Deciphering the response of heterotrophic, autotrophic and mixotrophic denitrification to enrofloxacin stress: Performance, denitrifying community and antibiotic resistance genes

Antibiotics frequently coexist with nitrate (NO3  -N) in wastewater. This study systematically explored the negative effects of enrofloxacin (ENR) on the microbial performance of autotrophic, heterotrophic and mixotrophic denitrification processes under parallel conditions. The ENR concentration was set at 0, 0.1, 1 and 10 mg/L for four phases in sequence. As the influent ENR concentration increased, the nitrate removal efficiency (NrE) in the pyrite-based autotrophic denitrification (PAD) system declined gradually to 11.08% (minimum) and the expressed narG/H/I and napA/B genes decreased. However, the NrE in the polycaprolactone-based heterotrophic denitrification (PHD) and mixotrophic denitrification systems declined slightly (the lowest NrE: 88.10% and 85.28%, respectively), and the expressed narG/H/I (0-1 mg/L) and napA/B genes increased gradually in the PHD system. The removal pathway of ENR was mainly through adsorption by substrates and/or biofilms rather than through biodegradation. Increasing the concentration of ENR (0-10 mg/L) reduced the relative abundance of functional genera (Syntrophomonas, Lentimicrobiaceae_unclassified, Bacteroidetes_vadinHA17_unclassified, and Clostridium) capable of degrading polymers in the PHD system, but elevated the relative abundance of the dominant genus Thiobacillus in the PAD system. Network analyses showed that Thiobacillus in the PAD system as well as Anaerolineaceae_unclassified and Simplicispira in the PHD system were resistant to ENR stress.