## 4 Discussion
The transmittance of the different digestates used in this work could explain why the two microalgal strains yielded different growth rates on the three tested digestates. Indeed, while the three digestates were diluted to obtain a similar ammonium concentration, transmittances were different as the digestates did not have the same particle composition and were produced from different feedstocks. [PERSON] et al. [56] demonstrated that microalgal growth was negatively correlated to the digestate transmittance; therefore, light availability could be one of the factors behind the different growth rates observed in the present experiment. The presented work demonstrated that microalgal strains can perform differently on different types of digestates, with a clear influence of environmental factors such as pH or light transmittance. Thus, when considering microalgae for digestate remediation, it is crucial to implement an initial strain screening to optimise the digestate uptake and, consequently, bioremediation. Composition analysis of _C. vulgaris_ and _S. obliquus_ showed a higher fatty acid content when the strains were grown on the different digestates in comparison to the F/2 control. Similar findings can be seen in the literature [57], especially when diluted digestate is used (which was the case in this work). Indeed, the lower nitrogen content of diluted digestate allows for an increase in lipid accumulation, and this was found for many microalgal species, including _Neochloris oleabundant_[58], _Synechocystis_ sp. and _Nannochloropsis salina_[59], and _Chorella_ sp., _Scendesmus_ sp. and _Nannochlorysis gaditana_[52; 14]. Additionally, fatty acid accumulation was different between the two microalgal strains studied; indeed, higher fatty acids concentrations were found after 10 days of the experiment for _C. vulgaris_, whereas the same result was observed after 6 days of experiment in cultures of _S. obliquus_. Fatty acid accumulation in microalgal cells is mainly based on nutrient deprivation [60; 46]. Inthis study, the nitrogen results showed very little consumption of the ammonium from the digestates until the end of the experiment due to an acclimation period. This could explain why _C. vulgaris_ had the highest fatty acids content at the end of the experiment, as nutrients from the upscaling phase were depleted and the microalgae had not acclimatised to the ammonium from digestate yet. _S. obliquus_ had the highest concentration of fatty acids at day 6, and ammonium results showed that the microalgae consumed ammonium almost instantly between day 8 and day 10 of the experiment. Therefore, fatty acids were accumulated until day 6, when nutrients were depleted and decreased once _S. obliquus_ acclimatised to the novel source of ammonium, shifting from nutrient depletion to nutrient consumption. However, the very limited consumption of nitrogen linked to the observed acclimation of both strains led to a decrease in proteins over the time of experiment. Indeed, protein accumulation is directly correlated to nitrogen uptake [61,62], which was very minimal in the present study. Carbohydrates, on the other hand, increased significantly during the experiment, reaching maximum concentrations after 10 days for all digestates and the F/2 control. Similarly to the lipid content, nitrogen starvation can increase the accumulation of carbohydrates [61,63,64,65]. Indeed, under nitrogen depletion, microalgae can transform proteins into carbohydrates by shifting the use of photosynthetic carbon from the metabolic pathway of protein synthesis to carbohydrates synthesis, hence accumulating this compound as energy storage [66,67]. This was also in accordance with the observed decrease in proteins during the present experiment. Additionally, the synchronic increase in lipids and carbohydrates can be explained by their parallel synthesis pathways, where energy is firstly stored as carbohydrates and then converted into lipids [68]. Similar results were found in the literature; for example, [PERSON] et al.