Ciprofloxacin Alters Cyanobacterial Competition through Extracellular Organic Matter Composition.

Antibiotic pollution poses increasing risks to freshwater ecosystems, yet its effects on phytoplankton competition, particularly cyanobacterial succession, remain poorly understood. This study investigates how ciprofloxacin (CIP) regulates the competitive relationship between Microcystis aeruginosa and Anabaena sp. by altering physiological traits and extracellular organic matter (EOM) composition. Growth and pigment assays showed that M. aeruginosa was highly sensitive to CIP, exhibiting substantial growth inhibition (39.42%) and chlorophyll a reduction (58.93%) even at 5 μg/L, with inhibition further increasing to 97.61% and 99.30%, respectively, at 15 μg/L. In contrast, Anabaena sp. showed limited growth inhibition and maintained relatively stable pigment levels under CIP exposure. Excitation-emission matrix fluorescence revealed that CIP strongly stimulated EOM production in Anabaena sp. and in M. aeruginosa but suppressed EOM release in the coculture. Liquid chromatography-mass spectrometry analysis identified 67 extracellular metabolites and showed that coculture specific metabolites decreased from 15 to 2 under CIP exposure, indicating weakened competition induced metabolic activity. Ciprofloxacin sharply reduced allelopathic and signaling compounds in M. aeruginosa, while Anabaena sp. maintained or enhanced stress related metabolites. By impairing photosynthesis and allelopathic capacity in M. aeruginosa and strengthening the stress tolerance and EOM mediated interactions of Anabaena sp., CIP shifted the direction of interference competition and reversed dominance from M. aeruginosa to Anabaena sp. These findings demonstrate that antibiotics can restructure phytoplankton communities, underscoring the need to incorporate species interactions into ecological risk assessments.