## 3 Results  
### Indoor Setup  
After the inoculation, the microalgae were suspended in the culture medium (Figure 2a). However, by day 14 the microalgae tended to form large aggregates (Figure 2b) that settle to the bottom of the pond, leading to the formation of a clear supernatant (OD\({}_{750}\) = 0.03) within few minutes after manual mixing. High production of acidic mucopolsaccharides was recorded in the aggregates, as shown in Figure 2c. As shown by TEM and SEM images, other microorganisms were also included in the aggregates (Figures 3 and 4). Figure 3: Transmission electron images of the microalgae grown indoor on the 20 th day. Other organisms (arrows) are visible in the medium. In (**a**) two cells surrounded by multiple layers of an electron-dense outer wall (W). (**b**) Detail of the microalgae outer wall and one of the microorganisms present in the medium. C, chloroplast. Bars 1 \(\upmu\)m.  
Figure 2: Light microscopy view of microalgae grown in the indoor setup at 0 (**a**) and 14 days (**b**) of cultivation. In (**c**), the presence of acidic mucopolsaccharides on the 14 th day is highlighted thanks to Alcian Blue staining. Bars, 5 \(\upmu\)m.  
Biomass concentration at the end of cultivation (0.129 g\({}_{\text{DW}}\) L\({}^{-1}\)) was significantly higher than the initial value (0.108 g\({}_{\text{DW}}\) L\({}^{-1}\); Figure 5). Another significant variation registered over time was the decrease between days 10 and 14, with the dry biomass concentration recorded on day 14 being not significantly different from the initial one. Experimental error due to the elevated aggregation of the biomass recorded on day 14 can be taken into account for the low value recorded on that day. For a report of the ANOVA analysis see Table S1 in the Supplementary Materials. The concentration of all photosynthetic pigments decreased since the third day of the experiment, with no other significant variations over time, except for the increase recorded on day 7 for both Chl \(a\) and Chl \(b\), with the concentration of both pigments not significantly different from that recorded at the beginning of the experiment (Figure 6a-c). The Chl/Car ratio fluctuated over time with values significantly higher than the initial one on days 7, 17, and 20. No significant changes were recorded in the Chl _a_/Chl \(b\) ratio (Figure 6d,e). Overall, the F\({}_{\text{V}}\)/F\({}_{\text{M}}\) ratio remained stable for the whole duration of the experiment, with a significant  
Figure 4: SEM images of the microalgae grown indoors on the 20 th day. In (**a**) a large microalgae aggregate (A) is visible, along with some filamentous-like organisms (O). In (**b**) other organisms (C) are visible near an aggregate (A). Figure 5: Biomass yield expressed as grams of algal dry weight per liter of the culture grown indoors. Data refer to means \(\pm\) standard deviations (n = 3). Different letters highlight significant differences between groups (\(p\leq\) 0.05) found using a one-way ANOVA. increase on the last day (Figure 6f). For a report of the ANOVA analyses, see Table S1 in the Supplementary Materials. Regarding the concentrations of N and P in the medium, different trends were recorded: nitrate concentration significantly decreased since day 3, while ammonium concentration significantly decreased only after day 14 (Figure 7a,b). In contrast, phosphate concentration fluctuated. In fact, a significant decrease was recorded only on day 3. Then, the P concentration increased, reaching significant peaks on days 10 and 14. The concentration recorded at the end of the experiment was higher than the initial one  
Figure 6: Photosynthetic pigment content (ug pigment mg\({}^{-1}\) dry biomass) and PSII maximum quantum yield (\(\rm F_{V}/F_{M}\) ratio) of microalgae cultivated indoors for 20 days. (**a**) Chlorophyll \(a\), (**b**) chlorophyll \(b\), (**c**) carotenoids, (**d**) chlorophyll \(a\) on chlorophyll \(b\) ratio, (**e**) total chlorophyll on carotenoid ratio, (**f**) PSII maximum quantum yield.