Electron Respiratory Enhancement Limited Sulfamethoxazole Metabolic Potential: An Overlooked Easily Biodegradable Substrate in MABRs

A low concentration of an easily biodegradable substrate (EBS) promotes the cometabolism of antibiotics in the receiving water. However, a high EBS concentration in wastewater treatment plants is associated with a negative effect on antibiotic metabolism. In this study, we aimed to determine the effects of different concentrations of EBS (at lab-scale using synthetic wastewater and acetate as the sole carbon source besides sulfamethoxazole) on sulfamethoxazole (SMX) biodegradation performance and the underlying molecular mechanisms of SMX biodegradation. We observed decreased SMX biodegradation efficiency (approximately 65%) at a high EBS concentration (approximately 100% in the absence of EBS). Metagenomic analysis suggested that the catalytic activity of the enzyme E.2.7.11.1 (coded by the K12132) decreased to one-third at high EBS concentration compared to the case where EBS was absent (P < 0.01). In contrast, the enzyme E.7.1.1.2, which can participate in the electron transfer chains of mitochondria and aerobic bacteria, is top enriched. Furthermore, the enzymes E.2.7.11.1 and E.3.4.16.4 that contribute to drug resistance in the functional species (e.g., Microbacterium_sp. and Pseudomonas_sp._A-1) showed significantly decreased activity in the EBS concentration. These results demonstrate that the significant decrease in resistance enzyme activity caused by electronic respiration enhancement is the main factor limiting the metabolic potential of SMX.