The Regulatory Mechanism Of Element Zinc On Anaerobic Clostridium Carboxidivorans P7 Fermentation Solvent

Fossil energy is consumed by the astonishing speed,and it is estimated that global oil resources will be depleted in 50 years.According to the current level of consumption,coal resources will be maintained for 200 years at most.Natural gas resources will be exhausted in 80 years if the annual exploitation quantity is maintained at 2.3 trillion cubic meters.The extensive utilization of fossil fuels increased the concentration of carbon gas and cause serious harm to environment.Facing fossil energy depletion and environmental pollution,the utilization of renewable energy has been widely concerned.It already become extremely urgent problems to seek the competitive renewable clean energy for mankind.The synthesis of chemicals such as ethanol and butanol with syngas（CO,CO₂ and H2）as raw materials has been paid attention.At the same time,the development of syngas fermentation industry can promote the strategic shift of industrial material line and control industrial pollution from the begining,which is significant to solve environmental pollution and resource constraints.There are two methods for synthesis of syngas: chemical synthesis and microbial fermentation.Compared with chemical synthesis,microbial fermentation has the advantages of low cost,wide adaptability,simple operation and great specificity of products.There is a class of anaerobic bacteria named acetogen in nature,which can use H2+CO₂ or / and CO as carbon source for autotrophy.They can synthesize final metabolites with the value of industrial application and liquid fuels such as ethanol and butanol in the process of autotrophy.In this study,we use Clostridium carboxidivorans P7 as the subject,which is a kind of acetogens.Their metabolites are ethanol,butanol,hexanol,acetic acid and butyric acid.The metabolic pathway of products consists of two stages: carbon fixation and product synthesis.With the use of CO₂ or CO,we synthesize intermediate metabolite-acetyl coenzyme A（AcetylCoA）by the Wood-Ljungdahl pathway,which can produce ethanol,butanol,hexanol,acetic acid and butyric acid by the metabolic pathway of products and synthesize cytoskeleton.In this study,we report the regulation and molecular mechanism of C.carboxidivorans P7 syngas fermentation under Zn stress for the first time.Within a certain concentration range,C.carboxidivorans P7 fermentative biomass and alcohol yield were increased along with initial Zn+ concentration（0⁵60 μM）growth.We research the system that Zn+ concentration was 280 μM by real-time fluorescent quantitative PCR.According to expression level changes of functional genes in the Wood-Ljungdahl pathway and the Acid-Alcohol metabolic pathway of C.carboxidivorans P7,it idicates that relative expression level of genes involved in carbon fixation-acsA,acsB,fdhI,fdhⅡ,fdhⅢ and fdhⅤ genes involved in product synthesis pathway-buk,bdh10,bdh35,bdh4 and bdh50 were increased.Meawhile,expression level of adh gene and CoAT gene were also significantly increased.With the increase in expression level of genes involved in carbon fixation and Adh gene,C.carboxidivorans P7 can improve overall efficiency of syngas fermentation under Zn stress.Most of enzymatic active centers contain metal elements in the Wood-Ljungdahl pathway and the Acid-Alcohol metabolic pathway.Therefore,we select metal elements-Zn,Mo,Ni,which play an important role in organism,to optimize culture medium to improve the alcohol content of products of C.carboxidivorans P7 and provide the theoretical basis for industrial production and the function of mental elements in organism.According to the medium optimization result of single metal element,it indicates that C.carboxidivorans P7 biomass accumulation and target product yield were increased when Mo was removed from the medium and Ni content was 8.4 μM.C.carboxidivorans P7 biomass accumulation and target product yield were increased along with Zn concentration growth,which was different from Mo and Ni.At the same time,element combination optimized medium-Mo（0 μM）/Zn（70 μM）/Ni（8.4 μM）was most favorable to improve the alcohol content of products of C.carboxidivorans P7,which means that different metal elements have interaction in organism.The final alcohol content reached 7.13 g/L,which was 3 times higher than the alcohol content of standard medium.