Mechanisms governing photogranule formation during the rapid start-up of a photogranular bioreactor in high-salinity synthetic mariculture wastewater: The critical roles of EPS and microbial community dynamics

Photogranulation technology has great potential in the mariculture wastewater treatment. This study aimed to explore the microbial characteristics changes associated with photogranulation during the start-up of photogranular sequencing bioreactor (PSBR) system. The result showed that the photogranules with clear boundaries, dense cores and regular shapes were formed within 65 d. Compared with loosely bound – extracellular polymeric substances (LB-EPS) (29.16 mg/L), the tightly bound - extracellular polymeric substances (TB-EPS) (238.16 mg/L) was the main component of EPS in the system. The protein to polysaccharide (PN/PS) ratio revealed a distinct functional shift: proteins in the outer layer (LB-EPS) drove the initial microbial aggregation, while the continuous accumulation of proteins in the inner layer (TB-EPS), supported by a stable polysaccharide framework, became decisive for structural stabilization during the maturation period. The molecular weight of the EPS showed a gradual increase and finally remained stable during the photogranulation process. The fluorescence intensity of aromatic and tryptophan substances enhanced remarkably, implying that they noticeably contributed to the formation and the stability of photogranules. High-throughput sequencing results showed that the diversity of microbial communities significantly decreased and the prevalent species emerged with the progression of photogranulation in the PSBR system. Furthermore, Proteobacteria and Cyanobacteria were the dominant prokaryotic species, and the Chlorophyta and Bacillariophyta were the crucial microalgae for the photogranulation. They have formed a good mutually beneficial symbiotic relationship. The result of functional prediction showed that Photoautotrophy was the main metabolic pathway of photogranules.