## 3 Results  
### Impact of Sodium bicarbonate Supplementation on Chlorella Growth  
The growth of _Chlorella sorokiniana_ was monitored for 72 h as changes in optical density (OD\({}_{800}\)), cell number, and cellular size. The addition of sodium bicarbonate (NaHCO\({}_{3}\)) as 1, 2, and 3 g L\({}^{-1}\), occurred in a single administration at T0 (lag phase). At T0 the number of cells for each culture treatment was about \(2.5\times 10^{6}\) cell mL\({}^{-1}\) so that the administration of a different amount of NaHCO\({}_{3}\) (1, 2, and 3 g L\({}^{-1}\)) corresponded to 0.4, 0.8, 1.2 ng NaHCO\({}_{3}\) cell\({}^{-1}\), respectively.  
Based on the optical density (Figure 1), during the first three hours, no significant change was observed among experimental conditions. After 24 h, the treatment NaHCO\({}_{3}\) 3 g L\({}^{-1}\) showed the highest growth. After 48 h, treatments 2 and 3 NaHCO\({}_{3}\) g L\({}^{-1}\) showed a significant increase in biomass compared to the control. However, at 72 h, both 1 and 2 g L\({}^{-1}\) treatments displayed even a significantly lower OD as the control, while 3 g L\({}^{-1}\) treatment showed a value similar to that of control cells.  
Figure 1: _Chlorella sorokiniana_ growth determined by optical density (OD\({}_{800}\)) estimation in control cells and after different bicarbonate addition NaHCO\({}_{3}\) 1 g L\({}^{-1}\), 2 g L\({}^{-1}\), and 3 g L\({}^{-1}\). Error bars represent SD (\(n=3\)). The values with the same letter were not significantly different (\(p\leq 0.001\), ANOVA, [PERSON]’s multiple comparison).  
In terms of the number of cells (Figure 2), no significant differences emerged in the first 4 h. After 24 h, the control showed the highest number of cells with a strong difference with other treatments, among which NaHCO\({}_{3}\) 3 g L\({}^{-1}\) displayed higher values. After 48 h, the control was still characterized by the highest number of cells, but NaHCO\({}_{3}\) 3 g L\({}^{-1}\) showed a net increase with a value similar to that of the control. Treatments 1 and 2 g L\({}^{-1}\) presented a lower number of cells, with 1 g L\({}^{-1}\) being the lowest. At the end of the experiment, the treatment with the highest number of cells was 3 g L\({}^{-1}\) that exceeded the control. As regards 1 g L\({}^{-1}\) and 2 g L\({}^{-1}\), both displayed the same pattern observed at 48 h, with 2 g L\({}^{-1}\) characterized by a higher number of cells. The difference in the average cells' size is shown in Figure 3. Already after 1 h from NaHCO\({}_{3}\) addition, the size of cells supplemented with 3 g L\({}^{-1}\) appeared larger (3.0 \(\pm\) 0.03 \(\upmu\)m) with respect to control (2.4 \(\pm\) 0.3 \(\upmu\)m) and to the other conditions. Until 24 h, the cells added with NaHCO\({}_{3}\) 3 g L\({}^{-1}\) showed a larger diameter than the cells of the other conditions. From 48 to 72 h, the cells of the other two experimental conditions (NaHCO\({}_{3}\) 1 and 2 g L\({}^{-1}\)) also increased their diameter compared to the control (Figure 3). In Table S1, the data concerning the variations of growth rate and doubling time in the control cells and in the experimental cultures are shown.  
Figure 2: Cellular density (cells mL\({}^{-1}\)) in _Chlorella sorokiniana_ control and bicarbonate enriched cultures (NaHCO\({}_{3}\) 1 g L\({}^{-1}\), 2 g L\({}^{-1}\), and 3 g L\({}^{-1}\)). Error bars represent SD (\(n\) = 3). The values with the same letter were not significantly different (\(p\)\(\leq\) 0.001, ANOVA, [PERSON]’s multiple comparison).