Metabolic traits of Clostridium ljungdahlii during syngas fermentation to ethanol

Thermochemical conversion of biomass into syngas (a mix of carbon monoxide, hydrogen, and carbon dioxide) and subsequent fermentation by carboxydotrophic, homoacetogenic bacteria is a promising approach for production of renewable compounds from biomass or organic waste substrates. We have designed, built, and operated a 2-stage continuous system with Clostridium ljungdahlii to undergo syngas fermentation to ethanol. This system achieves promising cell density, ethanol selectivity, and productivity. Clostridium ljungdahlii grows acidogenic (produces acetate from syngas) in the first stage. In the second stage, stationary cells convert acetate and syngas into ethanol - a process referred to as solventogenesis. Using liquid chromatography-mass spectrometry (LC-MS) methods to perform proteomic and metabolomic analyses of the cells from each stage, we have compared the abundance of nearly 2000 cellular proteins and important key metabolites to determine factors that are essential for efficient ethanol production. Our results suggest that the formation of either acetate or ethanol from syngas is not determined via regulation of expression of central metabolic pathway enzymes. Instead, we found the following characteristic traits of solventogenesis during syngas fermentation: 1) enhanced expression of enzymes for biosynthesis of ammonium and vitamins likely caused by limitation of the respective nutrients; 2) enhanced expression of enzymes for metabolism of sulfur-containing amino acids; and 3) a change of the redox state of the cells to a more reduced one, possibly triggered by a lack of an electron sink under no-growth conditions. Our results may provide insights into novel approaches to improve syngas-to-ethanol fermentation.