Low CO2 Levels Boost Biodegradable Plastic Output

In an innovative gas fermentation process, reducing the concentration of carbon dioxide was found to significantly improve microbial production of the biodegradable plastic, poly[(R )-3-hydroxybutyrate]. Researchers found that hydrogen-oxidizing bacteria grown under safe, nonflammable gas conditions enable more efficient production of biodegradable plastic at lower CO₂ levels. The study provides a promising strategy for sustainable carbon recycling and efficient CO₂ utilization.

How CO₂ Levels Affect the Production of Biodegradable Plastic P(3HB)

As the efforts to reduce carbon dioxide (CO₂) emissions accelerate worldwide, scientists are exploring ways to transform this abundant greenhouse gas into useful products. One of these approaches is microbial CO₂ conversion, which uses naturally occurring microorganisms to convert CO₂ into sustainable materials. Particularly, Ralstonia eutropha (a hydrogen-oxidizing bacterium) is widely used in this process, and uses hydrogen, oxygen, and CO₂ for synthesis of biodegradable plastics such as poly[(R )-3-hydroxybutyrate] (P(3HB)).

Conventional gas fermentation systems often require high hydrogen concentrations in flammable range, which affects the safety of the process. To address this, a research team from Institute of Science Tokyo (Science Tokyo), Japan, had previously developed a noncombustible gas culture system. Now, the group used the noncombustible system and investigated how adjusting the concentration of CO₂ could improve the production of P(3HB) under safe operating conditions. The study was led by Assistant Professor Yuki Miyahara from the Department of Materials Science and Engineering, Science Tokyo, in collaboration with graduate student Chih-Ting Wang, Postdoctoral Researcher Ramamoorthi M Sivashankari, and Professor Takeharu Tsuge, all from Science Tokyo. The findings were made available online on April 17, 2026, and published in Volume 14, Issue 16 of the journal ACS Sustainable Chemistry & Engineering on April 27, 2026.

"We observed that reducing CO₂ concentration resulted in higher production of P(3HB)," explains Miyahara.

On the contrary to the conventional expectations, the researchers discovered that lowering the supply of CO₂ to approximately 1.4% by volume, significantly increased the accumulation of P(3HB) inside the cells. Moreover, the bacteria not only produced more plastic but also converted the CO₂ more efficiently than the cultures grown under higher CO₂ concentrations.

To further understand why low CO₂ concentrations improved polymer production, the team investigated the role of carbonic anhydrase (Can), which is an enzyme that rapidly converts CO₂ into bicarbonate. Since this reaction plays an important role in supplying inorganic carbon for cellular metabolism, the researchers tested whether increasing the enzyme's activity could enhance the production of P(3HB). The results revealed that increasing Can expression boosted the accumulation of P(3HB), but only under low CO₂ conditions. This suggests that efficient carbon processing within the cells is very important when external CO₂ is limited. The increased expression of Can enzyme ensured ample supply of inorganic carbon, allowing the cells to produce larger amounts of biodegradable plastic.

"The combined effect of low CO₂ and enhanced Can activity reveals an effective strategy for improving microbial carbon utilization, making it safer and more efficient," comments Miyahara.

According to the authors, low CO₂ availability triggers adaptive cellular responses within the bacterial cells, which improve the carbon utilization efficiency. Therefore, instead of limiting the growth, moderate CO₂ scarcity encourages the cells to use available carbon more effectively, which results in greater polymer accumulation. However, under higher CO₂ concentrations, these adaptive responses become less pronounced as carbon is already readily available.

Overall, the study inspires the development of industrial processes that are capable of converting low-concentration CO₂ sources, such as exhaust gases, into biodegradable plastics. By combining safer gas handling along with improved carbon conversion, the approach offers a promising path for sustainable carbon recycling and reducing the emission of greenhouse gases, while also producing eco-friendly materials.

In the future, the researchers plan to further improve the process and extend this strategy to other microorganisms and products. These advances could accelerate the development of new processes that could transform waste carbon into a wide range of renewable materials, supporting the transition towards a circular carbon economy.

Authors:

Chih-Ting Wang¹, Ramamoorthi M Sivashankari¹, Yuki Miyahara¹*, and Takeharu Tsuge¹*

Title:

Impact of Low CO₂ Concentration on Autotrophic Production of Poly[(R)-3-hydroxybutyrate] by Ralstonia eutropha H16 and Synergistic Effect of Carbonic Anhydrase

Journal:

ACS Sustainable Chemistry & Engineering

DOI:

10.1021/acssuschemeng.6c00126

Affiliations:

¹Department of Materials Science and Engineering, Institute of Science Tokyo, Japan