Production of medium-chain carboxylic acids by Megasphaera sp. MH with supplemental electron acceptors

Abstract

Background: C5-C8 medium-chain carboxylic acids are valuable chemicals as the precursors of various chemicals and transport fuels. However, only a few strict anaerobes have been discovered to produce them and their production is limited to low concentrations because of product toxicity. Therefore, a bacterial strain capable of producing high-titer C5-C8 carboxylic acids was strategically isolated and characterized for production of medium chain length carboxylic acids. Results: Hexanoic acid-producing anaerobes were isolated from the inner surface of a cattle rumen sample. One of the isolates, displaying the highest hexanoic acid production, was identified as Megasphaera sp. MH according to 16S rRNA gene sequence analysis. Megasphaera sp. MH metabolizes fructose and produces various medium-chain carboxylic acids, including hexanoic acid, in low concentrations. The addition of acetate to the fructose medium as an electron acceptor increased hexanoic acid production as well as cell growth. Supplementation of propionate and butyrate into the medium also enhanced the production of C5-C8 medium-chain carboxylic acids. Megasphaera sp. MH produced 5.7 g L-1 of pentanoic acid (C5), 9.7 g L-1 of hexanoic acid (C6), 3.2 g L-1 of heptanoic acid (C7) and 1.2 g L-1 of octanoic acid (C8) in medium supplemented with C2-C6 carboxylic acids as the electron acceptors. This is the first report on the production of high-titer heptanoic acid and octanoic acid using a pure anaerobic culture. Conclusion: Megasphaera sp. MH metabolized fructose for the production of C2-C8 carbon-chain carboxylic acids using various electron acceptors and achieved a high-titer of 9.7 g L-1 and fast productivity of 0.41 g L-1 h-1 for hexanoic acid. However, further metabolic activities of Megaspahera sp. MH for C5-C8 carboxylic acids production must be deciphered and improved for industrially relevant production levels.