Isolation,identification And Degradation Characteristics Of Azoxystrobin-degrading Strains

Azoxystrobin is one of strobilurin fungicides,which are used to control almost all the fungal diseases caused by ascomycetes,basidiomycetes,imperfect fungi and oomycetes fungus,and so on.However,continuous use of azoxystrobin have increased the public concern on environmental contamination problems and potential human health risk.It is of great significance to remove the residues of azoxystrobin from the environment.Recently,bioremediation has emerged as a great potential alternative approach to degrade pesticide residues because of its cost-effective and eco-friendly properties.In this study,we aim to isolate and identify a highly efficient degrading bacterium,and then investigate the degradation characteristics.In addition,the biodegradation metabolites of azoxystrobin were identified by gas chromatography-mass spectrometry(GC-MS).The biodegradation pathway of azoxystrobin were proposed based on the degradation metabolites.Finally,the biodegradation effects of selected strain were evaluated in contaminated soils.Sprcifically,the main results are as follows:(1)Isolation,screening and identification of degrading strains:83 colonies were isolated from the soil samples by using enrichment and domestication methods,and their degradation abilities were determined by high performance liquid chromatography(HPLC).One strain,named ZX-3,was selccted for its highly efficient degradation ability.It could efficiently degrade more than 80%of 50 mg·L^-1 azoxystrobin within 5 days.The degrading strain ZX-3 was identified as Paracoccus communis based on the morphology,physio-biochemical tests,16S r DNA sequence and Biolog automatic identification system.(2)Degradation characteristics of strain ZX-3:Through one-factor experiment,the optimum rang of temperature,p H and inoculum size(OD₆₀₀)on degradation abilities of strain ZX-3 were 30?-35??7.0-8.0 and 2.000-2.300,respectively.Strain ZX-3 could utilize azoxystrobin as the sole carbon and energy source for growth,and approximately83.8%of the azoxystrobin(50 mg·L^-1)was degraded by strain ZX-3 within 7 days.In addition,strain ZX-3 effectively degraded 50 mg·L^-1 of azoxystrobin,kresoxim-methyl and pyraclostrobin with the degradation of 83.8%,50.7%and 53.7%,respectively within 7 days of incubation.(3)Identification of degradation products and analysis of degradation pathways:The degradation metabolites of azoxystrobin were identified by GC-MS.Three main metabolic intermediates(named compound A,B and C,respectively)were detected during the azoxystrobin degradation by strain ZX-3.And these compounds(A,B and C)were identified as 2-amino-4-(4-chlorophenyl)-3-cyano-5,6-dimethyl-pyridine(A),2-amino-4-hydroxy-6,8-dimethyl-7(8H)-pteridinone(B)and 3-quinolinecarboxylic acid,6,8-difluoro-4-hydroxy-,ethyl ester(C),respectively.Moreover,based on analysis of the chemical structures of azoxystrobin and the metabolites,the biodegradation pathway of azoxystrobin in strain ZX-3 was proposed.Azoxystrobin was transformed by the degradation of the aromatic ring and hydrolysis of its ester linkage to yield compound A,B and C.(4)Degradation ability of strain ZX-3 in azoxystrobin-contaminated soil:The biodegradation experiment in azoxystrobin-contaminated soil was carried out with the inoculation of strain ZX-3.The results showed that the strain ZX-3 could accelerate the degradation of azoxystrobin in soil.After 10 days of treatment,approximately 77.1%of azoxystrobin was degraded with the inoculation of strain ZX-3,while the control only showed about 26.4%of degradation.