Abstract
The thermal resistance of fermenting microbes is a key characteristic of stable fermentation at high temperatures. Therefore, the effects of various metal ions on the growth of Zymomonas mobilis TISTR 548, a thermotolerant ethanologenic bacterium, at a critical high temperature (CHT) were examined. Addition of Mg2+ and K+ increased CHT by 1°C, but the effects of the addition of Mn2+, Ni2+, Co2+, Al3+, Fe3+, and Zn2+ on CHT were negligible. To understand the physiological functions associated with the addition of Mg2+ or K+, cell morphology, intracellular reactive oxygen species (ROS) level, and ethanol productivity were investigated at 39°C (i.e., above CHT). Cell elongation was repressed by Mg2+, but not by K+. Addition of both metals reduced intracellular ROS level, with only K+ showing the highest reduction strength, followed by both metals and only Mg2+. Additionally, ethanol productivity was recovered with the addition of both metals. Moreover, the addition of Mg2+ or K+ at a non-permissive temperature in 26 thermosensitive, single gene-disrupted mutants of Z. mobilis TISTR 548 revealed that several mutants showed metal ion-specific growth improvement. Remarkably, K+ repressed growth of two mutants. These results suggest that K+ and Mg2+ enhance cell growth at CHT via different mechanisms, which involve the maintenance of low intracellular ROS levels.
Highlights
Bioethanol has gained attention as an alternative to fossil fuel because as a carbonneutral fuel, it can potentially delay the progress of global warming (Hahn-Hägerdal et al, 2006; Chisti, 2008)
We focused on Z. mobilis TISTR 548, one of the thermotolerant Z. mobilis strains that grew at 39◦C (Sootsuwan et al, 2007), and developed thermotolerant mutants by thermal adaptation enhancement of its critical high temperature (CHT), an upper limit for survival, up to 2◦C (Matsushita et al, 2016; Kosaka et al, 2019)
The effect of the addition of metal ions was evaluated with two-step cultivation, wherein only viable and culturable cells grow, whereas dead or viable but non-culturable cells do not grow in fresh medium at the second cultivation (Kosaka et al, 2019)
Summary
Bioethanol has gained attention as an alternative to fossil fuel because as a carbonneutral fuel, it can potentially delay the progress of global warming (Hahn-Hägerdal et al, 2006; Chisti, 2008). We focused on Z. mobilis TISTR 548, one of the thermotolerant Z. mobilis strains that grew at 39◦C (Sootsuwan et al, 2007), and developed thermotolerant mutants by thermal adaptation enhancement of its critical high temperature (CHT), an upper limit for survival, up to 2◦C (Matsushita et al, 2016; Kosaka et al, 2019). We subsequently used this mutant strain with HTF using a model fermentation and distillation system to reveal the effectiveness of this method and bioethanol productivity by HTF with Z. mobilis (Murata et al, 2015)
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