Acetaldehyde Stimulation of the Growth of Zymomonas mobilis Subjected to Ethanol and Other Environmental Stresses: Effect of Other Metabolic Electron Acceptors and Evidence for a Mechanism

Frank Vriesekoop,Neville B Pamment

doi:10.3390/fermentation7020080

Frank Vriesekoop, Neville B Pamment

Open Access

https://doi.org/10.3390/fermentation7020080

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Journal: Fermentation	Publication Date: May 21, 2021
Citations: 1	License type: CC BY 4.0

Affiliation: University of Melbourne, Harper Adams University

Abstract

Ethanol-stressed cultures of Z. mobilis showed greatly reduced lag times in growth when supplemented with small amounts of acetaldehyde. This effect could be mimicked by other metabolic electron acceptors, including propionaldehyde and oxygen, indicating a redox-based mechanism. Added propionaldehyde was rapidly and stoichiometrically converted to 1-propanol, suggesting that added acetaldehyde is also reduced during early growth. Acetaldehyde addition measurably accelerated glycolysis in nongrowing cells and also slightly stimulated cultures subjected to temperature change, osmotic shock and salt and acetate stress. Acetaldehyde’s stimulatory effect appears to be due to its ability to accelerate glycolysis via its effect on the cellular redox balance. Acetaldehyde reduction opposes the drain on NAD+ concentrations caused by oxidation of the added ethanol, accounting for the particularly strong effect on ethanol-stressed cells. This study provides evidence for our earlier proposed redox-based mechanism for acetaldehyde’s ability to reduce the lag phase of environmentally stressed cultures and suggests that the effect may have applications in industrial fermentations, especially those inhibited by ethanol and toxic compounds present in, for instance, lignocellulosic hydrolysates.

Highlights

Ethanol produced by fermentation continues to hold promise as a significant contributor to future global transport fuel requirements, especially when it is produced from low-cost and abundant lignocellulosic feedstocks such as wood and crop residues [1].For this purpose, the Gram-negative, hyperethanologenic bacterium Zymomonas mobilis has many advantages over conventional yeasts, including the ability to achieve higher fermentation rates and ethanol yields, as well as a very high ethanol tolerance
We demonstrated that the addition of very small quantities of acetaldehyde is successful in ameliorating growth inhibition due to a variety of environmental stresses in Saccharomyces cerevisiae fermentations [9,10,11], including substantial lag reductions in media containing furfural, acetate and other toxic chemicals found in lignocellulosic hydrolysates
We examine the ability of added acetaldehyde to reduce growth inhibition due to a variety of environmental stresses applied to Z. mobilis cultures and provide evidence for a redox-based explanation for acetaldehyde stimulation in this organism

Summary

Introduction

Ethanol produced by fermentation continues to hold promise as a significant contributor to future global transport fuel requirements, especially when it is produced from low-cost and abundant lignocellulosic feedstocks such as wood and crop residues [1]. For this purpose, the Gram-negative, hyperethanologenic bacterium Zymomonas mobilis has many advantages over conventional yeasts, including the ability to achieve higher fermentation rates and ethanol yields, as well as a very high ethanol tolerance (see reviews by Panesar [2] and Weir [3]). These inhibitors include alcohols, acetic and other organic acids, furfural, phenolic compounds, heavy metals and salts, including sodium chloride [6,7,8].

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