Degradation of a fluoroquinolone antibiotic in an urbanized stretch of the River Tiber

The widespread detection of antibiotics in terrestrial and aquatic systems has engendered significant scientific and regulatory concern. Overall, knowledge concerning the ecotoxicology and sub-lethal effects in water is scarce, but some experimental studies show that antibiotics can induce pathogen resistance and they can also have detrimental effects on natural microbial communities and their key functions. The main aim of this study was to investigate the occurrence of the biodegradation and photodegradation processes of the fluoroquinolone ciprofloxacin (CIP) in the River Tiber waters, in a stretch highly impacted from human pressure. Two set of microcosms consisting of river water containing the natural microbial community and treated with 500 μg/L of CIP in absence or presence of UV-light were performed. Moreover, some microcosms were filled with river water previously sterilized and then treated with the antibiotic. The combined experimental set made it possible to evaluate if the antibiotic CIP could be photodegraded and/or biodegraded. CIP residual concentrations were measured over time by using HPLC coupled to fluorescence detection (FLD) and the effects of the antibiotic on the natural microbial community were assessed in terms of live cell abundance. The key role of light in CIP disappearance was confirmed, but also its biodegradation in natural river water was demonstrated. In fact, differently from other experiments we found a higher degradation rate (DT50= 10.4 d), in presence of both light and the natural river bacterial populations than in the same sterilized river water (DT50= 18.4 d). Moreover, even in the dark, a partial CIP biodegradation was also observed (DT50= 177 d). The overall results were supported by the increase in live cell numbers with the decrease of CIP concentrations both in the dark and light condition.