Assistant Professor Yuki Miyahara and her team at the Institute of Science Tokyo made a surprising discovery: sometimes, giving bacteria less of something helps them make more of something useful. Their study, published in ACS Sustainable Chemistry & Engineering, found that reducing carbon dioxide to just 1.4% of the gas mixture helped bacteria produce significantly more biodegradable plastic.
The research focuses on a type of bacteria called Ralstonia eutropha, which naturally eats hydrogen, oxygen, and carbon dioxide to produce a plastic called poly[(R)-3-hydroxybutyrate] — or P(3HB) for short. This plastic is biodegradable, meaning it can break down naturally instead of sitting in landfills for hundreds of years.
Most gas fermentation systems — where microbes convert gases into useful products — require high levels of hydrogen, which is flammable and makes the process dangerous. Miyahara's team had previously developed a safer system that avoids this fire risk. Now, they wanted to see if tweaking carbon dioxide levels could improve plastic production without sacrificing safety.
The results surprised them. Lowering carbon dioxide from higher concentrations down to about 1.4% by volume caused the bacteria to accumulate much more P(3HB) inside their cells. Not only did the microbes make more plastic — they also converted the carbon dioxide more efficiently.
To understand why, the researchers looked at an enzyme called carbonic anhydrase, which helps cells turn carbon dioxide into a form they can use for building things. When they increased how much of this enzyme the bacteria produced, plastic production went up — but only when carbon dioxide was kept low. This suggests that when bacteria have limited access to carbon dioxide, they get better at processing whatever they do get, leading to more plastic output.
"Instead of limiting growth, moderate CO2 scarcity encourages the cells to use available carbon more effectively," Miyahara explained. "The combined effect of low CO2 and enhanced [carbonic anhydrase] activity reveals an effective strategy for improving microbial carbon utilization, making it safer and more efficient."
This matters because it opens the door to using exhaust gases from factories and power plants — which contain relatively low concentrations of carbon dioxide — as raw material for making eco-friendly plastics. Instead of pumping that carbon dioxide into the atmosphere, it could be fed to bacteria that turn it into products we can actually use.
The team, which also included graduate student Chih-Ting Wang, postdoctoral researcher Ramamoorthi M. Sivashankari, and Professor Takeharu Tsuge, plans to expand this approach to other microorganisms and materials. Their goal is to support something called a circular carbon economy — a world where waste carbon gets reused again and again instead of being released as pollution.