Researchers from Newcastle University in the UK have engineered Escherichia coli bacteria to capture carbon dioxide using hydrogen gas to convert it into formic acid. The research, accepted for publication in Applied and Environmental Microbiology raises the possibility of converting atmospheric CO2 to commodity chemicals.
Escherichia coli is gram-negative bacterium that is a workhorse for biotechnology. The organism naturally performs a mixed-acid fermentation under anaerobic conditions where it synthesises formate hydrogenlyase (FHL-1). The physiological role of the enzyme is the disproportionation of formate in to H2 and CO2. However, the enzyme has been observed to catalyse hydrogenation of CO2 given the correct conditions, and so has possibilities in bio-based carbon capture and storage if it can be harnessed as a hydrogen-dependent CO2-reductase (HDCR).
In this study, an E. coli host strain was engineered for the continuous production of formic acid from H2 and CO2 during bacterial growth in a pressurised batch bioreactor. Incorporation of tungsten, in place of molybdenum, in FHL-1 helped to impose a degree of catalytic bias on the enzyme. This work demonstrates that it is possible to couple cell growth to simultaneous, unidirectional formate production from carbon dioxide and develops a process for growth under pressurised gases.
Normally, an enzyme in E. coli catalyzes the reverse of this reaction—the production of H2 and CO2 from formic acid.
To reverse the normal reaction in E. coli, the investigators got the bacteria to switch out molybdenum, a metal that is normally a critical part of the enzyme, for tungsten, by growing the bacteria in an excess of the latter.
This is fairly easy to do as E. coli cannot readily tell the difference between the two. Swapping of tungsten for molybdenum changed the properties of our enzyme so that it was locked in CO2 capturing mode rather than being able to switch between CO2 capture and CO2 production.
The investigators used a special pressurized bioreactor filled with H2 and CO2 to make the gases available to the microbes. The bacteria grew under gas pressure and generated formic acid from the CO2, said Dr. Sargent.
Dr. Sargent said he developed the idea from reading about the emergence of life on Earth, both in primary literature and popular science books. Three and a half billion years ago, there was no oxygen in the atmosphere, but there were high levels of CO2 and H2, and cellular life had begun evolving 10,000 meters below the ocean’s surface.
Back then, these compounds would have needed to be converted into the carbohydrates on which all life depends. That could have been accomplished by an enzyme “such as the one we found in E. coli, hydrogenating carbon dioxide into an organic acid,” said Dr. Sargent. “We wanted to try this in the lab.”
The ultimate aim would be to capture wasted CO2 using renewable hydrogen gas from biohydrogen—as in this research—or electrolysis powered by renewable electricity, and convert it to formic acid. The key is for a microbe to use formate as its sole carbon source. Then we can make fuel, plastic or chemicals. This is the vision of a truly cyclic bioeconomy where CO2 is constantly produced, captured and returned to the market.
Magali Roger, Thomas C. P. Reed and Frank Sargent (2021) “Harnessing Escherichia coli for bio-based production of formate
under pressurized H2 and CO2 gases.” AEM Accepted Manuscript doi: 10.1128/AEM.00299-21