## 3 Results
### Composition Analysis: Fatty Acids, Proteins, and Carbohydrates  
The analysis of the biomass composition was performed on the cultures where pH was manipulated. Indeed, cultures under pH\({}^{+}\) yielded more biomass (Figure 2), and efforts were therefore concentrated towards analysing these specific samples.  
In cultures of _C. vulgaris_, the total fatty acids increased significantly over the time of the experiment for all conditions tested (\(p\) < 0.001), with a most significant increase between day 6 and day 10 for the F/2 control and both SU and Innolab digestates, whereas the FAs concentration was the highest after 6 days in the Cooperl digestate, reaching 4.91 \(\pm\) 0.3% DW (Figure 5). A similar trend was observed in the total MUFAs and PUFAs concentrations, especially for the SU digestate, with a five-fold increase between day 0 and day 10 (Supplementary Material, Figure S4). Additionally, most of the fatty acids measured as part of this work were found in higher concentrations in cultures grown using the different digestate than in the F/2 control (\(p\) < 0.001).  
In cultures of _S. obliquus_, the highest concentrations of fatty acids were measured on day 6 of the experiment for all tested conditions. Additionally, the highest fatty acids content was recorded in cultures grown on the Cooperl digestate (6.94 \(\pm\) 0.33% DW); however, there were no significant differences between the F/2 control and the digestates conditions on day 6 (\(p\) > 0.05, Figure 5). Finally, the statistical analysis showed that the total fatty acid content was not significantly different between the two tested strains at the beginning of the experiment, but that, over the course of the study, concentrations of fatty acids were significantly higher in cultures of _S. obliquus_ (\(p\) < 0.001).  
The protein content generally decreased over time or maintained a constant concentration for all digestates studied and the control for both cultures of _C. vulgaris_ and _S. obliquus_. Apart from the odd increase on day 6 of the experiment, the protein content after 10 days of culture was significantly lower (\(p\) < 0.001) or similar (\(p\) > 0.05) to the protein concentration recorded at the beginning of the experiment. For example, in the Cooperl digestate, the protein content decreased from 46.8 \(\pm\) 0.001 to 27.8 \(\pm\) 0.001% DW in cultures of _C. vulgaris_, and  
Figure 4: Concentration of phosphorus PO\({}_{4}\)\({}^{3-}\) in cultures of _C. vulgaris_ and _S. obliquus_ grown on three different digestates under pH manipulation and no pH manipulation for 10 days of experiment. Error bars represent the standard deviation on three replicates.  
from 43.5 \(\pm\) 0.001 to 29.6 \(\pm\) 0.001% DW in cultures of _S. obliquus_ (Figure 5). The observed results were in accordance with the low nutrient consumption described previously and shown in Figure 3. Additionally, there were no significant differences between the protein content of the two strains (_p_ > 0.05).  
Carbohydrates increased during the experiment in both cultures and for all conditions tested. In cultures of _C. vulgaris_, the most significant increases occurred between day 6 and day 10 of the experiment in the F/2 control and cultures grown on the Cooperl and Innolab digestates (_p_ < 0.001), with the highest carbohydrate concentration of 33 \(\pm\) 2% DW measured in the F/2 control (Figure 5). In cultures of _S. obliquus_, an increase in carbohydrate content was most significant between day 0 and day 6 (_p_ < 0.001) for the F/2 control and the SU and Innolab digestates. In cultures grown on the Cooperl digestate, carbohydrates increased the most between days 6 and 10 and reached a highest recorded carbohydrate concentration of 28.8 \(\pm\) 1.0% DW. Statistically, carbohydrates were significantly higher in cultures of _C. vulgaris_ in the F/2 control (_p_ < 0.001, Figure 5); however, no significant differences were found between the two strains in the digestates conditions (_p_ > 0.05).