Study On The Degradation Characteristics Of Polychlorinated Biphenyls By Rhodococcus Biphenylivorans TG9

Polychlorinated Biphenyls(PCBs)are a class of typical persistent organic pollutants in the environment,while soil and sediment are the sinks of PCBs.In recent years,biodegradation of PCBs has attracted great attention in the field of environmental science,while the crucial part is to screen out strains capable of removing PCB congeners at a high biotransformation rate and the clarification of degradation mechanism.A strain named as Rhodococcus biphenylivorans TG9 which could degrade PCBs was isolated from the PCBs contaminated sediment by resuscitation promoting factor.In this study,the transformation potential of different PCB congeners by TG9 has been determined.In addition,the PCB congeners which could be utilized as the sole carbon and energy source were screened and the degradation kinetics and intermediate metabolites were determined and identified.Finally,the complete genome sequencing results were analyzed and the activities of the key enzymes of upper pathway were explored.The main results are as follows:(1)The TG9 cells induced by biphenyl could degrade 94.4%of Aroclor1221,58.5%of Aroclorl242,27.3%of Aroclor1254 and 3.3%of Aroclor1260 at the concentration of 10 mg/L,respectively.Eighteen typical PCB congeners were chosen to evaluate the degradation characteristics of TG9,results showed that degradation rates decreased with the increasing number of chlorine substitution.It was found that the degradation selection order of TG9 was 2,2’-CB>3,3’-CB>4,4’-CB.Additionally,the monomers of PCB with symmetrical structure were transformed slower than that with unsymmetrical structure.(2)When exposed to 50 mg/L,250 mg/L and 500 mg/L Aroclor1242,TG9 could remove 61.1%,29.0%and 12.1%of PCBs in seven days,demonstrating its strong durability against toxic contaminants.Meanwhile,the degradation of PCB31 showed that the degradation rates of PCBs were decreased when the concentrations of pollutants were too high or too low.(3)The preliminary screening result of 96-well cell culture plates showed that strain TG9 could utilize 18 kinds of PCB monomer as sole carbon and energy source.The results of conical flask culture system showed that PCB1,PCB3,PCB9,PCB12,PCB14 and PCB18 could be utilized by TG9 while PCB10 and PCB31 could not serve as the sole carbon and energy source.The obtained results were consistent with the preliminary screening results.(4)The biodegradation kinetics of PCB monomers showed that the number of chlorine substitution was the main influencing factor of biodegradation rate.The degradation pattern of PCB 1,PCB3 and PCB 12 conformed to the first-order kinetic equation,and the maximum degradation rate constant was 0.139.After biphenyl activation,TG9 could degrade PCB31,and the maximum degradation rate constants of PCB31,PCB8 and PCB11 were 0.197,0.470 and 0.453,respectively.The possible reason was that the Bph enzymes in TG9 were strongly induced.(5)Varies amount of chlorobenzoic acid were produced from differentially substituted PCBs,it showed that the biodegradation pathway of PCBs belonged to bph.Moreover,TG9 could continue to utilize some chlorobenzoic acids,even the 2,5-CBA which has more than one chlorine atoms on aromatic ring could be degraded completely.(6)The biphenyl-catabolic(bph)genes encoding enzymes for the degradation of PCBs were analyzed in TG9.The results showed that TG9 has diverse biphenyl/PCBs degradative genes,including three bphC genes in the upstream pathway and the gene cluster in TG9 may be a new cluster.Plasmid extraction results indicated that plasmid was not present in TG9.(7)The transcription level of bph genes in normal active state and the dormancy state in the simulated environment were compared,it showed that the bph genes were up-regulated except the bphB gene while TG9 was under the normal active state.The activity of BphA and BphC in TG9 were increased significantly under the condition of biphenyl,PCB1 and PCB 12,while the activities of these enzymes did not change in PCB31 culture system.Those results further clarified the mechanism of enzymes expression when TG9 was under different metabolic modes.