3.3.1 Growth profile, sugar consumption, OUR and SOUR
Figure 3a shows the growth profile and sugar consumptions of the
transgenic rice cell culture grown in NB+0.5xS medium in the pilot-scale
bioreactor using single-stage batch operation with simplified bioreactor
processes. The rice cells continued growing until day 8 of cultivation;
no lag phase was not observed in this run (data not shown). The initial
and final biomass concentrations in the growth phase were 2.05 ± 0.05
and 5.85 ± 0.13 g DW/L, respectively (Table S1). The
µmax and τD were 0.16 ± 0.03
day-1 and 4.4 ± 0.9 day, respectively (Table S1),
which are comparable to the µmax of 0.20 ± 0.04
day-1 and τD of 3.5 ± 0.8 day obtained
in the lab-sale bioreactor run under similar kLa and
simplified bioreactor operation (Macharoen et al., 2020) but
substantially different from what was obtained in the cyclical
semicontinuous operation as described earlier. Tanaka et al., reported
that Catharanthus roseus cell suspension cultures in the shake
flask with two baffles had lower growth rate and maximum biomass
concentration than the shake flask without baffles (Tanaka et al.,
1988). Thus, the presence of baffles in the semicontinuous run likely
generates different hydrodynamic stress and oxygen transfer capability
compared to the batch run, resulting in different µmaxand τD.
Sucrose concentration dramatically declined during the first few days of
cultivation and reached undetectable levels by day 5 of cultivation,
while glucose concentration became undetectable at day 8 of cultivation,
indicating the start of the induction phase, or 0 dpi (Figure 3a). This
sugar consumption pattern is also observed in the lab-scale culture
using the same bioreactor process and kLa (Macharoen et
al., 2020). In other words, maintaining kLa as a
scale-up parameter likely provides equivalent growth kinetic parameters,
i.e. µmax, τD and rate of sugar
consumption between lab-scale and pilot-scale transgenic rice cell
suspension cultures.
The OUR pattern in this run (Figure 3b) is similar to cycles 3-4 in the
semicontinuous run presented in previous Section 3.1.2 (Figure 1b) in
which the maximum OUR of 1.76 mmol O2L-1 h-1 (Table S1) was reached
simultaneously with the start of sucrose depletion (2-4 days prior to
the induction phase). In addition, the OUR and the metabolic activity
(Figure S1b) profiles are alike, indicating that rice cells are likely
most metabolically active at the beginning of sucrose depletion. The
SOUR reached its maximum at 0.46 mmol O2 g
DW-1 h-1 (Table S1), which is
equivalent to what obtained in the lab-scale culture under the same
conditions (Macharoen et al., 2020), at the early exponential growth
phase and then gradually decreased toward the end of growth phase
(Figure 3b). A possible explanation for this is that the portion of
active cell aggregates to total cell aggregates at the beginning of this
run was greater than for cycles 3-4 in which the proportion of
accumulated nonviable cells may increase over the long cultivation
period. During the induction phase, OUR, SOUR and metabolic activity
dramatically decreased as expected. The DO and pH profiles in this run
(Figure S2) are also similar to the profiles found in the lab-scale
study (Macharoen et al., 2020) suggesting that maintaining
kLa in this study is adequate to reproduce growth
kinetics (e.g. µmax, τD, OUR and SOUR)
and culture conditions (e.g. DO and pH patterns) between a lab-scale
(5-L) production to a pilot-scale (40-L) production.