Deciphering ecological drivers and assembly mechanisms of sulfonamide-degrading bacteria in constructed wetlands.

Constructed wetlands (CWs) are increasingly utilized for sulfonamide remediation, yet the ecological drivers governing sulfonamide-degrading bacteria (SDB) remain inadequately characterized. To systematically evaluate how oxygen and nutrient availability interact to shape SDB communities, this study classified CWs into three distinct functional types: high-dissolved-oxygen-high-nutrient (HD-HN), high-dissolved-oxygen-low-nutrient (HD-LN), and low-dissolved-oxygen-high-nutrient (LD-HN) systems. Analysis of medium-to-large CWs in South China demonstrated pervasive SDB colonization (relative abundance: 0.104 %-0.503 %), with comparative analysis across the three CW types revealing that aerobic regimes synergized with elevated nutrient loads fostered stochastic community assembly (R2 = 0.813 for HD-HN) and expanded niche breadth of SDB, culminating in functionally robust consortia. The classification framework demonstrated that stochastic processes dominated high-resource environments (HD-HN), starkly contrasting with deterministic selection in oxygen-depleted systems (LD-HN) and nutrient-limited conditions (HD-LN). Co-occurrence network pinpointed SDB as keystone taxa with significantly higher connectivity (average degree >30), facilitating sulfonamide elimination through cross-functional collaboration with non-SDB taxa. Mesocosm experiments validated field observations by replicating key environmental gradients under controlled conditions, achieving >50 % sulfamethazine removal in optimized HD-HN configurations, confirming the generalizability of the identified SDB response patterns. These findings advance predictive frameworks for optimizing CW design through targeted manipulation of oxygen and nutrient regimes to enhance co-contaminant remediation.