Energy Conservation and Carbon Flux Distribution During Fermentation of CO or H2/CO2 by Clostridium ljungdahlii

Abstract

Both CO and H₂ can be utilized as energy sources during the autotrophic growth of <i>Clostridium ljungdahlii</i>. In principle, CO is a more energetically and thermodynamically favorable energy source for gas fermentation in comparison to H₂. Therefore, metabolism may vary during growth under different energy sources. In this study, <i>C. ljungdahlii</i> was fed with CO and/or CO₂/H₂ at pH 6.0 with a gas pressure of 0.1 MPa. <i>C. ljungdahlii</i> primarily produced acetate in the presence of H₂ as an energy source, but produced alcohols with CO as an energy source under the same fermentation conditions. A key enzyme activity assay, metabolic flux analysis, and comparative transcriptomics were performed for investigating the response mechanism of <i>C. ljungdahlii</i> under different energy sources. A CO dehydrogenase and an aldehyde:ferredoxin oxidoreductase were found to play important roles in CO utilization and alcohol production. Based on these findings, novel metabolic schemes are proposed for <i>C. ljungdahlii</i> growing on CO and/or CO₂/H₂. These schemes indicate that more ATP is produced during CO-fermentation than during H₂-fermentation, leading to increased alcohol production.