Screening And Identification Of A Novel Oxyfluorfen-degrading Bacterial Strain And Its Biochemical Degradation Pathway

Oxyfluorfen is a diphenyl ether herbicide, used to control the growth ofbroadleaf weeds in rice field. Because of its high herbicidal activity and lowapplication rates, oxyfluorfen has been widely used in rice fields. However, persistentuse of oxyfluorfen may seriously increase the health risks and ecological safetyproblems. Therefore, to remove the residue contaminate of oxyfluorfen inenvironment is becoming more important.In this study,23bacterial strain have been isolated from active soil. The bacteriumR-21can degrade and utilize oxyfluorfen as the sole carbon source for growth. R-21was identified as Chryseobacterium aquifrigidense by morphology,physio-biochemical characteristics according to those described in Bergey’s Manualof Determinative Bacteriology and genetic analysis based on16S rRNA genesequence.Response surface methodology using central composite rotatable design wassuccessfully applied to optimize the cultural conditions. Under the optimum culturalconditions (pH6.9, temperature33.4oC, and inoculum size0.2g L-1), this strain R-21could degrade92.1%of oxyfluorfen at50mg L-1within5days. In addition, we alsoinvestigated the relationship between the initial oxyfluorfen concentration and thedegrading rate. The results show that when the initial concentration of oxyfluorfenwas55.8946mg L-1, the degrading rate was maximum specific degrading rate. Whenthe initial concentration of oxyfluorfen was lower than55.8946mg L-1, degrading ratewas gradually increased. At a higher initial concentration, degrading rate wasdecreased. The strain R-21also could degrade other diphenyl ethers herbicides suchas fomesafen, bifenox, acifluorfen, and fluoroglycofen. Degradation of oxyfluorfen,fomesafen, bifenox, acifluorfen, and fluoroglycofen was fitted to the bi-exponentialkinetic model. With the strain R-21, the t1/2was,0.7,1.0,1.7,1.2, and1.4days foroxyfluorfen, fomesafen, acifluorofen, fluoroglycofen and bifenox, respectively. During oxyfluorfen degradation, four metabolitesN-(2-ethoxy-4-(2-hydroxy-4-(trifluoromethyl) phenoxy) phenyl) acetamide,3-ethoxy-4-nitrophenol,4-nitrobenzene-1,3-diol and4-amino-3-ethoxyphenol weredetected and identified by atmospheric pressure gas chromatography coupled toquadrupole-time of flight mass spectrometry (APGC-Q-TOF-MS) andultra-performance liquid chromatography coupled to quadrupole–time of flight massspectrometry (UPLC-Q-TOF-MS), and a plausible degradation pathway was deduced.The main possible pathways involved cleavage of the ether linkage, dechlorination,reduction of nitro group and amino acetylation.