Microbial cell facilities have the potential to decrease dependence on fossil fuels by converting waste into useful chemicals.
How fossil fuels contribute to climate change is now well understood: burning hydrocarbons releases carbon dioxide (CO 2 ), which accumulates in the atmosphere and causes global warming.
The challenge now is to reduce our dependence on fossil fuels or even reverse the process and transform CO 2 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 these fossil fuels as a source of chemicals. Instead, we need to use renewable biomass, especially non-edible sources such as agricultural or food waste, or even CO 2 itself. These byproducts can be converted into useful fuels and chemicals,” says Sang Yup Lee, a distinguished professor in the Department of Chemical and Biomolecular Engineering at KAIST.
As pioneers in the field of metabolic systems engineering (EMS), Lee’s partners are combining the traditional approach of metabolic engineering with the tools and strategies of systems biology, synthetic biology, and evolutionary engineering in search of solutions. For example, the team successfully reconciled 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 microorganisms have the ability to directly modify compounds, incorporating a single carbon atom to convert into more complex hydrocarbons. For example, Eubacterium limosum is an acetogen, a microorganism that has the ability to convert carbon monoxide (CO) or CO 2 to acetate through a unique metabolic pathway in which acetyl coenzyme A (acetyl-CoA) is an intermediate. However, even for acetogens, CO delays growth, which explains why your tolerance to it needs to be increased.
Evolution in the laboratory
By cultivating E. limosum in increasingly higher concentrations of CO for nearly 400 generations, researchers led by Byung-Kwan Cho, professor in the Department of Biological Sciences at KAIST, were able to produce a bacterial strain adapted to high levels of it.
The total realignment of the genetic code indicated that the change related to increased tolerance was located in acetyl-CoA synthase, a component of the CO dehydrogenase/acetyl-CoA synthase enzymatic complex.
Using this knowledge, Cho and team introduced an artificial biosynthesis pathway to produce 2,3-butanediol (2,3-BDO) in the CO-tolerant strain. This resulted in the rapid conversion of CO to 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, says Cho.
“We can now use harmful waste gases to produce chemicals that previously could only be obtained from oil,” he adds. “Because the pathway is truly artificial, it is not regulated by intrinsic or original regulatory pathways within the bacteria, meaning we can fully predict how it works.”
Now that these useful microbes have been found, the next challenge is to transform laboratory-scale experiments into industrial-scale factories. “The constraints are similar to any scale-up process, such as mixing gas transfer and heat exchange, and vary between different production systems,” says Lee.
“We can no longer depend on fossil resources,” he continues. “We have to establish sustainable systems for the future.”
