The Mechanisms Of The Start-up And Enhancement Of CO₂ Reduction For Acetate Production In Microbial Electrosynthesis System

With the growth of the population,human beings consumed a large number of fossil fuels and discharged a large amount of CO₂.CO₂ is the main component of greenhouse gas,which lead to global warming.To reduce CO₂ emissions,the technologies of carbon capture,utilization,and storage（CCUS）had attracted researchers’attention.Microbial electrochemical synthesis（MES）is a novel carbon utilization technology,which could convert CO₂ into organic matter.However,the application of MES was limited,such as the long startup period,low acetate production,and low electron recovery rate.Previous studies focused on the effects of cathode materials and operating parameters on the performance of acetate production,but further clarifications were needed on the microbial community structure of biocathode,enhancement of electron transfer and the mechanism of microbial energy metabolism.Based on the reducing CO₂ to acetate in MES,this study investigated the impact of startup and enhancement strategies on acetate production,detected the population and quantity of key microbial,analyzed the change rule of the microbial community structure,and explored the influence mechanism of the key metabolic pathway.To clarify the mechanism of startup and enhancement strategies on the acetate production in MES.The main research contents of this paper are as follows:（1）For exploring an efficient startup strategy,two strategies,i.e.heterotrophic precultivation（HP）and polarity reversal（PR）,for the startup of acetate production MES were investigated in this study.After 9 cycles operation,the acetate production of HP was 79%higher than that of PR.Higher abundances of 16 S r RNA gene and fhs gene on the biocathode of HP than that of PR were revealed.Functional prediction indicated that HP had higher capacities of electron transfer and energy metabolism,and lower capacities of other pathways for acetyl-Co A competition.Though the biocathodes of HP and PR had relatively similar electrochemical characteristics,the advantages in the electrode microorganism abundance,microbial community structure and functions,resulted in the better acetate production performance of HP than PR.The results revealed that heterotrophic precultivation rather than polarity reversal is a better startup strategy of acetate microbial electrosynthesis reactor.（2）For exploring the effect of electron shuttle molecules（ESMs）on microbial interactions,riboflavin（B2）,methyl viologen（MV）and neutral red（NR）were added in the MES.The acetate production of B2 and NR addition were 41%and 51%higher than that of the control.Butyrate was detected in the MV and NR dosed groups.The microbial community of 16S r RNA gene and the transcriptional expression of microbial electron receptors were analyzed.The acetogens relative abundance of control,B2,MV and NR were 0.29%,5.68%,22.78%and42.89%,respectively.The microbial function profile of the microbial community on the biocathodes indicated that the effect of ESMs addition on the expression on electron transport was greater than the Wood-Ljungdahl（WL）pathway.The B2 was coupled with the NADH complex and hydrogenase,while NR was coupled with the Rnf complex to support electron transfer and energy conversion via various electron transfer pathways.The results revealed that the ESMs coupled with different electron acceptors of microorganisms to achieve electron transfer,resulting in product changes.（3）The effect of Fe²⁺and Ni²⁺addition on acetate production in MES was investigated.Both Fe²⁺and Ni²⁺addition enhanced acetate production of the MES,which was 76.9%and110.9%higher than that of control,respectively.Gene expression of‘Energy metabolism’,especially in‘Carbon fixation pathways in prokaryotes’was up-regulated by Fe²⁺and Ni²⁺addition.Hydrogenase was found as an important energy transfer mediator for CO₂ reduction and acetate synthesis.Fe²⁺addition and Ni²⁺addition respectively enhanced the expression of methyl branch and carboxyl branch of the WL pathway,and thus promoted acetate production.The analysis of Metatranscriptomics showed that Gene expression of M00023 and M00140,which were the metabolism of positive correlation WL pathway,were up-regulated by Fe²⁺and Ni²⁺addition.Meanwhile,the expression of fatty acid metabolism genes were up-regulated by Fe²⁺and Ni²⁺addition.However,the expression of methanogenic were inhibited by Fe²⁺and Ni²⁺addition,especially hydrogenotrophic methanogenesis.The carbon fixation pathways of the Calvin cycle,the reverse tricarboxylic acid cycle and the dicarboxylic acid 4-hydroxybutyric acid pathway were detected in MES.The results provided a metatranscriptomic insight into the effect of Fe²⁺and Ni²⁺on acetate production by CO₂ reduction in MES.（4）For exploring the performance and mechanisms of acetate synthesis in the Cyanobacterial biochar addition,the acetate concentration and electron recovery rate were examined,and microbial morphology and community structure were analyzed.The results showed that the acetate concentration of Cyanobacterial biochar（ABC）and hydrogen peroxide modified Cyanobacterial biochar（ABC-H₂O₂）were increased by 33.8%and 77.0%,respectively,and the electron recovery rate of ABC-H₂O₂ group was higher than that of ABC group.The analysis of electron transfer performance and redox activity of biochar showed that the oxygen-containing functional groups on the surface of ABC-H₂O₂ increased,enhanced the capacity of oxidation activity and electron transfer,which was beneficial for microorganisms to obtain electrons from biochar.The abundance of acetogens was reduced,while the abundance of electroactive and hydrogen-producing microorganisms were increased in the addition of biochar.The better acetogenic performance in biochar addition is mainly to provide more sufficient electrons for acetogens and increase the abundance of electroactive and hydrogen-producing microorganisms.This study clarified the factors which influenced the startup and enhancement strategies on acetate production in MES,analyzed the influence mechanism of various factors on acetate production,clarified the metabolic characteristics related to acetate synthesis,and analyzed the response characteristics of the enhancement strategies.The research was conducive to further understanding the mechanism of electron transfer,microbial metabolism and microbial interaction in the process of acetate synthesis in MES,and provided the theoretical basis and technical support for the upgrading and application of reducing CO₂ to acetate in MES.