title: Quantitative analysis of an engineered CO2-fixing Escherichia coli reveals great potential of heterotrophic CO2 fixation.
abstract: BACKGROUND
  Production of fuels from the abundant and wasteful CO2 is a promising approach to reduce carbon emission and consumption of fossil fuels. Autotrophic microbes naturally assimilate CO2 using energy from light, hydrogen, and/or sulfur. However, their slow growth rates call for investigation of the possibility of heterotrophic CO2 fixation. Although preliminary research has suggested that CO2 fixation in heterotrophic microbes is feasible after incorporation of a CO2-fixing bypass into the central carbon metabolic pathway, it remains unclear how much and how efficient that CO2 can be fixed by a heterotrophic microbe.
  
  
  RESULTS
  A simple metabolic flux index was developed to indicate the relative strength of the CO2-fixation flux. When two sequential enzymes of the cyanobacterial Calvin cycle were incorporated into an E. coli strain, the flux of the CO2-fixing bypass pathway accounts for 13 % of that of the central carbon metabolic pathway. The value was increased to 17 % when the carbonic anhydrase involved in the cyanobacterial carbon concentrating mechanism was introduced, indicating that low intracellular CO2 concentration is one limiting factor for CO2 fixation in E. coli. The engineered CO2-fixing E. coli with carbonic anhydrase was able to fix CO2 at a rate of 19.6 mg CO2 L(-1) h(-1) or the specific rate of 22.5 mg CO2 g DCW(-1) h(-1). This CO2-fixation rate is comparable with the reported rates of 14 autotrophic cyanobacteria and algae (10.5-147.0 mg CO2 L(-1) h(-1) or the specific rates of 3.5-23.7 mg CO2 g DCW(-1) h(-1)).
  
  
  CONCLUSIONS
  The ability of CO2 fixation was created and improved in E. coli by incorporating partial cyanobacterial Calvin cycle and carbon concentrating mechanism, respectively. Quantitative analysis revealed that the CO2-fixation rate of this strain is comparable with that of the autotrophic cyanobacteria and algae, demonstrating great potential of heterotrophic CO2 fixation.
authors.affiliation: CAS Key Laboratory of Microbial Physiological and Metabolic Engineering, Institute of Microbiology, Chinese Academy of Sciences, No. 1 West Beichen Road Chaoyang District, Beijing, 100101 China ; University of the Chinese Academy of Sciences, Beijing, China.
authors.forename: Fuyu
authors.initials: F
authors.lastname: Gong
authors.affiliation: CAS Key Laboratory of Microbial Physiological and Metabolic Engineering, Institute of Microbiology, Chinese Academy of Sciences, No. 1 West Beichen Road Chaoyang District, Beijing, 100101 China.
authors.forename: Guoxia
authors.initials: G
authors.lastname: Liu
authors.affiliation: CAS Key Laboratory of Microbial Physiological and Metabolic Engineering, Institute of Microbiology, Chinese Academy of Sciences, No. 1 West Beichen Road Chaoyang District, Beijing, 100101 China ; University of the Chinese Academy of Sciences, Beijing, China.
authors.forename: Xiaoyun
authors.initials: X
authors.lastname: Zhai
authors.affiliation: CAS Key Laboratory of Microbial Physiological and Metabolic Engineering, Institute of Microbiology, Chinese Academy of Sciences, No. 1 West Beichen Road Chaoyang District, Beijing, 100101 China.
authors.forename: Jie
authors.initials: J
authors.lastname: Zhou
authors.affiliation: CAS Key Laboratory of Microbial Physiological and Metabolic Engineering, Institute of Microbiology, Chinese Academy of Sciences, No. 1 West Beichen Road Chaoyang District, Beijing, 100101 China.
authors.forename: Zhen
authors.initials: Z
authors.lastname: Cai
authors.affiliation: CAS Key Laboratory of Microbial Physiological and Metabolic Engineering, Institute of Microbiology, Chinese Academy of Sciences, No. 1 West Beichen Road Chaoyang District, Beijing, 100101 China.
authors.forename: Yin
authors.initials: Y
authors.lastname: Li
keywords: CO2-fixation rate; Carbon fixation; Carbonic anhydrase; Heterotrophic microbe; Rubisco