Isobutyric Acid Production through Lactic Acid Chain Elongation by the Acetogen Clostridium luticellarii

Abstract Isobutyric acid is a platform molecule with a wide variety of applications ranging from food additives to plastics, but its production currently relies on petrochemical resources. However, recent studies have shown that isobutyric acid is often transiently produced as a by-product in lactic acid chain elongating microbiomes and consistently correlates with the occurrence of a genus closely related to the acetogen Clostridium luticellarii. In this study, we characterized the growth and product spectrum of C. luticellarii on lactic acid. Results showed that C. luticellarii converts lactic acid to mixtures of acetic, butyric, isobutyric, and caproic acid. Elevated lactic acid concentrations of 400 mM and above inhibited the strain’s growth, while the presence of elevated acetic acid concentrations (>50 mM) increased the product selectivity towards butyric and isobutyric acid. Using a fed-batch strategy, we demonstrated the production of isobutyric and butyric acid up to 15 and 10 g L−1, respectively, at a combined yield of 0.41 g gLA−1. The alcohols isobutanol and butanol were by-products of the fed-batch fermentation, reaching concentrations up to 0.66 and 0.29 g L−1, respectively. Further experiments suggested that C. luticellarii may be able to co-metabolize H2 and CO2 or methanol and CO2 alongside lactic acid, paving the way for the coupling of organic waste valorization to carbon capture and utilization through the production of butyric and isobutyric acid.