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

Byoung Seung Jeon,Okkyoung Choi,Youngsoon Um,Byoung-In Sang

doi:10.1186/s13068-016-0549-3

Abstract

BackgroundC5–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.ResultsHexanoic 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.ConclusionMegasphaera 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.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-016-0549-3) contains supplementary material, which is available to authorized users.

Highlights

C5–C8 medium-chain carboxylic acids are valuable chemicals as the precursors of various chemicals and transport fuels
Isolation of hexanoic acid‐producing bacteria For the isolation of hexanoic acid-producing bacteria, reinforced clostridia medium (RCM) supplemented with hexanoic acid was used as a selection medium
Hexanoic acid has been shown to be toxic for microbial growth [19, 20]; the suppression of bacteria that do not produce hexanoic acid was expected by supplementing hexanoic acid (5 g/L) to RCM

Summary

Introduction

C5–C8 medium-chain carboxylic acids are valuable chemicals as the precursors of various chemicals and transport fuels. 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. Medium-chain carboxylic acids have 5–8 carbon chains, such as pentanoic acid (valeric acid), hexanoic acid (caproic acid), heptanoic acid (enanthic acid), and octanoic acid (caprylic acid), which can be used as platform chemicals for a broad range of organic building blocks [1]. Production of these carboxylic acids has been rarely reported and only at low-titers because of product inhibition [2, 3]. It is postulated that hexanoic acid is produced by two consecutive condensation reactions: the first is the formation of butyric acid from two acetyl-CoAs, and the

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