Low Carbon Dioxide Boosts Microbes to Produce More Biodegradable Plastic

A new study suggests that tweaking the carbon dioxide supply in safe gas fermentation can markedly boost the production of a biodegradable plastic. Researchers focused on poly[(R)-3-hydroxybutyrate], or P(3HB), a polymer synthesized by hydrogen-oxidizing bacteria used in carbon-recycling biotechnology.

The work centers on Ralstonia eutropha H16, which converts hydrogen, oxygen, and CO₂ into intracellular bioplastic under autotrophic conditions. However, conventional approaches often rely on hydrogen concentrations that can fall into flammable ranges, creating safety constraints for industrial scale-up.

To overcome this, the team employed a previously developed noncombustible gas culture system. With that safer platform in place, they asked a key question: does CO₂ concentration merely limit growth, or can it actively reshape how efficiently cells incorporate carbon into P(3HB)?

Surprisingly, reducing CO₂ availability improved polymer accumulation. When CO₂ was lowered to about 1.4% by volume, cells accumulated substantially more P(3HB) than cultures fed with higher CO₂ levels. Alongside higher product formation, the bacteria also demonstrated more efficient conversion of CO₂ into polymer.

The researchers then probed the molecular reason for this effect by examining carbonic anhydrase, an enzyme that accelerates the conversion of CO₂ into bicarbonate. Because bicarbonate is a crucial inorganic carbon source for cellular metabolism, the team tested whether elevating carbonic anhydrase activity would change outcomes under different CO₂ regimes.

Increasing carbonic anhydrase expression boosted P(3HB) accumulation—but only when external CO₂ was low. This points to a synergy between external carbon scarcity and internal carbon processing: when CO₂ is limited, cells benefit most from faster enzyme-driven carbon conversion.

In essence, the study indicates that moderate CO₂ limitation triggers adaptive cellular responses that enhance carbon utilization efficiency. At higher CO₂ concentrations, carbon processing becomes less rate-limiting, making these adaptations less impactful.

The findings could help design industrial processes that utilize low-concentration CO₂ sources, such as exhaust gases, while maintaining safe reactor conditions. By improving both gas safety and carbon conversion efficiency, the approach offers a practical route toward circular carbon recycling and biodegradable materials.

Keywords

CO₂ utilization; gas fermentation; noncombustible culture; Ralstonia eutropha; poly[(R)-3-hydroxybutyrate] (P(3HB)); carbonic anhydrase; carbon recycling; biodegradable plastics