Bacteria convert carbon dioxide in the air into bioplastics!

A well-known bacterium can convert carbon dioxide from the air into bioplastics, tackling two global problems in one quick step, using a prototype system designed by chemical engineers in Korea.

Plastic-eating bacteria, which are capable of breaking down plastic waste in a matter of hours, have recently attracted a lot of attention as a microscopic solution to the world's growing plastic problem.

While cleaning up the mess we've already made is a big priority, finding new ways to make plastic from sources other than crude oil and its derivatives is also vital to reducing our dependence on fossil fuels. Plastic polymers are long chains of repeating subunits strung together, and the backbones of these chains are often carbon atoms.

Many chemical engineers have stuck with the bright idea that increasing levels of carbon dioxide in the Earth's atmosphere could be an untapped resource for making plastics or other carbon-based products, such as jet fuel or concrete - if only we could capture carbon dioxide from the air. And make something out of it.

One way to convert carbon dioxide into other useful carbon-containing compounds is by injecting electricity in a reaction called electrolysis. But this method, while promising, produces mostly short-chain starter compounds consisting of one to three carbons. Making chemicals with carbon chains longer than carbon dioxide is both a harder and more efficient task.

In this new effort, a team of chemical engineers at the Korea Advanced Institute of Science and Technology (KAIST) has developed a two-part system for converting carbon dioxide into a common type of bioplastic with the help of a bacterium called Cupriavidus necator. The first step in the system is the electrolyser, which converts the carbon dioxide gas into formate. Then it is fed into the fermentation tank, where the bacteria work.

C. necator is known for its ability to synthesize carbon compounds such as poly-3-hydroxybutyrate or PHB, a type of biodegradable and compostable polyester, from other carbon sources.

In this case, C. necator devours the formate feedstock from the electrolysis reaction and the granules of the PHB stock - which can then be extracted from the harvested cells.

The same solution circulates between the electrolysis reaction and the fermentation tank, with a membrane separating the two chambers so that bacteria are isolated from the by-products of the electrolysis reaction.

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And if the system is powered by renewable energy, it could be a fossil-fuel-free way to generate bioplastics that simultaneously use up carbon dioxide — which must be quickly removed from the air to reduce global warming.

Hongju Lee and Sang Yip Lee, biomolecular engineers at KAIST who led the study, are optimistic that their approach is scalable and could go some way to helping change the way plastics are made.

"The results of this research are technologies that can be applied to the production of various chemicals in addition to bioplastics and are expected to be used as key parts needed to achieve carbon neutrality in the future," they say.

Laboratory experiments showed that C. necator cells in the hybrid system could synthesize so much PHB that the polyester product represented up to 83% of the dry cell weight of the bacteria after 120 hours or 5 days of operation.

Based on these results, the researchers claim that their setup is up to 20 times more productive than similar systems previously tested.

The team also reported that their system could operate without interruption as long as the bacterial cells were replenished each day and the plastic product removed to keep the reactions going.

This continuous production will be key to making the system work at industrial levels. So far, the researchers have tested it for just 18 days and produced 1.45 grams worth of polyester.

But the researchers say their integrated system is an improvement over previous batch reactors or other setups that could only run one stage of reaction at a time and required additional separation and purification steps.

Meanwhile, other biochemical engineers are seeking to enhance C. necator's natural ability to produce PHB from carbon dioxide with some genetic modifications because they say the amount of polymer C. necator produces is still too low for marketability — at least for now.

The study is published in PNAS.