Physiology of S. cerevisiae in continuous VHG ethanol
fermentation
During CVEF, oscillatory behavior of fermentation parameters, such as
biomass, cell viability, ethanol, residual glucose, and glycerol, are
displayed periodically and sustained with a period of approximately 150
h at the dilution rate of 0.027 h-1 (Bai, Chen,
Anderson, et al., 2004; Wang et al., 2013). In order to focus on the
intracellular regulation mechanism of yeast cells during the oscillatory
process, a complete representative period was shown in Figure 1, in
which ethanol, glycerol, biomass, cell viability and living cells
exhibited the similar changing tendencies, while residual glucose
presented the opposite profile.
Considering the periodic wave characteristics, the different phases of
sinusoidal function (0π, π/2, π, and 3π/2) are appropriately applied to
describe the changing trends of parameters for better exploring and
comprehending their correlations (Figure S1). The oscillation curve of
residual glucose is configured as a 0π phase, so the profiles of ethanol
and glycerol are correspondingly considered as a π phase. Because
ethanol as the major product requires the consumption of glucose in
proportion, it naturally follows that there is an opposite tendency
between them. The parameters associated with cell growth (biomass, cell
viability, and living cells) are also classified as a π phase based on
the foundational knowledge that cell growth couples with the generation
of primary metabolites such as ethanol. However, the changes of cell
growth and viability happened prior to other parameters, reflecting a
delayed response of intracellular metabolites. In addition, both high
glucose and ethanol caused a synergy inhibition on viability. According
to the biomass curve, five representative sampling points (P1-5 in
Figure 1) were selected for subsequent metabolic profile analysis and
transcriptomic analysis.