Biodegradation of 17 alpha-methyltestosterone by microorganisms in sediment from masculinizing pond of nile tilapia fry

Abstract

17alpha-methyltestosterone (MT), an anabolic androgenic steroid, is used widely in reversing the sex of Nile tilapia (Oreochromis niloticus), an important freshwater aquaculture fish in many developing countries, to produce an all male population. MT is a suspect human carcinogen and can act as an endocrine disrupting compound interfering with the normal function of endocrine and reproductive systems of animals when exposed to MT levels as low as nanogram per liter. However, an understanding of the occurrence and fate of MT in the environment is very limited. This study investigated the biodegradation of MT and its androgenic potency by microorganisms in sediment under different electron acceptor conditions and by isolated MT-degrading bacteria from the sediment and water of a masculinizing pond. MT was degraded under aerobic, sulfate-reducing and methanogenic conditions, but it was hardly degraded under iron(III)-reducing condition and was not degraded under nitrate-reducing condition. Statistically, the first-order biodegradation rates of MT for aerobic, sulfate-reducing and methanogenic conditions were similar. However, the measured androgenic activities under methanogenic condition continued to persist for more than one month even though the measured MT concentrations were negligible. An unknown labile metabolite of MT was found under aerobic condition but not for other electron acceptor conditions. Based on the biodegradation results, it appeared that MT and its metabolites can potentially accumulate in environments under iron (III)-reducing, nitratereducing, and methanogenic conditions. Thirty strains of MT-degrading bacteria were isolated from the sediment and water of masculinization pond. The isolated bacteria were related to three genus including Pimelobacter, Rhodococcus, and Nocardioides. Three isolated strains, strains SB100-05, SB010-03, and WB100-05, were selected for further identification and characterization. Phylogenetic analysis, physiological properties and cell morphology of the three strains SB100-05, SB010-03 and WB100-05 suggested that they were closely related to Nocardioides aromaticivorans, Rhodococcus equi, and Nocardioides nitrophenolicus, respectively. Haldane’s model was found to fit the MT degradation kinetics of the three strains indicating the possibility of substrate inhibition. Of the three strains tested, strain SB10005 degraded MT faster but had lower resistance to high MT concentration than the other strains.