## 3 Results and Discussion  
Photoautotrophic Cultivations of M. salina in Miniaturized Stirred-Tank PBR at Constant Incident Light Intensities  
Photoautotrophic and pH-controlled growth of _M. salina_ were studied with varying constant incident light intensities using the newly designed illumination modules in conjunction with the bioreaction unit. To check the reproducibility of the microalgal growth, one row of eight stirred-tank bioreactors was illuminated at the same incident light intensity. The effects of altogether 12 different constant incident light intensities were investigated in the course of two automated experiments, each consisting of 48 stirred-tank bioreactors in parallel. The growth of _M. salina_ was monitored at-line by measuring OD\({}_{750}\) (Figure 5). Growth of _M. salina_ commenced with no lag phase in all 48 parallel PBRs immediately after placing the bioreaction unit with the inoculated stirred-tank bioreactors on the deck of the LHS and switching on the illumination modules (Figure 4c). The specified step time of 12 h for OD\({}_{750}\) measurements does not allow the identification of short lag phases. Biomass formation of _M. salina_ in the first 12 h, automatically measured at-line as OD\({}_{750}\), was significantly higher than in the subsequent exponential growth phases, independent of the varying incident light intensities between 100 and 1000 \(\upmu\)mol m\({}^{-2}\) s\({}^{-1}\). After inoculation, the microalgae culture has to adapt to the new environment with higher light intensity compared to the preculture preparation, which should actually cause an initial lag phase. The reasons for the rapid increase in OD\({}_{750}\) within the first 12 h are unknown. OD measurements may be affected by changes in the optical characteristics of the microalgae at the wavelength applied (e.g., morphological adaptation of the cells). Figure 5: Automated photoautotrophic batch cultivations 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 \(\pm\) 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 gray shading indicates the exponential growth phase used to estimate the individual exponential growth rates as a function of the incident light intensities. Individual constant exponential growth of _M. salina_ was observed in the following 3-3.5 d (process times with constant exponential growth rates are shown in Figure 5) after which a more or less linear increase in OD\({}_{750}\) was measured. Relative standard deviations of OD\({}_{750}\) were lower in the first 3 d of the batch processes (<8.5% on average) but increased until Day 6 to 10% on average at the maximum. These deviations are in the same range as previously reported with heterotrophic parallel cultivations of microorganisms in miniaturized stirred-tank bioreactors [43; 61; 62; 63]. It should be pointed out that the flashing light does not affect the growth of _M. salina_ in the parallel stirred-tank PBRs, as the PWM dimming frequency of the LEDs was set to 1000 Hz. In the literature, the flashing light effect was evaluated at frequencies of 1-100 Hz [64; 65]. For example, a frequency of 5-10 Hz indicated an increased yield of _Chlamydomonas reinhardtii_ biomass compared to constant incident light, but, at 100 Hz, the results were comparable [66]. Due to the long process times of microalgae cultivation, which can extend over several days, varying evaporation rates in the parallel PBRs may result in increased standard deviations.