## 3 Results and Discussion
As described above, evaporation is compensated for by employing individually measured liquid levels in the parallel stirred-tank PBRs followed by liquid level correction by the individual addition of sterile water. The total added volume was tracked for each of the parallel PBRs, and a mean evaporation rate was estimated. The individual evaporation rates are plotted in Figure 6.  
Figure 6: Evaporation rates during automated photoautotrophic batch cultivations of _M. salina_ in stirred-tank PBRs. The layout represents the positioning of the 48 parallel PBRs inside the bioreactor. The final evaporation rates were estimated by totaling the quantities of individual sterile water added for automated liquid level correction. The inlet of the gas supply cover of the bioreaction unit is located between the 3 rd and 4 th rows of Reactor Position 8 (Figure 4c). The evaporation rates varied over the range 94.0-119.4 \(\mu\)L h\({}^{-1}\), with an average of 104 \(\pm\) 5.7 \(\mu\)L h\({}^{-1}\). It is clear that automated individual liquid level corrections are necessary if dry air is used for gassing the parallel stirred-tank PBRs. Depending on the reactor position, the 5.5% mean difference in the evaporation rates may not reduce parallel reproducibility in typical heterotrophic batch processes from a few hours up to one day. However, the slow growth of microalgae in photoautotrophic batch processes between several days and several weeks will result in notable differences in the reaction volumes in the stirred-tank PBRs with non-individual compensation of evaporation. Thus, individual measurement of liquid levels in parallel PBRs followed by liquid level correction is sure to improve the parallel reproducibility of photoautotrophic bioprocesses. Incidentally, evaporation compensation is not only common but also an important factor in microalgae cultivation independent of the photobioreactors employed [67]. A second parallel photoautotrophic growth study with 48 stirred-tank bioreactors was performed with _M. salina_ (Figure 7). The incident light intensities were varied in each row of eight stirred-tank bioreactors over the range 40-1817 \(\mu\)mol m\({}^{-2}\) s\({}^{-1}\) to enable investigation of the effects of both lower and higher constant incident light intensities in comparison with the previous study. As before, no lag phase was observed at low-incident light intensities. Constant exponential growth of _M. salina_ following the initial high biomass formation was observed for up to 3.5 d before any linear increase in OD\({}_{750}\) was measured. In contrast, lag phases of 1.5 d after the initial high biomass formation were seen at high-constant-incident light intensities of 1400-1817 \(\mu\)mol m\({}^{-2}\) s\({}^{-1}\). The following exponential growth phases of _M. salina_ were reduced to 2 d before the linear increase in OD\({}_{750}\) took place. The relative standard deviations of the OD\({}_{750}\) measurements were higher at the end of the batch processes with the highest incident light intensity (up to 15% at 1400 \(\mu\)mol m\({}^{-2}\) s\({}^{-1}\), and up to 28% at 1800 \(\mu\)mol m\({}^{-2}\) s\({}^{-1}\), respectively). The pH in the parallel stirred-tank PBRs may have an influence on both exponential growth rates and OD\({}_{750}\) after 6 d. As an example, the at-line measured pH is shown in Figure 7 as a function of process time. The pH was measured at-line and controlled with a step time of 12 h.  
Even though the initial pH of the ASW medium was pH 8.0 before inoculation, the pH dropped to pH 7.3 at the beginning of the batch processes due to the 2% CO\({}_{2}\) in the air used for gassing. After three additions of a base by the LHS at process times of 0 d, 0.5 d, and 1.0 d, respectively, the lower boundary of the uncontrolled pH range (pH 7.7) was reached in all of the 48 parallel PBRs. Subsequently, the pH increased due to the fixation of CO\({}_{2}\) by the microalgae in suspension, and no further base or acid addition was necessary.