## 4 Discussion
[45], who demonstrated that the highest growth rate and biomass productivity in _S.obliquus_ were associated with a pH of 6. The better growth performances in acidic pH could be linked to a better nutrient availability as discussed above. Indeed, while an acclimation phase occurred, ammonium was available for the microalgae to uptake beyond this acclimation, rather than evaporating into the environment. This conveyed a significant advantage in terms of reducing ammonium leakage into the atmosphere, especially when considering a larger scale for industrial applications of waste remediation by microalgae. Furthermore, a higher ammonium availability allowed for greater biomass accumulation, which was reflected in the dry weight recorded at the end of the experiment. [PERSON] et al. [46] found that a neutral pH of 7 provided the best performances in terms of growth and DHA accumulation in _Schizochytrium limacinum_; however, they also tested a pH of 6, which yielded the second-best performances in cultures. On the contrary, higher pHs of 8 and 9 resulted in less growth, as well as a rapid ammonium decrease from the medium, linked to evaporation,and the authors directly linked the lesser growth to the lack of ammonium availability in the medium following evaporation. The presented work demonstrated that an acidic pH had clear advantages in terms of an improved nutrient availability and uptake, resulting in a more consistent and higher biomass production from microalgae grown on digestate, and this result could be further improved by looking at acclimation strategies to novel sources of nitrogen. Due to the naturally high pH of digestate, efforts should be made towards controlling this factor to ensure that ammonium recovery is mainly performed via microalgal growth, rather than evaporation. This is particularly true when considering the larger-scale system, for which, abiotic parameters are more difficult to control and evaporation can easily occur. Another aspect of this study was to assess if there was any specificity between microalgal strains and digestates, in an effort to further optimise remediation by tailoring the combination of digestate and strain. Regardless of pH manipulation, both microalgal strains yielded less growth on the modified F/2 control in comparison to the three tested digestates. This could be explained by ammonium chloride being less available to microalgae in comparison to ammonium from digestate. Additionally, digestate can be enriched with dissolved CO2 generated during the methane fermentation process; this CO2 can be utilised for photosynthesis [47]. Under no pH manipulation, _C. vulgaris_ grew better on the SU digestate, whereas growth was higher in the Inmolab digestate when an acidic pH was maintained. In cultures of _S. obliquus_, better performances were recorded for SU and Cooperl digestates in both pHM\({}^{+}\) and pHM\({}^{-}\), which was reflected in the absorbance and dry weight data. Therefore, it can be assumed that certain types of digestates can be favourable to some microalgal strains; however, this can be modified by environmental factors, and, in the case of the presented study, by pH manipulation, which was particularly true for _C. vulgaris_. It could hence be assumed from previous results that an acidic pH had the potential to modify digestates properties, notably by increasing the nutrient availability, and hence shifting strain performances from one digestate to another. Similarly, digestates yielding less growth could have inhibitory factors specific to the tested strains [48]. The literature focuses on ammonium toxicity [49; 50; 51], but the digestate composition can be extremely complex and other inhibitory elements can be at play. Another well-known property of digestate is its dark colour, limiting light availability for photosynthesis, and this has been mainly counteracted by dilution (including in this study) [52; 53; 54; 14; 55; 52].