## 3 Results and Discussion
After 5 d, the pH started to decrease in the stirred-tank PBRs with constant incident light intensities of 150 \(\mu\)mol m\({}^{-2}\) s\({}^{-1}\) or higher, but pH titration was not necessary (>pH 7.7). The decreasing pH correlates with the decreasing growth rate (see Figure 7), as less CO\({}_{2}\) is consumed toward the end of the process. The pH control adjusts the initial pH rather slowly, but most microalgae are tolerant of a neutral or slightly alkaline pH [68] and have a broad pH optimum for growth [31; 59]. In Figure 8, the estimated exponential growth rates of the individual batch processes are plotted as a function of the constant incident light intensities and the optical density OD\({}_{750}\) on Day 6. Figure 7: Automated photoautotrophic batch cultivation of _M. salina_ in miniaturized stirred-tank PBRs. Each row of eight bioreactors in the bioreaction unit was illuminated at six different incident light intensities I\({}_{0}\). Each bioreactor was initially filled with 10 mL of _M. salina_ suspension in an ASW medium with an initial OD\({}_{750}\) of 0.5 (initial pH 8 + 0.3). Magnetically induced S-stirrers were employed at 500 rpm. The head space of the 48 parallel stirred-tank bioreactors was rinsed with 288 L h\({}^{-1}\) of sterile air with 2% CO\({}_{2}\). The blue vertical shading indicates the exponential growth phase used to estimate the individual exponential growth rates as a function of the incident light intensities. Mean OD is represented by blue circles, whereas pH is represented by white circles. If the at-line-measured pH was below pH 7.7 or above pH 8.3 (horizontal gray-shaded area), either 45 \(\mu\)L of base (0.5 M NaOH) or acid (0.5 M HCl) was added to the relevant bioreactor by the LHS. Exponential growth rates of \(M\). _salina_ and OD\({}_{750}\) after 6 d increase non-linearly with increasing incident light intensities up to 250 umol m\({}^{-2}\) s\({}^{-1}\). The exponential growth rates and optical densities OD\({}_{750}\) attained were subsequently constant (\(\mu_{\text{opt}}\) \(\sim\)0.5 d\({}^{-1}\)) within the estimation error, indicating a slight limitation of the microalgae in suspension. A significant reduction in the exponential growth rate of \(M\). _salina_ can be clearly seen at the highest incident light intensity of 1830 umol m\({}^{-2}\) s\({}^{-1}\), indicating slight inhibition of the microalgae in suspension. Although the estimated exponential growth rates and the OD\({}_{750}\) data were generated in two separate experiments, the values fit the trend of the curves within the estimation errors, indicating sequential reproducibility. The maximum growth rates of \(M\). _salina_ observed in the parallel stirred-tank PBRs with constant illumination (\(\mu_{\text{opt}}\) = 0.525 d\({}^{-1}\)) were compared with the literature data (Table 1). _M. salina_ strains, media, illuminated layer thickness, and reactor types were found to vary considerably, making direct comparison difficult. Optimum growth rates of \(M\). _salina_ are reported between 0.1 and 1.3 d\({}^{-1}\)[17; 69; 70; 71; 72]. The maximum OD\({}_{750}\) measured in the stirred-tank PBRs is comparable to data published by [PERSON] et al. [30] reporting on photoautotrophic batch cultivations of \(M\). _salina_ using a flat plate gas-lift PBR on a liter-scale with a layer thickness of 2 cm. Figure 8: Automated photoautotrophic batch cultivations of \(M\). _salina_ in miniaturized stirred-tank PBRs (\(\bullet\) first experiment, \(\blacksquare\) second experiment). Estimated exponential growth rates of \(M\). _salina_ (blue) and OD\({}_{750}\) after 6 d as a function of the constant incident light intensities I\({}_{0}\) (red). Standard deviations are based on \(n=8\) parallel stirred-tank bioreactors. Photoautotrophic Cultivations of M. salina in Miniaturized Stirred-Tank PBRs with Day and Night Cycles  
Outdoor cultivations of microalgae are subject to variations in temperature and incident light intensities arising from natural day and night cycles.