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.