Studies On Extracellular Electron Transfer Mechanisms Of Enterobacter Cloacae SgZ-5T For The Biorecovery Of Palladium As Nanorods

Electrochemically active microorganisms?EAMs?can transfer electrons from substrate metabolism to extracellular electron acceptors,directly or indirectly,thereby achieving the purpose of pollutant degradation and metal recovery.Therefore,it is of significance to study extracellular electron transfer?EET?mechanism of EAMs for energy transformation and environment remediation.In this thesis,an electrochemically active strain,Enterobacter cloacae SgZ-5T,was isolated from microbial fuel cell.It can specifically reduce Pd???.Based on this point,the mechanism of how SgZ-5T reduce Pd???was investigated combining with electrochemistry,spectroscopy,metabolomics and transcriptomics techniques.The main conclusions are summarized as follows.?1?SgZ-5T could specifically reduce the Pd???ion to Pd?0?as nanorods.However,it had weak ability to reduce other metal ions,such as Au and Ag.Compared with SgZ-5T cells before exposed to Pd???,different electrochemical behaviors were observed from SgZ-5T after interacted with Pd????labelled as SgZ-5T-Pd?:i)obvious potential shifts were found on the two pairs of redox systems in SgZ-5T-Pd cells shown in electrochemical different pulse voltammograms;ii)the surface of SgZ-5T-Pd cells became much smoother than before reduction.The characteristic IR bands corresponding to membrane proteins changed a lot and the disappearance of the IR band of CO₂ indicated that some change occurred in cell metabolism;iii)SgZ-5T-Pd could not only secrete hydroquinone?HQ?,but also regulated its own metabolic pathway to secrete riboflavin?RF?under the induction of Pd???from gas chromatography-time-of-flight/mass spectrometry,high efficiency liquid chromatography and mass spectrometric results.In addition,no hydroquinone was detected in the supernatant in the presence of Pd???by the reason that HQ may adsorb on the surface of Pd nanorods.?2?The corresponding metabolic pathways were also analyzed using metabolomics and transcriptomics to further investigate the recovery process.No significant change had happened on the abundance of YqjG gene encoding glutathionyl-HQ reductase synthase in SgZ-5T-Pd,thus leading to the occurrence of few change in SgZ-5T-Pd cells compared to Pd???-free.On the other hand,the abundance of 6-phosphate gluconic acid,ribulose-5-phosphate,ribose-5-phosphate,fructose-6-phosphate and RF notably increased,indicating that Pd induced the production of riboflavin synthase.It would further cause up-regulation of the abundance of gene encoding RF synthase?ribE?in the present of Pd???.All these significant differences indicated that SgZ-5T could recover precious metal of Pd by regulating its metabolism.?3?The UV-Vis spectroscopy data demonstrated that the Pd???reduction rate was enhanced by 5%,5.5%and 30%by the addition of3.33?M HQ,3.33?M RF and the both,respectively.All these results revealed that the bi-mediators secreted by bacteria could significantly improve the recovery efficiency of Pd???.In summary,the EET mechanism of Enterobacter cloacae SgZ-5T via bi-mediators to recover noble metal of Pd was demonstrated.These findings developed and improved microbial EET mechanism,which was also meaningful for understanding metal biorecovery processes and natural biogeochemical processes.