Bio-entrapment of Chlorella vulgaris mimicking granular sludge: Insights into aggregated microalgae growth and symbiotic bacteria under antibiotic stress

Microalgal-bacterial granular sludge (MBGS) offers an efficient solution for low-carbon sewage treatment by eliminating challenging sludge-water separation processes. However, the impact of extracellular polymeric substances (EPS)-enveloped micro-environments on microalgae and the role of microalgae-derived phytohormones in bacterial signaling in MBGS remain poorly understood. This study investigated the characteristics of Chlorella vulgaris within an EPS-enveloped micro-environment using bio-entrapment in natural polysaccharide-protein (PS-PN) hydrogel granules. Results showed that PS-PN hydrogel bio-entrapment facilitates microalgal cell aggregation and proliferation, and upregulated pathways for photosynthesis, carbon metabolism, and antioxidant defense, suggesting that aggregated growth in PS-PN granules enhanced carbon fixation, organic metabolism, and environmental adaptation. In addition, tryptophan in the hydrogel activated the tryptamine pathway of C. vulgaris , increasing indoleacetic acid (IAA) production to 212.88 ± 7.86 μg/L (vs. 1.95 ± 0.10 μg/L in suspended cells). This IAA alleviated   Pseudomonas stutzeri   growth inhibition under sulfamethoxazole (SMX) stress. The microalgae-bacteria symbiotic system seeded with aggregated-growth microalgae exhibited superior growth and antioxidant enzyme activity under SMX stress. This study highlights the advantages of aggregated-growth microalgae in supporting both their survival and that of symbiotic bacteria under antibiotic stress, and provides new insights into the role of microalgae-derived phytohormones in microalgae-bacteria signaling and stress resistance within MBGS.