Ionic synergy of Ca2+, Mg2+, and Fe2+ balances hydrophobicity and microbial ecology in microalgal-bacterial granular sludge.

Microalgal-bacterial granular sludge (MBGS) faces structural stability challenges that limit its engineering application. To investigate the regulatory mechanisms of divalent metal ions under light-dark cycles, five experimental groups were established: a blank control, Ca2+ addition, Mg2+ addition, Fe2+ addition, and combined ion addition. Results showed that Fe2+ was crucial for maintaining granule integrity and promoting photosynthetic taxa through regulating hydrophobic carbon group accumulation in extracellular polymeric substances (EPS), though its sole addition caused community specialization and compromised nitrogen/phosphorus removal stability, especially under dark conditions. Ca2+ enhanced Proteobacteria proliferation and biomass accumulation, improving COD and TN removal, but this effect seemed to be insufficient to counteract granule disintegration caused by iron deficiency. Mg2+ exhibited limited direct community effects but correlated positively with COD removal and Chl-a/Chl-b ratio. Notably, combined addition achieved balanced EPS hydrophilic-hydrophobic properties, maintained community diversity and functional redundancy, and demonstrated stable pollutant removal through ionic synergy. This study elucidates metal ion regulatory pathways in MBGS, providing theoretical foundations for targeted ion optimization.