Among various ethanologenic microorganisms, thermotolerant Zymomonas mobilis has emerged as a promising candidate for industrial ethanol production at elevated temperatures. However, the comparative fermentation efficiency and the underlying molecular mechanisms driving thermotolerance in newly developed strains remain largely unexplored, hindering their industrial application. In this study, the recently developed thermotolerant strains Z. mobilis 200M and Z. mobilis PYK exhibited critical high temperatures for growth approximately 2.0 and 2.5 °C higher than the wild-type, respectively. While 40 °C represents severe heat stress that completely inhibits the growth of the wild-type, the thermotolerant strains remained viable, exhibiting significantly shorter cell lengths under these conditions. This study provides the first evidence of their superior multi-stress tolerance toward heat, ethanol, acetic acid, formic acid, and H2O2. Furthermore, the thermotolerant strains exhibited significantly higher ethanol fermentation efficiencies than the wild-type. At 40 °C, Z. mobilis 200M produced approximately 5.8-fold and 3.0-fold more ethanol than the wild-type after 24 and 48 h, respectively, while Z. mobilis PYK yielded 6.4-fold and 3.1-fold increases. Novel transcriptional insights via RT-qPCR revealed the simultaneous overexpression of genes involved in ethanol production, protein quality control, and signal transduction, particularly during the exponential phase under heat stress. Collectively, these findings bridge the gap between strain development and molecular understanding, suggesting that the coordinated upregulation of these genetic pathways enhances the adaptive capacity and fermentation efficiency of these thermotolerant strains during sustained growth at 40 °C.
Charoenpunthuwong et al. (Sat,) studied this question.