Microbial Electrosynthesis of Isobutyric, Butyric, Caproic Acids, and Corresponding Alcohols from Carbon Dioxide

Microbial electrosynthesis is potentially a sustainable biotechnology for the conversion of the greenhouse gas CO2 into carboxylic acids, thus far mostly limited to acetic acid (C2). Despite the environmental benefits of recycling CO2 emissions to counter global warming, bioelectrochemical production of acetate is not very attractive from an economic point of view. Conversely, carboxylates and corresponding alcohols with longer C content not only have a higher economical value as compared to acetate, but they are also relevant platform chemicals and fuels used on a diverse array of industrial applications. Here, we report on a specific mixed reactor microbiome capable of producing a mixture of C4 and C6 carboxylic acids (isobutyric, n-butyric, and n-caproic acids) and their corresponding alcohols (isobutanol, n-butanol, and n-hexanol) using CO2 as the sole carbon source and reducing power provided by an electrode. Metagenomic analysis supports the hypothesis of a sequential carbon chain elongation process comprised of acetogenesis, solventogenesis, and reverse beta-oxidation, and that isobutyric acid is derived from the isomerization of n-butyric acid.