Thesis (M.Sc.)--Chulalongkorn University, 2014

17α-methyltestosterone (MT) is a synthetic androgenic steroid, which is widely used in masculinization of aquaculture industry. Male population is preferred in a farming of Nile tilapia because of the faster growth rate, bigger body size and larger weight than the female. Although MT is beneficial for aquaculture production, the release of residual causes adverse effects to the endocrine and reproductive systems of natural aquatic organisms. This study aims to isolate the MT degrading microorganisms and to investigate the biodegradation characteristic, in order to enhance the biological removal. The isolation was performed with enrichment technique using MT as a sole carbon source at concentrations between 0.5 to 30 mg/L. The inoculum sources were sediment and biofilm from masculinizing ponds of Nile tilapia fry. Three strains, namely B051, B052 and S303 which affiliated to Acinetobacter sp., Ochrobactrum sp. and Nocardioides sp., respectively, were tested for the MT degrading kinetic and substrate versatility among other sex steroidal hormones which may coexist in farm environment. Michaelis-Menten model well explained the MT degrading kinetic for the 1 – 50 mg/L. The Vmax and Km value of strain B051, B052 and S303 were 0.34 ngL-1h-1cell-1 and 8.23 mg/L, 0.18 ngL-1h-1cell-1 and 1.48 mg/L, 0.24 ngL-1h-1cell-1and 1.0 mg/L, respectively. Strain S303 was distinguished MT degrader as it had higher Vmax and it had the lowest Km, which can imply that strain S303 had ability to degrade MT faster than the others at low concentration close to environmental condition. In addition, all isolates also removed testosterone while they had not reduced estrone (E1), 17β-estradiol (E2) or17α-ethynylestradiol (EE2) at the concentration of 10 mg/L during 7 days. The results inferred that all three isolates are specialized in androgenic hormone degradation.