Comparison of Five Acetogens for Production of C2 – C6 Alcohols and Acids from CO2

The use of fossil fuels, which results in huge CO₂ emissions causing negative environmental impacts, is unsustainable. In addition, there are over 15 billion gallons of corn ethanol produced per year in the United States generating over 43 million tons per year of high purity biogenic CO₂. This biogenic CO₂ can potentially be converted to additional ethanol using autotrophic microorganisms. The conversion of waste carbon streams to valuable products is an emerging research field that addresses the need to reduce greenhouse (GHG) emissions, improve carbon conversion efficiency and generate additional revenues towards a circular carbon economy. Thermochemical and electrocatalysis methods have been developed to convert CO₂ to formic acid, CO, H₂ and CH₄. However, these methods suffer from a low carbon conversion efficiency and catalyst poisoning. Biological CO₂ conversion is a novel process in which the cells directly convert CO₂ at near ambient temperatures and pressures into biofuels and biobased products. This can generate additional revenue from CO₂-containing waste streams for biorefineries and other industries. The objective of this study is to compare five different acetogens (Clostridium ragsdalei strain P11, C. carboxidivorans strain P7A and three new clostridial strains A, B and C) for their ability to convert CO₂ to C₂ – C₆ alcohols and acids via the acetyl-CoA pathway. In addition, the effects of inoculum preparation on cell activity and product profiles were examined. Experiments were performed with inocula prepared with syngas (CO:CO₂:H₂) and CO₂:H₂. Inoculum preparations and fermentations were performed in 250 mL bottle assays with 50 mL working volume at 37°C and 125 – 150 rpm. After inoculum preparation, fermentations were performed with a gas mix containing CO₂:H₂:N₂ (20:60:20) that was fed daily for 360 h. Results showed that all strains converted CO₂ into various amounts of C₂ – C₆ alcohols and fatty acids depending on the inoculum preparation technique. All five strains prepared with syngas inocula produced alcohol. However, acetic acid was mostly produced by all strains with CO₂:H₂ inocula. Strain P11 was the best CO₂ and H₂ consumer regardless of the inoculum preparation technique, producing 2 g/L ethanol and 6 g/L acetic acid from syngas inoculum. All strains with CO₂:H₂ inocula produced mostly acetic acid (~ 12 g/L) from CO₂, with the highest from strains P11, P7A and strain A. Notably, strain C produced ethanol, butanol, hexanol and acetic, butyric and hexanoic acids with either syngas or CO₂:H₂ inocula. Additional results from enzyme activities of the five strains from syngas and CO₂:H₂ inocula will be presented to explain the variations in C₂ – C₆ alcohols and fatty acids profiles during CO₂ conversion. This research is expected to advance our understanding of biological CO₂ conversion into valuable products with increased carbon utilization towards realizing the circular carbon economy.