Enantioselective Degradation And Enantiomerization Of The Chiral Mecoprop Herbicide In Soils

Chiral phenoxyalkanoic acid herbicides are widely used,but their excessive use bring pollution and threaten human health.There were many studies focusing on the selective degradation.However,there are few reports on the carbon isotope fractionation(13C/12C)of their degradation processes,the relative abundance of related functional genes and the changes of microbial communities in soils,and the mechanism of enantioselective degradation and enantiomerization was still unclear.In this paper,the racemic(Rac-),S-and R-type forms of the typical chiral phenoxyalkanoic acid herbicides mecoprop(2-(4-chloro-2-methylphenoxy)propanoic acid,mecoprop,MCPP)were investigated to investigate their degradation kinetics,enantiomerselective degradation and transformation,isotope fractionation effects,changes of relative abundance of related functional genes and microbial community structure in four soils.The main findings are as follows:(1)The degradation of Rac-MCPP,R-MCPP and S-MCPP in the soil accorded with the first-order reaction kinetics model,and the sterilized experiment showed that the dissipation and enantioselective degradation of MCPP was microbal.Under non-sterilized conditions,the dissipation half-life of Rac-MCPP was 3.12?6.93 days.Under non-sterilized conditions,racemic MCPP exhibited different degradation enantiomerselectivity in different soils.The R-enantiomer was preferential degradated in corn filed soil(Hefei)with an EF value of 0.36 ± 0.01 at the end of degradation.In mulberry field soil(Hangzhou),R-type was preferentially degraded first,but after few days the S-type degradation took precedence.The EF value was 0.57 ± 0.02 at the end of degradation.Both tea orchard soil(Hangzhou)and vegetable soil(Hangzhou)showed that S-type enantiomers were preferentially degraded and the EF values were 0.75 ±0.02 and 0.92 ± 0.01 at the end of degradation,respectively.Enantioselectivity and soil pH was significantly positive correlation.(2)Enantioselective conversion of MCPP was obsearved in soil incubated with pure R-or S-enantiomers,and all of them were favored by R-?S-type transformation.The enantiomerization equilibrium constant K(KRS/KSR)of mulberry field soil(Hangzhou)and tea orchard soil(Hangzhou)were 2.23,2.19,respectively.The highest enantiomerization rates were obsearved in mulberry field soil(Hangzhou).It was only exsited the transformation of R-?S-type in com field soil(Hefei)and vegeatable garden soil(Hangzhou).The actual degradation rate of each enantiomer in the four soils were consistent with the degradation of the racemate and were greater than the latter,indicated that enantiomeric conversion can affect enantioselectivity.(3)The significant carbon isotopic enrichment occurred in the degradation process of Rac-MCPP in soil.The results of enantiomer culture showed that except the corn filed soil(Hefei)and mulberry filed soil(Hangzhou)incubated with S-enantiomer,there were significant carbon isotopic enrichment in the other soils.The apparent kinetic constants AKIE calculated from chiral herbicides ranged from 1.008?1.032,and the apparent kinetic effect constants for the C-H bond breaks were AKIE = 1.00?1.03,indicating that the initial degradation reactions were all C-H bond breaks.(4)The results of qPCR and PCR showed that no rdpA gene was detected in all four soils,but sdpA gene could be detected and the relative abundance of sdpA gene were changed when incubated with the Rac-MCPP,R-MCPP and S-MCPP.The highest relative abundance was 7.3 in the mulberry field soil(Hangzhou)under the incubation of S-type enantiomer.The relative abundance of sdpA gene growth in racemic mecoprop was lower than that in the enantiomer,indicating that there was little effect of initial concentrations in the microbial degradation process,even when the concentration is too high,it would inhibit the activity of microorganisms containing functional genes;at the same time,the relative abundance of sdpA gene were changed under the incubation of R-type enantiomer also could proof the existence of enantiomerization.(5)16S rRNA Illumina high-throughput sequencing results showed that the species richness(Chaol value,shannon index),species abundance distribution(uniformity,simpson index)and microbial community composition in the four soils after MCPP addition were changed.However,as time goes on,the initial state is gradually restored.The effect of treated with racemic MCPP is longer than treated with enantiomers.The relative abundance of Proteobacteria was significant increased in all the soils,and it could be inferred that it was the dominant bacteria group,which could dig deeper for the information of degradation related bacteria.(6)The sdpA amplicon high-throughput sequencing results showed that the soil type was the main factor determining the soil microbial community,while the internal comparative analysis of the soil showing that the different treatments and incubation time have obvious influence on the genetic composition(species composition).The effect of racemic treatment was more lasting.The species annotation results showed that 120 strans had successfully compared with kown strains,including MCPP-degrading strain Sphingobium herbicidovorans accounted for 5%,ranking third in the whole known strains.However,the other known bacteria that could degrade the chiral phenoxyalkanoic acid herbicides had not been found,yet.Besides,most of the species obtained by sequencing were still unknown.