Conclusions
Continuous ethanol fermentation using VHG substrate by S.
cerevisiae in a stirred tank reactor was carried out in this study, and
oscillatory behavior of fermentation parameters was exhibited
periodically and in a sustained manner with a period of approximately
130 h. Here, we also analyzed the intracellular oscillation behavior and
demonstrated that there was a lag response between macro-parameters,
extracellular metabolites, and the expression level of genes. In central
carbon metabolism, the regulation of PFK was shown pointed to
play an important role in the regulation of the oscillations of
glycolytic metabolites. For ethanol formation, it was assumed that the
regulation of the expression of gene ADH1 and PDC1 was
closely related to the flux changes of catabolism, while the regulation
of the expression of genes ADH4 , PDC5 and PDC6 was
closely related to the flux changes of anabolism. Glycerol was formed
due to both osmotic stress response and redox homeostasis mechanisms,
but the flux of the TCA cycle was negligible during CVEF at low
oxygen-supplemented conditions. Oxidative stress response related genes
and the HSPs family genes were observed to be significantly changed and
mainly triggered by ethanol inhibition. In anabolic process, the
ribosome synthesis genes were prioritized and expressed before the
biomass change, suggesting that it needs time for cell construction.
Finally, we proposed a mechanism to describe the oscillatory process,
and the sustained VHG oscillation is considered as a result of the
competition of cell growth, ethanol formation and ethanol inhibition.