Biotransformation of nitrogen and tetracycline by counter-diffusion biofilm system: Multiple metabolic pathways, mechanism, and slower resistance genes enrichment

[Display omitted] •800 µg/L TC declined the NH4+-N removal by restraining AOB and nitrification genes.•TC could affect denitrification metabolism by influencing electron behaviors.•Good TC removal by MABR was found via degradation into 17 low toxic intermediates.•Bacteria and archaea worked together and contributed to the TC removal.•The relative abundance of ARGs did not got enrichment. Although various wastewater treatment systems have been reported to be capable of transforming antibiotics, there is limited knowledge regarding the potential degradation of antibiotics through microbiological processes in a membrane aerated biofilm reactor (MABR) system. Therefore, this study aimed to investigate the metabolism of tetracycline (TC) and nitrogen in a MABR system. Results showed that the TC can initially be adsorbed by extracellular polymeric substances (EPS) and subsequently biodegraded into 17 low toxic intermediates by microbes, demonstrating that MABR could complete TC biotransformation with disruption of its chemical structure. The biodegradation pathways of TC were proposed, which mainly contained demethylation, deamination, hydroxylation, dehydration, dihydroxylation, bond cleavage and ring opening. The analysis of high-throughput sequencing (HTS) technology showed the existence of a diverse microbial community with varied metabolic pathways in the counter-diffusion biofilm system. Results demonstrated that nitrogen removal in the MABR system occurred through multiple pathways, including traditional autotrophic nitrification-heterotrophic denitrification, heterotrophic nitrification, aerobic denitrification, and autotrophic denitrification. Additionally, both tetracycline degrading bacteria (TDB) and archaea (TDA) coexisted in the MABR system, among which Methylophilus could contribute to the demethylation, while Rhizobium, Hydrogenophaga and Ramlibacter were crucial in cracking of aromatic ring for TC biodegradation.