Environmental adaptation is considered as one of the most challenging subjects in biology to understand evolutionary or ecological diversification processes and in biotechnology to obtain useful microbial strains. Temperature is one of the important environmental stresses; however, microbial adaptation to higher temperatures has not been studied extensively. For industrial purposes, the use of thermally adapted strains is important, not only to reduce the cooling expenses of the fermentation system, but also to protect fermentation production from accidental failure of thermal management. Recent progress in next-generation sequencing provides a powerful tool to track the genomic changes of the adapted strains and allows us to compare genomic DNA sequences of conventional strains with those of their closely related thermotolerant strains. In this article, we have attempted to summarize our recent approaches to produce thermotolerant strains by thermal adaptation and comparative genomic analyses of Acetobacter pasteurianus for high-temperature acetic acid fermentations, and Zymomonas mobilis and Kluyveromyces marxianus for high-temperature ethanol fermentations. Genomic analysis of the adapted strains has found a large number of mutations and/or disruptions in highly diversified genes, which could be categorized into groups related to cell surface functions, ion or amino acid transporters, and some transcriptional factors. Furthermore, several phenotypic and genetic analyses revealed that the thermal adaptation could lead to decreased ROS generation in cells that produce higher ROS levels at higher temperatures. Thus, it is suggested that the thermally adapted cells could become robust and resistant to many stressors, and thus could be useful for high-temperature fermentations.
Keywords: acetic acid fermentation; adaptive evolution; comparative genome analysis; ethanol fermentation; thermotolerance.