3.1.2 Oxygen uptake rate (OUR) and specific oxygen uptake rate (SOUR)
The measurement of OUR is a simple non-invasive method to monitor metabolic activity of the culture, while the SOUR measurement is the OUR normalized by biomass dry weight. During the exponential growth phase, the OUR increased in all cycles, and the SOUR was fairly constant in cycles 2-4, while the OUR and SOUR trends in cycle 1 are somewhat different from the rest as seen in Figure 1b. The difference in the OUR and SOUR patterns in cycle 1 may be the result of differences in agitation rates, which influence overall oxygen transfer rates, between cycle 1 and other cycles. The maximum OUR and SOUR in cycles 1-4 were 0.96, 1.38, 1.49 and 1.91 mmol O2L-1h-1, respectively, and 0.44, 0.35, 0.25 and 0.24 mmol O2 g DW-1h-1, respectively (Table S1), which are generally higher than our previous report of 0.52-1.25 mmol O2L-1h-1 for the maximum OUR and 0.18-0.29 mmol O2 g DW-1h-1 for the maximum SOUR (Corbin et al., 2016) likely due to different bioreactor conditions. The decreasing OUR and SOUR during the induction phases correlated to the reduction of metabolic activity (Figure S1a) probably due to cell quiescence from sugar starvation and/or cell lysis. However, the OUR, SOUR, and metabolic activity increased again after the medium exchange to fresh sugar rich media (Figure 1b). The pattern of increasing OUR and SOUR during the growth phase but decreasing during the induction phase was also found in previous studies using a two-stage transgenic rice cell semicontinuous suspensions under the RAmy3D promoter (Corbin et al., 2016; Trexler et al., 2005).