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 β-oxidation, and
that isobutyric acid is derived from the isomerization of n-butyric acid.