Effects of the antibiotic ciprofloxacin on the bacterial community structure and degradation of pyrene in marine sediment

The ecological consequences of antibiotics in the aquatic environment have been an issue of concern over the past years due to the potential risk for negative effects on indigenous microorganisms. Microorganisms provide important ecosystem services, such as nutrient recycling, organic matter mineralization and degradation of pollutants. In this study, effects of exposure to the antibiotic ciprofloxacin on the bacterial diversity and pollutant degradation in natural marine sediments were studied using molecular methods (T-RFLP) in combination with radiorespirometry. In a microcosm experiment, sediment spiked with 14C-labelled pyrene was exposed to five concentrations of ciprofloxacin (0, 20, 200, 1000 and 2000 μg L−1) in a single dose to the overlying water. The production of 14CO2 (i.e. complete mineralization of pyrene) was measured during 11 weeks. Sediment samples for bacterial community structure analysis were taken after 7 weeks. Results showed a significant dose-dependent inhibition of pyrene mineralization measured as the total 14CO2 production. The nominal EC50 was calculated to 560 μg L−1, corresponding to 0.4 μg/kg d.w. sediment. The lowest effect concentration on the bacterial community structure was 200 μg L−1, which corresponds to 0.1 μg/kg d.w. sediment. Our results show that antibiotic pollution can be a potential threat to both bacterial diversity and an essential ecosystem service they perform in marine sediment.