Surface-Associated Bacteria Trigger Cyanobacterial Cell Lysis during Preozonation

Preozonation is widely used to enhance the effectiveness of coagulation and filtration in algae-laden water treatment, but cyanobacterial cell rupture and the subsequent release of intracellular organic matter and cyanotoxins can increase treatment burdens and pose health risks. In natural waters, cyanobacteria are often surrounded by symbiotic bacteria, whose influence on ozonation performance and underlying mechanisms remains unclear. Herein, we found that axenic filamentous cyanobacteria (<i>Leptolyngbya</i> sp.) exhibited strong resistance to ozonation (0.3 mg L^-1, 20 min), whereas the presence of surface-associated bacteria markedly increased the cell rupture rate from 12 ± 6% to 76 ± 2%. Removal of loosely bound extracellular polymeric substances (LB-EPS) significantly reduced ozonation resistance in axenic cyanobacteria but unexpectedly enhanced that of xenic cultures. By integrating reactive oxygen species identification, extracellular metabolomics, and metabolic reconstruction, we demonstrate that surface-colonizing bacteria degrade the algal LB-EPS envelope, releasing metabolites that facilitate hydroxyl radical formation during ozonation, thereby intensifying cell rupture. Our results highlight surface-associated bacteria as a critical yet overlooked factor shaping cyanobacterial responses to preozonation, underscoring the need to re-evaluate ozone application strategies in bloom-impacted waters to minimize cell rupture and byproduct formation.