Regulation Of Bidirectional Electron Transfer Of Electroactive Bacteria And The Environmental Application

Electroactive bacteria(EAB)are attractive for wastewater treatment,environmental remediation and chemicals/energy production due to their unique physiological metabolic patterns and abilities to transport electrons to or from external environment.However,the mechanisms of bidirectional electron transport processes,including both extracellular electron transfer(EET)and extracellular electron uptake(EEU),remain poorly understood and the electron transport abilities of EAB are generally low,which restricts their practical application.In this thesis,we investigated into the mechanisms of bidirectional electron transfer of EAB and based on which explored the regulation strategies and environmental application.The phenomenon and mechanism of pyrite oxidation by Geobacter sulfurreducens,a model EAB strain,was firstly explored.The pyrite oxidation by G.sulfurreducens was mediated by Fe2+/Fe3+,and the pyrite-derived electrons were mainly used for intracellular fumarate reduction.Accompanied with the fumarate reduction,a transmembrane proton gradient was developed to drive intracellular ATP synthesis.However,such EEU process with limited electron flow is insufficient to drive the intracellular NADH synthesis.We found that a low extracellular pH could enhance the EEU rate in a bio-cathode system and induce intracellular NADH synthesis in the coupled process of pyrite oxidation-fumarate reduction.This result implies that engineering the transmembrane proton gradient may present an effective strategy to promote microbial electrosynthesis of EAB.Many EAB are capable of both EEU and EET,rendering them a high metabolic versatility in variable environment.Next,we explore engineering strategies to strengthen the EET ability of EAB by using Shewanella oneidensis MR-1 as a model strain.A new genetic editing system(CRISPR-ddAsCpf1)was created and introduced into the S.oneidensis MR-1 to regulate the electron flux of its EET process.The electron flux rediverting strategy was successfully applied for manipulating four putative cytochrome gene targets(fccA,dmsE,cctA,SO2930),and six other targets(SO0717,napB,dmsE,cctA,SO2930,nrfA)to enhance the reduction of methyl orange and Cr(?),respectively.Therefore,such an electron flux rediverting strategy allows for exploring the specific functions of different genes and may lay a basis for engineering EAB for more efficient environmental remediation applications.Lastly,we explored the application of EAB for recovering sulfur from waste streams and generating electricity at the same time.A novel bioelectrochemical sulfur recovery system(BESRS)was developed by incorporating the redox couples of I-/I3-and Fe2+/Fe3+ to mediate the anodic and cathodic reactions,respectively.Specifically,combining the sulfide oxidation anode and a bio-cathode employing bio-catalyst to recover electricity and sulfur resource.H2S was selectively and efficiently converted into elemental sulfur particles at a I-/I3-mediated abiotic anode,while a Fe2+/Fe3+-mediated oxygen-reducing biocathode inoculated with Acidithiobacillus ferrooxidans was adopted to drive electricity generation.This system achieved efficient sulfur recovery,with 100%H2S removal and a sulfide oxidizing rate(799 mg L-1 d-1)surpassing all the existing bioelectrochemical systems,and stable power output of about 100 mW/m2 and during 50-h continuous operation.Due to the continuous acidity replenishment from H2S supply,the catholyte pH was kept at about 1.5 to favor an efficient Fe2+/Fe3+ cycling.Considering that H2S is primarily derived in anaerobic processes,we further expanded the system to microbial electrosynthesis application.To this end,Methanococcus maripaludis S2 was employed as bio-catalyst for CH4 production from CO2.The synchronous and highly efficient elemental sulfur recovery and CH4 production imply a great potential of this system for sustainable biogas upgrading and exhaust gas purification.Therefore,our work may favor an improved manipulation and extended application of EAB for practical energy/resource recovery and environmental pollution control applications.