EFFECTS OF OXYTETRACYCLINE ON NITRIFICATION IN A MODEL AQUATIC SYSTEM

Our objective was to look at the effects of oxytetracycline (OTC), whi ch is used in the fish farming industry to treat bacterial diseases, o n the nitrification process in a simple aquatic system. The experiment s were conducted in 94 aquaria containing sand substrate, synthetic fr eshwater, and active cultures of the nitrifying bacteria Nitrosomonas and Nitrobacter. The synthetic freshwater was amended with ammonia, at an initial concentration of approximately 5 mg/l NH3-N, to provide an energy source for the nitrifying bacteria. The effects of OTC were ev aluated by measuring changes in the conversion of ammonia to nitrite a nd nitrate in reference to a negative control (0 mg/l OTC). We determi ned that nitrification was inhibited at all levels of OTC tested (12.5 -75 mg/1). Concentrations of 50-75 mg/l OTC resulted in nearly complet e inhibition of nitrification within 7 days. We estimated the 95% conf idence interval for the 7-day EC50 to be 8.60-26.96 mg/l OTC. The pote ntial for disrupting nitrification, which could lead to a build-up of toxic ammonia and nitrite, should be considered by aquaculturists trea ting diseased fishes.