Reducing atmospheric carbon dioxide

A descendant of CO2-eating e coli could be modified to make organic carbon molecules

The reduction of atmospheric carbon dioxide (CO2) is vital to combat Climate Change, since CO2 contributes roughly half of all greenhouse gases. Research at Israel’s Weizmann Institute of Science indicates there could be a way to design bacteria to absorb atmospheric CO2.   

The natural mechanism for removing atmospheric CO2 is photosynthesis. Leaves contain chlorophyll, which uses energy from sunlight in a chemical reaction that creates sugars out of atmospheric CO2 and water.  

Sugars have a large number (“n”) of carbon atoms coupled to a large number of water molecules (H2O) and formulae on the lines of Cn(H2O)n. An important “waste product” in photosynthesis is excess oxygen, which is released by plants into the atmosphere. Plants and trees also absorb oxygen and release CO2 during hours of darkness. But net-net, plants release more oxygen (O) and consume more CO2. Photosynthetic cyanobacteria — aquatic microbes that produce oxygen — also use energy from light to fix CO2 and turn it into sugars, proteins and fats. But natural photosynthesis is insufficient to deal with the excessive production of CO2 by industrial processes. 

Now Israeli scientists claim they have found a way to engineer common bacteria to eat atmospheric CO2. Right now, these bacteria release more CO2 than they consume. But this is a transformative approach and it may lead to insights that help to fight Climate Change. 

Living organisms are either autotrophs (such as plants) that convert CO2 into biomass (wood), and heterotrophs that consume organic compounds (humans and other animals). The common bacteria Escherichia coli (E.coli) is found in the guts of many animals and it is heterotrophic. It is easy to genetically engineer, and its fast growth means changes through generations can be quickly tested and tweaked to optimise mutations. Normally it consumes sugars, and emits CO2.

However, in an experiment described in Cell magazine, (https://www.cell.com/fulltext/S0092-8674(16)30668-7), a Weizmann team described how they transformed the dietary habits of e coli. Lead author, Shmuel Gleizer said, “From a basic scientific perspective, we wanted to see if such a major transformation in the diet of bacteria — from dependence on sugar to the synthesis of all biomass from CO2 — is possible. Beyond testing the feasibility of such a transformation in the lab, we wanted to know how extreme an adaptation is needed in terms of the changes to bacterial DNA.” 

The engineered strain of e coli first harvested energy from formate (HCO2), a chemical which can be produced electrochemically from renewable sources (or from formic acid, which is found in ants). Formate has only one carbon atom, and does not normally serve as a food source for e coli. 

In 2016, the Weizmann team created an e coli strain that ate CO2, but it preferred sugars. Using genetic engineering they gave it genes that allow photosynthetic organisms to convert CO2 into carbon. Photosynthesis isn’t possible in bacteria but they managed to insert a gene that lets the bacterium eat formate for energy.

The scientists further modified the bacteria to inactivate key enzymes that made it heterotrophic and thus, more dependent on autotrophic methods. Eventually they succeeded in “building” a bacteria that consumed only CO2. They confirmed this by labelling food sources with marker chemicals and detecting changes in the marker-levels. 

Then they cultured successive generations of the modified bacteria, with minute quantities of sugar, and CO2 at very high concentrations (about 250 times that in the atmosphere). After about 200 days, bacteria capable of using CO2 as their only carbon source grew. After 300 days, these bacteria grew faster than those that could not consume CO2.

The autotrophic bacteria still prefer sugar. Also this is slow-growth. Normal e coli doubles in number every 20 minutes, while the autotrophic type divides every 18 hours in an atmosphere that is 10 per cent CO2. They can’t survive at current atmospheric levels of CO2 of about 0.0407 per cent (407 parts per million). The scientists are trying to make the bacteria grow faster and live on lower CO2 levels. They are also trying to understand how changes in just 11 genes allowed the switch. 

Biotech companies use cell cultures living on corn syrup to produce chemicals. If such cells — yeast or bacteria — could live on CO2 and renewable electricity, they could be weaned from corn syrup. They may be further adapted to take energy from a solar cell and store that energy for use as fuel in the form of carbon fixed in cells. Thus, a descendant of this CO2-eating e coli could be modified to make organic carbon molecules to use as biofuels. Such products would have lower emissions and ideally, they could remove atmospheric CO2.