Co-fermentation of cellobiose and xylose by mixed culture of recombinant Saccharomyces cerevisiae and kinetic modeling

Abstract

Efficient conversion of cellulosic sugars in cellulosic hydrolysates is important for economically viable production of biofuels from lignocellulosic biomass, but the goal remains a critical challenge. The present study reports a new approach for simultaneous fermentation of cellobiose and xylose by using the co-culture consisting of recombinant Saccharomyces cerevisiae specialist strains. The co-culture system can provide competitive advantage of modularity compared to the single culture system and can be tuned to deal with fluctuations in feedstock composition to achieve robust and cost-effective biofuel production. This study characterized fermentation kinetics of the recombinant cellobiose-consuming S. cerevisiae strain EJ2, xylose-consuming S. cerevisiae strain SR8, and their co-culture. The motivation for kinetic modeling was to provide guidance and prediction of using the co-culture system for simultaneous fermentation of mixed sugars with adjustable biomass of each specialist strain under different substrate concentrations. The kinetic model for the co-culture system was developed based on the pure culture models and incorporated the effects of product inhibition, initial substrate concentration and inoculum size. The model simulations were validated by results from independent fermentation experiments under different substrate conditions, and good agreement was found between model predictions and experimental data from batch fermentation of cellobiose, xylose and their mixtures. Additionally, with the guidance of model prediction, simultaneous co-fermentation of 60 g/L cellobiose and 20 g/L xylose was achieved with the initial cell densities of 0.45 g dry cell weight /L for EJ2 and 0.9 g dry cell weight /L SR8. The results demonstrated that the kinetic modeling could be used to guide the design and optimization of yeast co-culture conditions for achieving simultaneous fermentation of cellobiose and xylose with improved ethanol productivity, which is critically important for robust and efficient renewable biofuel production from lignocellulosic biomass.

Modeling Biowaste Biorefineries: A Review

“…Morales-Rodriguez et al 2011, for instance, consider the modified Escherichia coli ATCC-55124 strain in their SSCF model, while Krishnan et al (1999) consider the modified S. cerevisiae 1400(pLNH33) strain. Chen et al (2018) on the other hand modeled a co-fermentation process based on the co-culture of several genetically modified S. cerevisiae strains, each one specialized in metabolizing one particular type of substrate.…”

“…For the EJ2-strain, it was found that the initial cellobiose concentration has a major impact on the ethanol production by the recombinant S. cerevisiae strain. Therefore, a S 0 factor was included in the expression for the ethanol production growth rate ν, rendering the following (semiempirical) correlation (Chen et al, 2018): Chen et al 2018noticed that the SR8 strain was unable to uptake xylose when it was only in a small concentration present in the medium. Additionally, when xylose levels were low and xylose consumption had ceased, the consumption of ethanol by the SR8 strain became prevalent.…”

“…The inhibitory effect of xylose was neglected, as its effect was much smaller as that of the produced ethanol. The modified kinetic model of the substrate consumption is given by the following correlation (Chen et al, 2018):…”

“…The growth kinetics were also derived from the Monod equation, which was altered accordingly to account for product and substrate inhibitory effects. The model proposed by Sreemahadevan et al (2018) displays high similarities with the model proposed by Chen et al (2018) and is therefore not further discussed.…”

High Gravity Fermentation of Sugarcane Bagasse Hydrolysate by Saccharomyces pastorianus to Produce Economically Distillable Ethanol Concentrations: Necessity of Medium Components Examined

“…Complementary approaches have coupled cellobiose utilization with xylose utilization in recombinant S. cerevisiae with improved xylose to ethanol conversion rates [30]. Recombinant approaches have not yet yielded a successful approach to xylose utilization that overall surpasses the ethanol productivities of native species on glucose; however, work is getting closer [8,14,[31][32][33][34][35][36].…”