## 3 Results and Discussion
### Synthesis of Polysaccharides, Proteins, and Amino Acid Profile  
Regarding the ability to accumulate polysaccharides and proteins under these culture conditions, all strains showed remarkable yields (Table 2). The highest concentrations of both classes of cellular materials were achieved by _N. pyriformis_ PAT2.7, with S/x% = 14.3 \(\pm\) 0.3% (wt/wt) (similar results were also recorded for _P. oklahonense_ SAG4.4) and P/x% = 49.7 \(\pm\) 4.9% (wt/wt) at the end of culture. Although polysaccharides are synthesized through anabolic pathways competing with lipid biosynthesis [1], in this study, both metabolites were produced in significant quantities, indicating that the supply of light and nutrients was sufficient for the cultures of the selected microalgae. The results of the current study were comparable to those obtained from [PERSON] et al. (2023) [36], who recorded carbohydrate content of 5.2-20.6% for the two cultivated _Picochlorum_ strains in outdoor raceway ponds. Indoor and outdoor cultivation of _Nannochloropsis_ sp. for 8 days led to carbohydrate accumulation of 22 \(\pm\) 1.4% and 23 \(\pm\) 1.3%, respectively [38]. The protein content of the four examined microalgal strains was comparable to those examined by [PERSON] et al. (2015) [38] and [PERSON] et al. (2023) [36], as previously mentioned. Notably, the very high protein content of _N. pyriformis_ PAT2.7 in ORWP as well is a strong indication for the utilization of this microalga as a food/feed supplement. The nutritional value of microalgae, in terms of protein quality, depends significantly on its amino acid composition [65]. Regarding the amino acid profile of the tested microalgal strains, _N. pyriformis_ PAT2.7 presented the highest total content of essential (EAA) and non-essential amino acids (NEAA). However, the EAA to NEAA ratio remained balanced at approximately 1:1 for all the cultured microalgae, which is also observed in several microalgae species [10]. The most abundant EAA in all selected species was leucine, followed by lysine and arginine. The highest lysine values were observed at _P. costavermella_ VAS2.5 (3.5 \(\pm\) 0.0%) and the lowest in _P. okalonnense_ SAG4.4 (2.3 \(\pm\) 1.0%). The differences between the microalgae of the same genus could be explained by the considerable variability in amino acid profile, which is due to the different algal species, environmental growth conditions, nutrient content of the feeding medium, and the stage of the algal life cycle [66]. _N. pyriformis_ PAT2.7 showed the highest concentration of methionine (1.0 \(\pm\) 0.1%), while the rest of the tested strains showed levels of approximately 0.6%. A similar trend appeared regarding the threonine concentration. The essential amino acid profile comparisons presented in the studies conducted by [PERSON] et al. (2023) [67] and [PERSON] et al. (2024) [68] revealed similarities among the Chlorophyta phylum. In this study, _P. costavermella_ VAS2.5, _P. okalonnense_ SAG4.4, and _N. pyriformis_ PAT2.7 were included. The presence of high leucine content was also highlighted in other Chlorophyta and Ochrophyta species, such as _Nannochloropsis granuloma_, _Nannochloropsis culata_, _Tetraselmis chui_, and _Tetraselmis sucecia_[69,70]. [PERSON] et al. (2021) [71] reported lower levels of arginine in the Heterokontophyta (e.g., _Nannochloropsis oceanica_) and Chlorophyta (e.g., _Tetraselmis suecia_) microalgae when investigating the potential of microalgal strains as a dietary supplement for aquafeeds, in comparison to the findings of the present study. Regarding the NEAA, glutamic acid was the most prevalent amino acid, followed by aspartic acid and glycine; analogous dominance in glutamic acid content was also observed in the research of [PERSON] et al. (2023) [67], where 56 different microalgal strains were analyzed. Similar results for glutamic acid were also demonstrated in the research of [PERSON] et al. (2024) [68], where _Nannochloropsis_ sp.