Abstract
BackgroundProduction 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.ResultsA 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).ConclusionsThe 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.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-015-0268-1) contains supplementary material, which is available to authorized users.
Highlights
Production of fuels from the abundant and wasteful CO2 is a promising approach to reduce carbon emission and consumption of fossil fuels
As the metabolic flux of the central metabolism for a given strain is quite stable, the relative metabolic flux of the CO2-fixing bypass pathway over that of the central carbon metabolic pathway may give a quantitative understanding on the efficiency of CO2 fixation
Quantitative analysis in this study revealed that an engineered heterotrophic E. coli could assimilate CO2 at a rate comparable to that of the autotrophic cyanobacteria and algae
Summary
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 Their slow growth rates call for investigation of the possibility of heterotrophic CO2 fixation. Examples include introduction of two enzymes of Calvin cycle into Escherichia coli and Saccharomyces cerevisiae, Gong et al Biotechnology for Biofuels (2015) 8:86 which resulted in enhanced CO2 recycling in an air-tight fermentor [19] and an increased ethanol yield [20], respectively. These preliminary data suggested that heterotrophic CO2-fixation is feasible, little is done to quantitatively analyze and evaluate the process. Due to lack of quantitative analysis, it remains unclear where the bottleneck for heterotrophic CO2-fixation is and whether the rate of heterotrophic CO2-fixation is higher, lower, or comparable with that of autotrophic CO2-fixation
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