Thesis (M.Sc.)--Chulalongkorn University, 2007

To produce an all-male population, a very common practice is to feed the Nile tilapia (Oreochromis niloticus) fry with 17alpha-methyltestosterone (MT)-impregnated food. MT residues in fish feed for the masculinization of Nile tilapia fry may accumulate in masculinizing ponds and be released into natural receiving water. A recent study showed that MT concentration in soils were between 2.8 and 2.9 ng/g for nearly three months after cessation of treatment demonstrating the persistence of MT in soil. The objectives of this study were to investigate the biodegradation of MT under aerobic and anaerobic conditions and isolation of responsible microorganisms. Bacterial seed used was aerobic sludge, anaerobic sludge taken from wastewater treatment plants and sediment taken from a masculinizing pond of tilapia fry. Aerobic and anaerobic biodegradation test suggested that MT was biodegradable. Under aerobic condition, first-order degradation rate constants using aerobic sludge were 1.43±0.12, 0.48±0.04, 0.37±0.01, 0.24±0.17 and 0.36±0.17 day-1, for an initial MT concentrations of 0.3, 1.0, 5.0, 7.0, and 10.0 mg/l, respectively. Under aerobic condition, first-order degradation rate constants using sediment were 0.52±0.02, 0.23±0.04, 0.17±0.02, 0.13±0.01 and 0.10±0.01 day-1 for the initial MT concentrations of 0.3, 1.0, 5.0, 7.0, and 10.0 mg/l, respectively. Under anaerobic degradation, first-order degradation rate constants using anaerobic sludge were 0.19±0.01, 0.21±0.01, 0.28±0.03, 0.19±0.01 and 0.10±0.01 day-1 for the initial MT concentrations of 0.1, 1.0, 3.0, 5.0, and 10.0 mg/l, respectively. Under anaerobic degradation, first-order degradation rate constants using sediment were 0.09±0.02, 0.34±0.01, 0.33±0.01, 0.23±0.08 and 0.09±0.01 day-1 for the initial MT concentrations of 0.1 1.0, 3.0, 5.0 and 10.0 mg/l, respectively. MT-degrading bacteria strain MT 3/10, MT 5/10 and 1/500 were isolated from aerobic sludge. Analysis of 16S rRNA gene sequences suggested that the strain MT 5/10 related closely to Acidovorax sp. RCPCd1 which is in the genus Acidovorax. And the strains MT 3/10 and MT 1/500 related closely to Methylophilus leisingeri strain RCP5 which is in the genus Methylophilus. The activities of both strains were not so effective. After 2.5 days of operation, MT concentrations were left over at 46 % for strains MT 3/10 and 55 % for MT 1/500.