Dual carbon source driven metabolic coupling shapes microalgal-bacterial granular sludge stability

Microalgal-bacterial granular sludge (MBGS) is a viable technology for wastewater treatment, yet its operational stability is often limited under single-carbon conditions due to metabolic imbalance. In this study, six dual carbon strategies were evaluated to investigate their roles in regulating system stability and pollutant removal. The results showed that carbon source composition strongly influenced reactor performance, potentially by pH buffering, thereby reshaping microenvironmental conditions and microbial community structure. Among all conditions, the acetate-glucose system achieved the highest stability, with simultaneous removal of COD (91.1%), NH₄⁺-N (96.8%), and PO₄^3--P (96.9%). Metagenomic analysis and system performance indicated that proton consumption during acetate assimilation likely offset acidification from glucose fermentation, maintaining a favorable alkaline niche (pH 10.0-10.2) that enriched functional bacteria (e.g., Thauera, 4.1%) and enabled simultaneous nitrogen and phosphorus removal. In contrast, the glycerol-glucose system induced severe acidification (pH < 4.0), which suppressed bacterial activity and shifted the community toward acid-tolerant fungi (e.g., Fusarium, 38.9%), resulting in functional deterioration. These findings suggest that pH buffering likely serves as a key regulatory parameter linking carbon metabolism to system stability. Rational pairing of carbon sources with complementary proton fluxes may provide a practical strategy to enhance MBGS robustness and offers a generalizable framework for carbon-source design in biological wastewater treatment.