Upgrading dilute ethanol from syngas fermentation to n-caproate with reactor microbiomes

Abstract

Fermentation of synthesis gas (syngas, which is a gas mixture including CO, H2, and CO2) from a renewable biomass source is gaining momentum. However, energy-intensive distillation of ethanol at dilute concentrations of 2-4% (w/w), which are anticipated for syngas fermentation with carboxydotrophic bacteria, is one impediment towards widespread adaptation. n-Caproic acid is a carboxylic acid with a chain length of six carbon atoms, and can be extracted more easily compared to ethanol. This because of its hydrophilic nature and its charge as n-caproate beyond a pKa of 4.88 while ethanol is completely miscible due to its short 2-carbon chain and hydrogen-bonding interactions. n-Caproic acid can be produced from ethanol in an anaerobic open culture (reactor microbiome) by chain elongation via the reversed β oxidation pathway. Here, we show a proof-of-concept to utilize diluted ethanol and acetic acid in real syngas fermentation effluent as the sole substrate for chain elongation into the product n-caproic acid. This concept, therefore, integrates the syngas platform and the carboxylate platform within a biorefinery. We observed with a bioreactor study that lowering the pH to slightly acidic conditions was necessary to shift the metabolic flux from production of methane to production of n-caproic acid. The highest concentration of n-caproic acid of ~ 1 g L-1 was produced at pH 5.44.