Microalgae Exposed to Pb(II) Stress

Lead (Pb) is a persistent environmental contaminant that poses a significant risk to aquatic primary producers by disrupting photosynthesis, cellular metabolism, and physiological homeostasis. This study investigated the physiological and biochemical responses of two freshwater green microalgae, Mucidosphaerium pulchellum and Micractinium pusillum, exposed to Pb(II) concentrations ranging from 0 to 30 mg L−1. Species-specific responses were evaluated through growth kinetics, chlorophyll fluorescence, lipid accumulation, and fatty acid profiling to elucidate mechanisms underlying Pb stress tolerance. Pb(II) exposure resulted in concentration-dependent inhibition of growth and photosynthetic activity in both species, with marked reductions in quantum yield and increases in photosynthetic inhibition at the highest exposure concentration (30 m L−1), reaching approximately 95% in M. pulchellum and 93% in M. pusillum. Lipid metabolism exhibited distinct species-specific responses: M. pulchellum showed a progressive decline in total lipid content, whereas M. pusillum exhibited enhanced lipid accumulation under moderate Pb exposure followed by depletion under severe stress. Fatty acid analysis revealed significant membrane lipid remodeling, characterized primarily by reductions in polyunsaturated fatty acids, particularly α-linolenic acid (C18:3), with stronger alterations observed in M. pulchellum. These findings demonstrate that Pb toxicity involves interconnected effects on photosynthetic efficiency, carbon allocation, and membrane lipid composition, with species-specific differences in physiological resilience. The combined application of chlorophyll fluorescence and lipid-related biomarkers provides a sensitive approach for assessing heavy-metal stress responses and improving understanding of Pb tolerance mechanisms in freshwater microalgae. These findings provide a physiological basis for the development of sustainable biomonitoring approaches for freshwater ecosystems.