CO₂ drives ecological shifts through asymmetric selection in microalgae-bacteria granular for enhanced wastewater treatment and carbon fixation

Microalgae-bacteria granular sludge (ABGS) systems show promising potential for simultaneous pollutant removal and CO₂ fixation in wastewater treatment, aligning with carbon neutrality goals. However, the ecological mechanisms underlying CO₂-introduced ABGS system performance adaptations remain poorly understood, hindering biosystem optimization. Here, we systematically investigated multi-level ecological adaptation mechanisms in ABGS systems under external CO₂ in low-carbon wastewater. For the first time, our study reveals that CO₂ acts as a selective force driving community evolution through asymmetric patterns: increasing phylogenetic distances among competing species while decreasing among cooperating species. This evolutionary strategy optimizes niche differentiation and functional coordination, facilitating community transition to functionally diverse Bacteroidota and Proteobacteria. Such adaptation manifests in ecological pattern shift from Zipf (55.56 % of samples) to Lognormal distribution (66.67 % of samples) reflecting enhanced resource utilization, while network reorganizes to module-hub structure, with species interactions transforming from antagonistic to synergistic relationships, improving robustness through enhanced positive species interactions. These adaptations are underpinned by coordinated molecular responses, including enrichment of carbon fixation (RbcL/RbcS), quorum sensing and chemotaxis pathways. Consequently, system achieved the carbon fixation of 415.52 mg/L, increased TN and COD removal by 17.64 % and 11.30 %, and increased self-aggregation potential by 13.3 % supported by elevated key metabolites. This study establishes a new framework for understanding ecological mechanisms through environmental selection, offering strategies for achieving efficient wastewater treatment and carbon neutrality goals.