Inhibitory effects and biotransformation potential of ciprofloxacin under anoxic/anaerobic conditions

•Inhibition of denitrification by CIP gradually increased with long-term incubation.•CIP was biotransformed in a denitrifying culture and two products were determined.•The quinolone structure in CIP was not transformed under denitrifying conditions.•Sulfate reduction was inhibited by CIP at 10mg/L or higher but recovered later.•The methane production decreased more than 30% with 80 and 100mg/L CIP amendment. The inhibitory effects and biotransformation potential of the fluoroquinolone antibiotic ciprofloxacin (CIP) under anoxic (i.e., nitrate reducing) and anaerobic (i.e., sulfate reducing and methanogenic) conditions were investigated. Fermentation and sulfate reduction was inhibited in 10–80mg/L CIP-amended sulfate-reducing cultures but recovered with prolonged incubation. Methanogenesis in the mixed culture was significantly inhibited at 80–100mg CIP/L. No significant decrease of CIP concentration was observed under both sulfate-reducing and methanogenic conditions. However, a low degree of CIP biotransformation was observed in a fed-batch denitrifying culture after a lag time even though the microbial, denitrifying activity was gradually inhibited at 24–40mg CIP/L. Furthermore, the degradation of CIP was accelerated with a CIP reamendment of the denitrifying culture. Two CIP biotransformation products in the denitrifying culture were detected and their proposed chemical structures suggest that the antibiotic quinolone moiety of CIP was intact.