## 3 Results
### Nutrient Consumption: Ammonium and Phosphorus  
The concentration of NH\({}_{4}\)\({}^{+}\) decreased rapidly after the start of the experiment in both cultures of _C. vulgaris_ and _S. obliquus_, and for all three digestates tested when the pH was not manipulated. The decrease continued steadily and the final NH\({}_{4}\)\({}^{+}\) concentration in the different media was significantly lower than at the beginning of the experiment (\(p\) < 0.001, Figure 3). No decrease in the NH\({}_{4}\)\({}^{+}\) concentration was observed in the F/2 control, which maintained a consistent content of ammonium throughout the experiment and for both strains. The observed decrease in ammonium, however, was not in accordance with the dry weight data measured previously (Figure 2), and it could be assumed that the recorded decrease was mainly linked to the evaporation of the ammonium, most likely induced by the naturally high pH of digestate. Indeed, NH\({}_{4}\)\({}^{+}\) did not decrease in the F/2 control where the pH remained neutral. However, the microalgae did not consume this source of nitrogen, despite its availability. Under pH manipulation, the concentration of NH\({}_{4}\)\({}^{+}\) remained the same in cultures of _C. vulgaris_ for all three digestates and the control, with a concentration averaging 83.7 \(\pm\) 1.83 mg L\({}^{-1}\) (\(p\) > 0.05). Therefore NH\({}_{4}\)\({}^{+}\) was not consumed by _C. vulgaris_ during the 10 days of the experiment. The same result was observed in cultures of _S. obliquus_, up to day 8 of the experiment. However, NH\({}_{4}\)\({}^{+}\) decreased significantly between day 8 and day 10 (\(p\) < 0.001) and reached values below 20 \(\pm\) 3.93 mg L\({}^{-1}\) in the three tested digestates, showing a rapid consumption of ammonium by the microalgae (Figure 3). In this instance, it was demonstrated that the ammonium evaporation did not occur under an acidic pH;  
Figure 2: Dry weight of _C. vulgaris_ and _S. obliquus_ grown on three different digestates under pH manipulation and no pH manipulation on day 0 and 10 of the experiment. Error bars represent the standard deviation on three replicates. however, NH4+ was still not consumed by the microalgae, or only until a later stage of the experiment. Therefore, it could be hypothesised that both _C. vulgaris_ and _S. obliquus_ were acclimating to the novel source of nitrogen (ammonium from digestate or ammonium chloride in the F/2 control). Phosphorus decreased over time in cultures of _C. vulgaris_ when the pH was not manipulated, showing a consumption of the nutrient, especially in the Cooperl and Innolab digestates. A decrease in the phosphorus concentration was also observed in cultures of _S. obliquus_ in pHM- for all of the conditions tested, except for cultures grown in the Innolab digestate, where phosphorus increased after 4 days of cultivation, reaching 8.86 \(\pm\) 0.154 mg L-1 (Figure 4). The growth data presented previously showed that _S. obliquus_ performed less well when grown on the Innolab digestate (Figures 1 and 2). Microalgal cells are known to release phosphorus when decaying; therefore, the observed increase in phosphorus could be linked to cells of _S. obliquus_ degrading in the Innolab digestate and releasing stored phosphorus. Under an acidic pH, phosphorus increased during the time of the experiment in cultures of _C. vulgaris_, but decreased overall in cultures of _S. obliquus_ (Figure 4). While regulating the pH using HCl, lower pHs were recorded in cultures of _C. vulgaris_, and a pH lower than 6 was measured several times across the time of experiment. Under an acidic pH, particles dissolve more easily, and, in this study, it was possible that particles remaining in the different digestate would have released phosphorus when dissolving in the acidic environment, explaining the observed results. Additionally, growth data showed that _S. obliquus_ performed better in pHM+, which could also explain the decrease in phosphorus for this strain, using the nutrient for growth purposes.