## 4 Discussion  
In this study, two microalgal strains were grown on three digestates resulting from the anaerobic digestion of different feedstocks. Digestates were treated to suit microalgal needs, and a concentration of 2% of the three digestates was used to grow _C. vulgaris_ and  
Figure 5: Total fatty acids, proteins, and carbohydrates content in cultures of _C. vulgaris_ and _S. obliquus_ grown on three different digestates under pH manipulation on day 0, 6, and 10 of the experiment. Error bars represent the standard deviation on three replicates. S. obliquus_ for 10 days (digestate optimisation based on [PERSON] et al. [12]). Due to the volatile nature of ammonium, especially at a high pH, which is a characteristic of digestate, the pH was manipulated to reach acidic values of 6-6.5 and a comparison was made against cultures where the pH was not modified. Results showed that pH manipulation to limit NH\({}_{4}\)+ evaporation from the different tested media generated better performances from the microalgae _S. obliquus_, with an overall better growth rate and dry weights recorded across all digestates. The same pH manipulation did not necessarily yield better growth in cultures of _C. vulgaris_, which demonstrated signs of stress, mainly via the clumping of cells. However, the dry weight was higher when the pH was maintained to be acidic in comparison to no pH manipulation. Additionally, nutrient data revealed that evaporation likely took place when the pH was not manipulated; indeed, the observed decrease in ammonium during the experiment did not coincide with a corresponding gain in the biomass dry weight. A delayed consumption of ammonium was observed for _S. obliquus_ under an acidic pH, and the gain in dry weight was more in accordance with recorded ammonium data. The slow consumption of ammonium by both _S. obliquus_ and _C. vulgaris_ could be the result of an acclimation to the ammonium present in digestate; the same acclimation occurred in the modified F/2 control where the nitrogen source was supplied by ammonium chloride, demonstrating that ammonium was the main factor resulting in the observed acclimation and that digestate did not present other toxicity to both microalgal strains. Acclimation to a newly introduced source of nitrogen has been described in the literature; for example, [PERSON] et al. [33] showed that 6 weeks of acclimation time to a high ammonium concentration was needed for both _C. vulgaris_ and _S. obliquus_. In the present study, _C. vulgaris_ did not appear to consume ammonium from digestate after 10 days of the experiment; however, _S. obliquus_ consumed ammonium almost instantly after 8 days of acclimation, showing better acclimation capabilities in the presented work. Phosphorus was mainly consumed under no pH manipulation, except in decaying cultures, where increasing concentrations of phosphorus were recorded and likely linked to a release from dying cells [34; 35]. In cultures of _C. vulgaris_, an acidic pH induced a phosphorus release via the dissolution of digestate particles; thus, an increase in the compound over the course of the experiment was observed [36; 37; 38]. However, in cultures of _S. obliquus_, a decrease in phosphorus was observed after several days, showing consumption by the microalgae. In environments with high concentrations of phosphorus, the phenomenon of P luxury uptake can occur [39; 40], and phosphorus is accumulated under the form of polyphosphate in microalgal cells [41; 42]. Polyphosphates facilitate the incorporation of metal ions and, consequently, can contribute to the remediation of heavy metals in waste streams [43]; additionally, the excess phosphorus uptake by microalgae promotes phosphorus removal from waste [44]. In the presented work, an acidic pH seemed to promote luxury uptake in _S. obliquus_, demonstrating the relevance of pH control in waste bioremediation systems. In this study, an acidic pH provided better growth performances, especially in cultures of _S. obliquus_; this was in accordance with findings from [PERSON] et al.