Mildly acidic pH boosts up CO₂ conversion to isobutyrate in H₂ driven gas fermentation system

Area of Science:

• Biotechnology
• Environmental Science
• Microbiology

Background:

• Wastewater treatment generates significant carbon dioxide (CO2), a greenhouse gas.
• Microbial gas fermentation can capture CO2 and produce valuable chemicals.
• Production of branched or medium-chain fatty acids via gas fermentation is limited.

Purpose of the Study:

• To investigate the production of isobutyrate and hexanoate using microbial gas fermentation.
• To evaluate the effect of pH on product selectivity and microbial community composition.

Main Methods:

• Utilized membrane biofilm reactors (MBfRs) for gas fermentation with H2 and CO2.
• Operated reactors at neutral (pH 7) and mildly acidic (pH 6) conditions.
• Analyzed microbial communities using high-throughput 16S rRNA gene amplicon sequencing.

Main Results:

• Mildly acidic pH (6) favored isobutyrate (57%) and hexanoate (42%) production over acetate.
• A high concentration of isobutyrate (266 mmol C/L) was achieved at pH 6.
• Microbial community shifted from acetate-producing Acetobacterium at pH 7 to chain elongation microorganisms at pH 6.

Conclusions:

• Mildly acidic pH is crucial for enhancing bioisomerization and chain elongation in gas fermentation.
• This study demonstrates a novel green route for producing valuable isobutyrate and hexanoate.
• Findings provide insights into microbial community dynamics influencing product formation.