Biodegradation,Chemotaxis And Genome Study Of Polychlorinated Biphenyls Degrading Strain Rhodococcus Sp.

Polychlorinated biphenyls(PCBs)is a typical persistent organic pollutant.Microbial degradation of PCBs pollutants is an attractive clean-up strategy for its environmental friendly and cost-effective.Previous studies about the biodegradation of PCBs mainly focused on the isolation of degrading strains and the analysis of metabolic pathways.However,the biodegradation efficiency of PCBs can be limited by its strong hydrophobicity.The chemotactic motility of bacteria could increase pollutants bioavailability,which plays a key role in bioremediation.Although most of the reported degrading bacteria with chemotactic capability are flagella bacteria,there are few studies about the chemotaxis of degrading bacteria such as Rhodococcus sp.,which is flagella-free and widely distributes in environment.In this paper,Rhodococcus ruber SS1,Rhodococcus pyridinivorans SS2 and Rhodococcus biphenylivorans TG9 were studied,focusing on their biodegradability of biphenyl and PCBs,as well as the chemotaxis to biphenyl and its metabolites benzoate,PCBs and its metabolites chlorobenzoate and TCA cycle intermediates.The novel species TG9,which had a strong biodegradability,was selected for genome sequencing.RT-qPCR was used to study the expression of degradation key genes,chemotaxis genes and gas vesicle protein genes.The PCBs attenuation was investigated as well as the abundance of Dehalococcoides spp.,aerobic degradation key gene,chemotaxis gene and flagellin gene in paddy soil with floody and dry treatment.The main results are as follows:(1)Strains SS1,SS2 and TG9 exhibited high biphenyl tolerance and degradation abilities,and TG9 could tolerate more than 3000 mg/L biphenyl.Three strains could degrade the intermediate metabolites benzoate and chlorobenzoate,suggesting a complete biphenyl/PCBs metabolic pathway.(2)Both SS1 and SS2 had high cell surface hydrophobicity,which might be one of the reasons for its high degradation efficiency.Although the cell surface hydrophobicity of TG9 was low,it had a strong biodegradability.The underlying mechanism needs further study.(3)Drop assays showed that the three strains did not show chemotaxis towards the tested PCBs.But they had the chemotaxis ability to biphenyl,benzoate,2-CBA and TCA cycle intermediates,and the chemotaxis of TG9 to biphenyl was induced.This might be an important reason for their high degradation efficiencies.The capillary chemotaxis assays showed that three strains did not show chemotaxis towards biphenyl,benzoic acid,2-CBA and TCA cycle intermediates in liquid environments.The electron microscopy results showed that the strains did not have flagella and pilus.It is speculated that they may have chemotaxis ability by gliding on solid surface.(4)The complete genome of strain TG9 is composed of one gapless circular chromosome of 5,034,221 bp and contains 4882 protein coding genes,53 tRNA genes,and 12 rRNA genes,with 68.04%GC content.The TG9 genome has a complete bph genes encoding the enzyme required for biphenyl metabolic.And the genome of TG9 contains mcp,cheR and cheY genes and a gas vesicle genes gene cluster.(5)RT-qPCR results showed that the expression of bphA,cheY4,gas vesicle genes gvpA,gvpJ,and gvpS of TG9 were up-regulated when cultured by PCB1;The bphA,mcp,cheR,cheY4,and gvpJ genes were significant up-regulated when cultured by biphenyl;The bphA and cheY4 were significant up-regulated when cultured by benzoate;The cheYl,cheY4,gvpA,and gvpJ genes were significant up-regulated when cultured by citric acid.It is speculated that the mcp,cheR and cheY4 genes may be involved in the chemotaxis regulation,and there may be an intrinsic link between the degradation and chemotaxis.(6)High chlorinated PCBs and low-chlorinated PCBs decreased with the passage of time in floody or dry paddy soil.The change of cheA copies number was not significant,and the fliC copies number decreased in floody condition.Multivariate statistics(RDA)revealed that Dehaloccoides spp.was positively influenced by the higher chlorinated PCBs.bphC and fliC gene were affected with moisture contents and low-chlorinated PCBs,suggesting a close connection between the abundance of potential degraders and degradation activity.The results will help to reveal the mechanism of chemotaxis and degradation of Rhodococcus,and also provide a new guidance for the remediation of PCBs contaminated soil.