Optimization Condition for Ethanol Production from Sweet Sorghum Juice by Recombinant Zymomonas mobilis Overexpressing groESL Genes

Abstract

High-temperature ethanol fermentation (HTEF) using high-potential thermotolerant ethanologenic microorganisms is a promising platform for ethanol production in tropical or subtropical areas. This study aims to evaluate the ethanol production potential of recombinant Zymomonas mobilis R301 overexpressing groESL genes under normal and high-temperature conditions and the expression of genes involved in the heat shock response and ethanol production pathway during ethanol fermentation using sweet sorghum juice (SSJ) as feedstock. Growth characterization analysis revealed that the recombinant Z. mobilis R301 exhibited multi-stress tolerance toward heat, acetic acid, and furfural. Based on the statistical experimental design, the optimum conditions for ethanol production from SSJ by the recombinant R301 at 30 °C were a sugar concentration of 171.67 g/L, cell concentration of 9.42% (v/v), and yeast extract concentration of 10.89 g/L, while those at 40 °C were a sugar concentration of 199.48 g/L, yeast extract concentration of 10.88 g/L, MgSO4 concentration of 1.05 g/L, and initial pH of 6.8. The maximum ethanol concentrations and productivities achieved in this study were 63.26 g/L and 1.17 g/L.h at 30 °C and 58.62 g/L and 1.22 g/L.h at 40 °C. The overexpression of the groES and groEL genes and upregulation of other heat shock-responsive genes at 40 °C enhanced cell growth, viability, and fermentation capacity of recombinant Z. mobilis R301 under heat stress. The current study demonstrated that recombinant Z. mobilis R301 exhibited high potential for ethanol production from SSJ or other sugar-based raw materials under high-temperature conditions.