Effects of oxytetracycline on nitrification in a model aquatic system

Abstract

Our objective was to look at the effects of oxytetracycline (OTC), which is used in the fish farming industry to treat bacterial diseases, on the nitrification process in a simple aquatic system. The experiments were conducted in 9-l aquaria containing sand substrate, synthetic freshwater, 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 NH 3N, to provide an energy source for the nitrifying bacteria. The effects of OTC were evaluated by measuring changes in the conversion of ammonia to nitrite and nitrate in reference to a negative control (0 mg/l OTC). We determined that nitrification was inhibited at all levels of OTC tested (12.5–75 mg/l). Concentrations of 50–75 mg/l OTC resulted in nearly complete inhibition of nitrification within 7 days. We estimated the 95% confidence interval for the 7-day EC50 to be 8.60–26.96 mg/l OTC. The potential for disrupting nitrification, which could lead to a build-up of toxic ammonia and nitrite, should be considered by aquaculturists treating diseased fishes.