The Determination Of The Threshold And The Fate Of Antibiotic Resistance Genes During Nitrifying Sludge Co-metabolism Degradation Of Antibiotics

The extensive use of antibiotics has caused serious environmental pollution.Meanwhile,the selective pressure from antibiotics and some of their transformation products play a critical role in the emergence and spread of antibiotic resistant bacteria(ARB)and antibiotic resistance genes(ARGs).Antibiotics can be biodegraded in activated sludge via co-metabolism and metabolism.However,the relationship between microbial community(e.g.,autotrophic and heterotrophic microorganisms)and the degradation of antibiotics and the transfer of ARGs,has yet to be elucidated.However,contradictory results,whether or not antibiotics can be effectively removed by sludge systems,have been reported in previous studies.It can be hypothesized that there is a threshold concentration to initiate antibiotic degradation by microorganisms,which remains to be further verified in activated sludge systems.In this study,we investigated the biodegradation pathways of sulfamethoxazole(SMX)and ARGs’ fate in different sludge systems,including aerobic sludge,mixed sludge,and nitrifying sludge.The mechanism of degradation threshold and degradation pathway was further verified by nitrifying sludge.The main conclusions include:(1)Although all three sludge systems could efficiently remove SMX after activating the degradation pathways by high concentration SMX stimulation,NH4+-N removal efficiency decreased slightly due to its competition with SMX.Heterotrophic bacteria also played an important role in SMX biodegradation,especially in aerobic sludge.However,ammonia-oxidizing bacteria(AOB)had a faster metabolic rate,which was about five times higher than heterotrophic bacteria,contributing much to SMX removal via co-metabolism.As SMX concentration increases,the amoA gene and AOB relative abundance decreased in aerobic sludge due to the enrichment of functional heterotrophic bacteria,while increased in nitrifying sludge owing to the advantages of AOB in removing SMX.Microbial community analysis showed that SMX pressure selected the functional bacteria to increase SMX removal and antibiotic resistance.SMX degradation species carried ARGs could increase their resistance to the biotoxicity of SMX and maintain system performance.(2)The threshold concentrations for effective SMX degradation were determined in the activated sludge systems,which is 0.2～0.5 mg/L in the nitrifying sludge system.EPS can protect microorganisms from the toxic effects of low concentration of SMX.When the concentration of SMX approach the threshold,EPS can promote the degradation of SMX.AHLs-mediated QS changes significantly during the degradation of SMX.QS activity is an important factor in the activation of SMX degradation pathway.Nitrosomonas europaea is an absolutely dominant species in the reactor due to its ability to oxidize various hydrocarbons,which plays an important role in the co-metabolism degradation of SMX.The selection pressure of antibiotics leads to the proliferation of ARGs and promotes the horizontal transfer of ARGs.These findings provide fundamental supports for the underlying mechanism and the proliferation of ARGs during antibiotics biodegradation,which opens up helpful guidance for antibiotics wastewater treatment and ARGs control.