Augmenting Pentose Utilization and Ethanol Production of Native Saccharomyces cerevisiae LN Using Medium Engineering and Response Surface Methodology.

Shalley Sharma,Anju Arora,K.N Singh,Surender Singh,Debarati Paul,Eldho Varghese,Lata Nain

doi:10.3389/fbioe.2018.00132

Abstract

Economics of ethanol production from lignocellulosic biomass depends on complete utilization of constituent carbohydrates and efficient fermentation of mixed sugars present in biomass hydrolysates. Saccharomyces cerevisiae, the commercial strain for ethanol production uses only glucose while pentoses remain unused. Recombinant strains capable of utilizing pentoses have been engineered but with limited success. Recently, presence of endogenous pentose assimilation pathway in S. cerevisiae was reported. On the contrary, evolutionary engineering of native xylose assimilating strains is promising approach. In this study, a native strain S. cerevisiae LN, isolated from fruit juice, was found to be capable of xylose assimilation and mixed sugar fermentation. Upon supplementation with yeast extract and peptone, glucose (10%) fermentation efficiency was 78% with ~90% sugar consumption. Medium engineering augmented mixed sugars (5% glucose + 5% xylose) fermentation efficiency to ~50 and 1.6% ethanol yield was obtained with concomitant sugar consumption ~60%. Statistical optimization of input variables Glucose (5.36%), Xylose (3.30%), YE (0.36%), and peptone (0.25%) with Response surface methodology led to improved sugar consumption (74.33%) and 2.36% ethanol within 84 h. Specific activities of Xylose Reductase and Xylitol Dehydrogenase exhibited by S. cerevisiae LN were relatively low. Their ratio indicated metabolism diverted toward ethanol than xylitol and other byproducts. Strain was tolerant to concentrations of HMF, furfural and acetic acid commonly encountered in biomass hydrolysates. Thus, genetic setup for xylose assimilation in S. cerevisiae LN is not merely artifact of xylose metabolizing pathway and can be augmented by adaptive evolution. This strain showed potential for commercial exploitation.

Highlights

Second generation bioethanol, produced by fermentation of sugars present in lignocellulosic biomass is a promising alternative fuel, due to the declining availability of fossil fuels and their negative impact on the environment (Cai et al, 2012; Lin et al, 2012; Yadav et al, 2018)
Glucose/Xylose Fermentation Results (Table 2) showed that S. cerevisiae LN could utilize xylose and ferment it to ethanol with fairly high efficiency but its uptake was very less
This study focuses on the augmenting innate machinery and metabolism of native strain of Saccharomyces for mixed substrate fermentation

Summary

Introduction

Second generation bioethanol, produced by fermentation of sugars present in lignocellulosic biomass is a promising alternative fuel, due to the declining availability of fossil fuels and their negative impact on the environment (Cai et al, 2012; Lin et al, 2012; Yadav et al, 2018). Saccharomyces cerevisiae, widely used in starch and sucrose based 1st generation bioethanol production, is believed to be the most appropriate candidate for lignocellulosic bioethanol production due to its highly efficient hexose fermentation under anaerobic conditions, high tolerance to both ethanol and inhibitory compounds present in lignocellulosic hydrolysates. Some bacteria, such as Zymomonas mobilis and genetically modified Escherichia coli, are capable of fermenting diverse sugars (Dien et al, 2003), the yeast S. cerevisiae is still the preferred organism for industrial production of ethanol because of its high ethanol tolerance, GRAS status, tolerance of low pH, as well as resistance to bacteriophage infection making it relevant in large industrial processes (Albergaria and Arneborg, 2016; Moysés et al, 2016). To obtain an economically feasible industrial process for bioethanol production, it is imperative to efficiently ferment both the sugars into ethanol (de Sales et al, 2015)

Methods

Results

Conclusion