Guoshuo Chen, Yixin Pan, Zijia Bai, Yanling Zheng, Yanjie Wei
Low carbon-to-nitrogen (C/N) ratio wastewater poses a major challenge to biological nitrogen removal due to insufficient electron donors for denitrification. In this study, an algae-bacteria membrane-aerated biofilm reactor (AB-MABR) was established to enhance nitrogen removal under carbon-limited conditions, and its performance was compared with that of a conventional bacterial MABR (B-MABR). The results showed that the AB-MABR achieved superior pollutant removal performance, with COD, NH4 +-N, and TN removal efficiencies being 4.0, 21.9, and 12.3% higher, respectively, than those of the B-MABR. Overall, AB-MABR outperformed B-MABR in pollutant removal. The removal efficiencies of COD, NH4 +-N, and TN were 92.3, 77.2, and 66.6%, respectively, which were markedly higher than those achieved by B-MABR (88.8, 55.3, and 54.3%). The incorporation of microalgae significantly enhanced microbial metabolic activity, as evidenced by higher ATP content, electron transport system activity (ETSA), and cytochrome c (Cyt-c) levels. Meanwhile, EPS production increased by 25% in the AB-MABR, accompanied by greater accumulation of protein-like and humic-like substances. SEM and CLSM analyses revealed that microalgae promoted the formation of a denser and more stratified biofilm with higher biomass and stronger structural stability. Metagenomic analysis further demonstrated that pathways associated with microbial metabolism, secondary metabolite biosynthesis, and environmental adaptation were enriched in the AB-MABR system, indicating enhanced metabolic potential and ecological resilience. Overall, microalgal incorporation strengthened electron transfer, stimulated EPS secretion, improved biofilm development, and enhanced microbial metabolic functions, thereby promoting nitrogen transformation and removal under low C/N conditions. These findings provide new insights into the synergistic mechanisms of algae-bacteria biofilms and demonstrate the potential of AB-MABR technology for sustainable nitrogen removal from carbon-limited wastewater.