Biophysical and Biochemical Assessment of Azithromycin-Induced Phytotoxicity in Giant Duckweed Spirodela polyrhiza

The increasing occurrence of antibiotic residues in aquatic ecosystems raises concerns regarding their phytotoxic effects on non-target primary producers. This study investigated the impact of azithromycin on growth, photosynthetic performance, and oxidative stress responses in Spirodela polyrhiza. Azithromycin exposure caused a concentration- and time-dependent inhibition of growth, accompanied by a decline in chlorophyll content, indicating reduced light-harvesting capacity. Chlorophyll fluorescence analysis revealed a widespread impairment of Photosystem II, characterized by reductions in the maximum quantum yield of primary photochemistry, electron transport efficiency, and overall photosynthetic performance. In contrast, only minor changes were observed in fluorescence parameters associated with the electron transport chain, suggesting the absence of a localized blockage. Instead, electron transport efficiency was reduced, leading to increased excitation pressure and a shift toward nonphotochemical energy dissipation. Photosynthetic disruption was closely associated with oxidative stress, as moderate azithromycin exposure enhanced the activities of the antioxidant enzymes superoxide dismutase and catalase, whereas higher concentrations caused oxidative damage, as indicated by increased malondialdehyde accumulation. Flavonoids and anthocyanins exhibited a biphasic response, reflecting an initial activation of antioxidant defenses followed by metabolic suppression under severe stress. These effects are attributed to inhibition of chloroplast protein synthesis, which compromises Photosystem II functionality and disrupts cellular redox balance. Overall, azithromycin induces phytotoxicity through coordinated alteration of photosynthetic efficiency and oxidative homeostasis, highlighting chloroplast translation as a critical target and emphasizing the ecological risks of macrolide antibiotics in aquatic environments.