How microbes might help us reverse climate change

How fossil fuels contribute to climate change is now well understood: the burning of hydrocarbons releases carbon dioxide (CO₂), which accumulates in the atmosphere to cause global warming.

The challenge now is to reduce our dependence on fossil fuels or even reverse the process and turn CO₂ back into useful hydrocarbons. At the Korea Advanced Institute of Science and Technology (KAIST), in Daejeon South Korea, researchers are using microbes to do just that.

“We need to move away from fossil fuels as a source of chemicals. We need to utilize renewable biomass instead, especially non-edible sources, such as agricultural or food waste, or even CO₂ itself. These byproducts can be converted to useful fuels and chemicals,” says Sang Yup Lee, a distinguished professor in KAIST’s Department of Chemical and Biomolecular Engineering.

As pioneers in the emerging discipline of systems metabolic engineering (SME), Lee’s colleagues are integrating traditional metabolic engineering with the tools and strategies of systems biology, synthetic biology and evolutionary engineering in search of solutions. For example, the team has successfully combined the electrolytic conversion of CO₂ into another substance, with microbial fermentation by the bacterium Cupriavidus necator to produce useful hydrocarbon products such as the biodegradable polymer, poly-3-hydroxybutyrate.

Other microbes are able to directly convert compounds containing a single atom of carbon into more complex hydrocarbons. For example, Eubacterium limosum is an acetogen, a microorganism that can convert carbon monoxide (CO) or CO₂ to acetate via a unique metabolic pathway in which acetyl coenzyme A (acetyl-CoA) is an intermediate. However, even for acetogens, CO retards growth, so their tolerance to it needs to be increased.

Evolution in the lab

By growing E. limosum at increasingly higher concentrations of CO for nearly 400 generations, researchers led by Byung-Kwan Cho, a professor in the Department of Biological Sciences at KAIST, were able to produce a bacterial strain adapted to high levels of it.

Whole genome resequencing showed that the mutation responsible for the increased tolerance was in the acetyl-CoA synthase component of the CO dehydrogenase/acetyl-CoA synthase enzyme complex.

Using this knowledge, Cho and the team introduced an artificial biosynthesis pathway to make 2,3-butanediol (2,3-BDO) in the CO-tolerant strain. This resulted in rapid conversion of CO into 2,3-BDO, a four-carbon molecule.

Such an approach could be used to induce similar mutations in other acetogenic bacteria and introduce other synthetic pathways to produce a range of chemicals, Cho says.

“We can now use harmful waste gases to produce chemicals that previously could only be obtained from petroleum,” he adds. “As the pathway is truly artificial, it is not regulated by intrinsic or original regulatory pathways inside the bacteria, which means we can fully predict how it works.”

Now that these useful microbes have been found, the next challenge is turning lab-scale experiments into industrial-scale factories. “The constraints are similar to any scale-up process such as gas transfer mixing, heat exchange and vary between different production systems,” says Lee.

“We can no longer rely on fossil resources,” he continues. “We have to establish sustainable systems for the future.”