Fate and effects of enrofloxacin in aquatic systems under different light conditions.

The fate and effects of fluoroquinolone antibacterials (FQ) in the environment is of significance because of apparent increased FQ resistance in environmental and clinical organisms. Here we simultaneously assessed the fate and effects of enrofloxacin (enro), an FQ often used in agriculture, on the chemistry and in situ microbial communities in receiving waters. We added enro to 25 microg/L in nine outdoor mesocosms maintained under three light conditions (in triplicate): full sunlight typical of the upper epilimnion (100% full-light exposure, FLE), partial shading typical of the lower epilimnion (28% FLE), and near-complete shading typical of the hypolimnion (0.5% FLE). Enro disappearance and ciprofloxacin (cipro) formation were monitored over time using LC/MS, and water chemistry and ambient microbial communities (using denaturing gradient gel electrophoresis; DGGE) were characterized. Enro half-lives were 0.8, 3.7, and 72 days for the 100%, 28%, and 0.5% FLE treatments, respectively, creating three distinct FQ exposure scenarios. Although FQ exposures ranged from approximately 6 microg/L for 24 h to approximately 21 microg/L for 30 days, no statistically significant exposure effects were noted in water quality or microbial communities (as indicated by whole-community 16S rDNA DGGE analysis and specific amplification of the QRDR region of gyrase A). Small changes in water chemistry were noted over time; however, changes could not be specifically attributed to FQs. In general, enro addition had minimal effect on water column conditions at the levels and durations used here; however, further investigation is needed to assess effects in aquatic sediments.