Interspecies Interactions Reverse the Hazard of Antibiotics Exposure: A Plankton Community Study on Responses to Ciprofloxacin hydrochloride

The ecotoxicological effects of Ciprofloxacin hydrochloride (CIP) were tested on population densities of plankton assemblages consisting of two algae (Isochrysis galbana and Platymonas subcordiformis) and a rotifer (Brachionus plicatilis). The I. galbana showed a significant decrease in densities when concentrations of CIP were above 2.0 mg L(−1) in single-species tests, while P. subcordiformis and B. plicatilis were stable in densities when CIP were less than10.0 mg L(−1). The equilibrium densities of I. galbana in community test increased with CIP concentrations after falling to a trough at 5.0 mg L(−1), showed a completely different pattern of P. subcordiformis which decreased with CIP concentrations after reaching a peak at 30.0 mg L(−1). The observed beneficial effect was a result of interspecies interactions of trophic cascade that buffered for more severe direct effects of toxicants. The community test-based NOEC of CIP (2.0 mg L(−1)), embodying the indirect effects, was different from the extrapolated one derived by single-species tests (0.5 mg L(−1)), but all lacked confidence interval. A CIP threshold concentration of obvious relevance to ecological interaction was calculated with a simplified plankton ecological model, achieving a value of 1.26 mg L(−1) with a 95% bootstrapping confidence interval from 1.18 to 1.31 mg L(−1).