Photosynthesis-driven interactions in the phycosphere enhance bacterial extracellular superoxide production

Extracellular superoxide generated by heterotrophic bacteria influences aquatic redox transformations, yet its regulation within the phycosphere remains poorly understood. Here, we show that the cyanobacterium Microcystis aeruginosa enhances extracellular superoxide production by Pseudomonas sp. QJX-1 under illumination. Cocultivation increased extracellular superoxide production approximately threefold over bacterial monoculture, with maximum enhancement during carbon starvation. Membrane separation localized detectable superoxide production to the bacterial chamber, indicating that diffusible algal products stimulated bacterial superoxide production. Mechanistically, illuminated cocultures showed 20.4% higher photocurrent than algal monocultures and accumulated the highest extracellular NADP(H) levels, indicating intensified photosynthesis-associated extracellular redox activity. Reduced nicotinamide cofactors stimulated diphenyleneiodonium-sensitive QJX-1-associated superoxide production, consistent with an oxidoreductase-associated route for one-electron reduction of molecular oxygen. In parallel, coculture accumulated catecholate siderophores, consistent with intensified microscale Fe competition; this siderophore-enhanced response required metabolically active bacterial cells and was attenuated by respiratory-chain inhibition. Representative extracellular redox-active and algal photoactive components further enhanced the bacterial response, with cytochrome c and chlorophyll a stimulating QJX-1-associated superoxide production under defined conditions. Functionally, during the carbon-starvation interval coinciding with peak superoxide production, the apparent sulfamethoxazole removal rate constant in coculture was 2.6 times that observed for QJX-1 alone. Moreover, quenching extracellular reactive oxygen species impaired metabolic activity under diverse chemical stressors and reduced carbon, nitrogen, and phosphorus substrate utilization. Collectively, these findings identify photosynthesis-dependent phycosphere interactions as regulators of bacterial superoxide production, linking biogenic reactive oxygen species to contaminant transformation, chemical-stress resilience, and elemental nutrient turnover.