Microalgal-bacterial granular sludge enhances oxytetracycline removal: Microbial responses, degradation pathways, and adaptive mechanisms

Oxytetracycline (OTC), an emerging “low-concentration, high-toxicity” contaminant, presents considerable hurdles to wastewater treatment processes. This study systematically evaluated for the first time the impacts of OTC on the operational performance, sludge characteristics, and microbial metabolic activity across three treatment systems: microalgal-bacterial granular sludge (MBGS), aerobic granular sludge (AGS), and activated sludge (AS). Results demonstrated that MBGS exhibited superior treatment efficiency, maintaining stable removal of 500 μg/L OTC at 88.06 ± 1.45 % ( p  < 0.05). MBGS adapted to OTC exposure by increasing ATP content and reducing lactate dehydrogenase release. Acclimated MBGS primarily removed OTC through biodegradation. Moreover, multiple OTC transformation products with reduced toxic potential were detected, signifying that MBGS systems achieve efficient microbial degradation. Metagenomic analyses revealed that Pseudomonadota in MBGS displayed high adaptability under OTC exposure. Additionally, OTC exposure upregulated carbohydrate and energy metabolism in MBGS, thereby enhancing overall microbial metabolic activity. Alphaproteobacteria contributed most significantly to key functional genes, underscoring their critical role in contaminant removal in the MBGS. Redundancy analysis highlights a robust association between Alphaproteobacteria and the abundance of antibiotic resistance genes. This study confirms the MBGS's resilience to OTC-contaminated wastewater, highlighting its potential for efficient antibiotic wastewater treatment.