## 3 Results and Discussion
### Experiment 2: Effect of Biomass-Specific Photon Supply Rate under Nitrogen Starvation  
The light distribution per biomass unit was studied in flat panel reactors subjecting cultures inoculated at two different densities, 0.25 and 0.65 g L\({}^{-1}\), to two light intensities, 1300 and 1800 \(\upmu\)mol m\({}^{-2}\) s\({}^{-1}\). These conditions resulted, as shown in Table 2, in four biomass-specific photon supply rates (BSPSR) which have a clear effect on growth and starch accumulation under nitrogen deprivation. Figure 4a,b, show the absorbance and dry weight over 96 h from starvation. The overall increase in dry weight and absorbance may be explained by the accumulation of metabolites such as starch and consequent increase in weight and granulometry [68] despite nitrogen starvation inhibiting growth. The cultures which received fewer photons per biomass unit and are also those that had a higher density at starvation, had higher final biomass productivity, 0.2 and 0.3 g L\({}^{-1}\) d\({}^{-1}\), compared to the lower irradiance treatments that were ranging between 0.12 and 0.15 g L\({}^{-1}\) d\({}^{-1}\). As reported in the FTIR screening, also in flat panels the starch content in the cells had increased significantly in all cultures within the first 12 h (Figure 4c) followed by degradation. Maximum starch concentration reached up to 30% in cultures with high BSPSR, whereas the cultures that received a lower light supply rate did not accumulate more than 15%. The relationship between BSPSR and starch content at 12 h was confirmed by a strong positive correlation of correlation coefficient 0.88 (Figure 5a). \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**BSPSR** & **Irradiance** & **Inoculum** & **Starch\%2.h** & **Starch\%2.h** & **Productivity\%8.h** \\ **(\(\upmu\)mol g\({}^{-1}\) s\({}^{-1}\))** & **(\(\upmu\) mol m\({}^{-2}\) s\({}^{-1}\))** & **(g L\({}^{-1}\))** & **(g g\({}^{-1}\) dw)** & **(g L\({}^{-1}\))** & **(g L\({}^{-1}\) dw\({}^{-1}\))** \\ \hline
102 & 1300 & 0.65 \(\pm\) 0.01 & 15.0 \(\pm\) 0.6 & 0.17 \(\pm\) 0.03 & 0.42 \(\pm\) 0.06 \\
141 & 1800 & 0.65 \(\pm\) 0.01 & 15.5 \(\pm\) 3.0 & 0.15 \(\pm\) 0.03 & 0.27 \(\pm\) 0.10 \\
268 & 1300 & 0.25 \(\pm\) 0.04 & 28.9 \(\pm\) 1.6 & 0.17 \(\pm\) 0.01 & 0.25 \(\pm\) 0.09 \\
371 & 1800 & 0.25 \(\pm\) 0.04 & 27.5 \(\pm\) 0.2 & 0.13 \(\pm\) 0.02 & 0.21 \(\pm\) 0.05 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Experimental parameters and results from experiment 2. The experimental parameters were biomass-specific photon supply rate (BSPSR), light intensity and inoculum density, vs the resulting starch concentration in the biomass 12 h after N starvation, starch concentration in the culture 24 h after N starvation, and biomass productivities after 48 h. Average values \(\pm\) SD. The peak of starch concentration in the biomass did not coincide with the peak of maximum starch accumulation in the culture, expressed as g L\({}^{-1}\), which happened at 24 h rather than 12 h (Figure 4d), and was very similar for all four treatments. This discrepancy happened because in starch-rich cultures, at 24 h from starvation, the starch was still contained at a high percentage in the cells (>25%), while we see a small increase in biomass dry weight. At the same time, the cultures with lower starch content, which had a higher initial biomass, higher productivity and less cell mortality reached similar starch production values as the cultures with a high starch content, which on the other hand had lower productivity and suffered the impact of light stress on cell growth. The photon supply rate, in this case, showed the opposite effect with a weak negative correlation of \(-0.65\) (Figure 5b). These two tendencies may both be interesting for industrial production depending on the desired final product. In a biorefinery context, where starch is to be extracted, a lower BSPSR may be chosen as a strategy to produce starch alongside other interesting metabolites.