Notable mixed substrate fermentation by native Kodamaea ohmeri strains isolated from Lagenaria siceraria flowers and ethanol production on paddy straw hydrolysates

Shalley Sharma,Debarati Paul,Surender Singh,Anju Arora,Lata Nain,Pankhuri Sharma

doi:10.1186/s13065-018-0375-8

Shalley Sharma, Debarati Paul + Show 4 more

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https://doi.org/10.1186/s13065-018-0375-8

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Abstract

BackgroundBioethanol obtained by fermenting cellulosic fraction of biomass holds promise for blending in petroleum. Cellulose hydrolysis yields glucose while hemicellulose hydrolysis predominantly yields xylose. Economic feasibility of bioethanol depends on complete utilization of biomass carbohydrates and an efficient co-fermenting organism is a prerequisite. While hexose fermentation capability of Saccharomyces cerevisiae is a boon, however, its inability to ferment pentose is a setback.ResultsTwo xylose fermenting Kodamaea ohmeri strains were isolated from Lagenaria siceraria flowers through enrichment on xylose. They showed 61% glucose fermentation efficiency in fortified medium. Medium engineering with 0.1% yeast extract and peptone, stimulated co-fermentation potential of both strains yielding maximum ethanol 0.25 g g−1 on mixed sugars with ~ 50% fermentation efficiency. Strains were tolerant to inhibitors like 5-hydroxymethyl furfural, furfural and acetic acid. Both K. ohmeri strains grew well on biologically pretreated rice straw hydrolysates and produced ethanol.ConclusionsThis is the first report of native Kodamaea sp. exhibiting notable mixed substrate utilization and ethanol fermentation. K. ohmeri strains showed relevant traits like utilizing and co-fermenting mixed sugars, exhibiting excellent growth, inhibitor tolerance, and ethanol production on rice straw hydrolysates.

Highlights

Bioethanol obtained by fermenting cellulosic fraction of biomass holds promise for blending in petroleum
Commercial strains of S. cerevisiae, the most widely used organisms for ethanol production are exclusively involved in glucose fermentation, completely utilizing cellulosic fraction while xylose is left unfermented
Inhibitor tolerance of K. ohmeri strains For exploitation of K. ohmeri strains for fermentation of biomass hydrolysates, it is important to check their capability to grow in presence of HMF, furfural, formic acid and acetic acid, the predominant by-products of biomass pretreatment which are present in hydrolysates and reported to inhibit growth

Summary

Introduction

Bioethanol obtained by fermenting cellulosic fraction of biomass holds promise for blending in petroleum. Commercial strains of S. cerevisiae, the most widely used organisms for ethanol production are exclusively involved in glucose fermentation, completely utilizing cellulosic fraction while xylose is left unfermented. To overcome this drawback of S. cerevisiae, recombinant strains capable of utilizing xylose have been developed since 1980s but ethanol yield was found to be low [4]. Several genetic engineering approaches have been adopted for developing a recombinant strain capable of mixed substrate fermentation but with limited success [5] This is due to the constraints associated with co-fermentation, like aerobic process of xylose fermentation, co-factor (NADH) imbalance [6] and glucose repression [7, 8].

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