Differential effects of environmentally relevant norfloxacin on Microcystis aeruginosa and Chlorella vulgaris: Growth and metabolism

The ecological effects of antibiotics at environmentally relevant low concentrations have attracted considerable attention, but how they regulate algal growth and influence bloom risks through selective, dose-dependent, and algae-specific mechanisms remains poorly understood. Moreover, most conventional toxicological studies have used high concentrations at the mg/L level, which are orders of magnitude higher than actual environmental concentrations (ranging from ng/L to μg/L). To address this gap, the present study systematically compared the selective effects and metabolic mechanisms of norfloxacin on the cyanobacterium Microcystis aeruginosa and the green alga Chlorella vulgaris under environmentally relevant concentrations. Low NOR concentrations (0.01–0.5 μg/L) stimulated the growth of Microcystis aeruginosa , increased its photosynthetic pigment content, enhanced photosystem activity, and activated metabolic pathways related to nucleotide and amino acid synthesis. Interestingly, higher NOR concentrations (5–100 μg/L) suppressed photosynthesis, induced oxidative stress, promoted the release of extracellular organic matter, and markedly disrupted central energy metabolism, including carbon fixation, glycolysis, the pentose phosphate pathway, and the tricarboxylic acid cycle. In contrast, Chlorella vulgaris exhibited greater tolerance, as evidenced by maintained photosynthetic homeostasis, moderate metabolic reorganization, and no significant fluctuation in cell abundance. These findings indicate that environmentally relevant concentrations of norfloxacin may selectively stimulate the growth of the cyanobacterium Microcystis aeruginosa through a “low-dose promotion, high-dose inhibition” pattern mediated by key metabolic pathways. The differential algae-specific responses suggest that norfloxacin pollution could potentially increase the risk of cyanobacterial blooms, although this possibility remains to be validated through co-culture or field studies.