Due to lack of quantitative analysis, it remains unclear where the bottleneck for heterotrophic CO 2-fixation is and whether the rate of heterotrophic CO 2-fixation is higher, lower, or comparable with that of autotrophic CO 2-fixation. To date, simple approaches capable of evaluating the CO 2 flux in heterotrophic microbes are still lacking, since the metabolites of the CO 2-fixing bypass pathway are indistinguishable from those of the central metabolic pathway. Examples include introduction of two enzymes of Calvin cycle into Escherichia coli and Saccharomyces cerevisiae, which resulted in enhanced CO 2 recycling in an air-tight fermentor and an increased ethanol yield, respectively.Īlthough these preliminary data suggested that heterotrophic CO 2-fixation is feasible, little is done to quantitatively analyze and evaluate the process. Recent studies indicated that incorporation of several steps of a natural carbon fixation pathway into a heterotrophic microbe may create a CO 2-fixing bypass pathway which enables the host to assimilate CO 2 at the expense of carbohydrates. Heterotrophic microbes usually do not assimilate CO 2 through the central metabolism. Apart from the light, autotrophic microbes can also use hydrogen and/or sulfur as the energy source for CO 2 assimilation under mild conditions. During the past 5 years, a variety of chemicals including ethanol, n-butanol, acetone, isobutyraldehyde, lactic acid, isoprene, 1,2-propanediol, methane, and biodiesel have been produced from CO 2 by engineered autotrophic microbes such as cyanobacteria and algae, using light as the energy resource.
To this end, energy input is required since the carbon in CO 2 is in its highest oxidation state. The wasteful greenhouse gas carbon dioxide (CO 2) is a potential raw material for production of chemicals and fuels. Quantitative analysis revealed that the CO 2-fixation rate of this strain is comparable with that of the autotrophic cyanobacteria and algae, demonstrating great potential of heterotrophic CO 2 fixation. coli by incorporating partial cyanobacterial Calvin cycle and carbon concentrating mechanism, respectively. The ability of CO 2 fixation was created and improved in E. This CO 2-fixation rate is comparable with the reported rates of 14 autotrophic cyanobacteria and algae (10.5–147.0 mg CO 2 L −1 h −1 or the specific rates of 3.5–23.7 mg CO 2 g DCW −1 h −1). coli with carbonic anhydrase was able to fix CO 2 at a rate of 19.6 mg CO 2 L −1 h −1 or the specific rate of 22.5 mg CO 2 g DCW −1 h −1. The value was increased to 17 % when the carbonic anhydrase involved in the cyanobacterial carbon concentrating mechanism was introduced, indicating that low intracellular CO 2 concentration is one limiting factor for CO 2 fixation in E. coli strain, the flux of the CO 2-fixing bypass pathway accounts for 13 % of that of the central carbon metabolic pathway. When two sequential enzymes of the cyanobacterial Calvin cycle were incorporated into an E. ResultsĪ simple metabolic flux index was developed to indicate the relative strength of the CO 2-fixation flux. Although preliminary research has suggested that CO 2 fixation in heterotrophic microbes is feasible after incorporation of a CO 2-fixing bypass into the central carbon metabolic pathway, it remains unclear how much and how efficient that CO 2 can be fixed by a heterotrophic microbe. However, their slow growth rates call for investigation of the possibility of heterotrophic CO 2 fixation. Autotrophic microbes naturally assimilate CO 2 using energy from light, hydrogen, and/or sulfur. Production of fuels from the abundant and wasteful CO 2 is a promising approach to reduce carbon emission and consumption of fossil fuels.
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