## 3 Results and Discussion  
### Growth and Biomass Production  
Cultures were carried out in an ORWP photobioreactor for selected microalgal strains, namely _P. costavermella_ VAS2.5, _M. gaditana_ VON5.3, _P. okalomense_ SAG4.4, and _N. pyriformis_ PAT2.7. Specifically, the selection of the four strains, previously isolated and cultured under laboratory conditions [21], was based on their interesting biochemical features as well as the lack of sufficient relative literature data. These cultures had been carried out in bioreactor volumes ranging from 0.1 L to 2 L.  
The ORWP photobioreactor was chosen because it is the most commonly used for commercial microalgae production, as the shear stress exerted on the cells is limited due to the reactor configuration and the low rotational speeds of the paddles of the wheel. In addition, an ORWP has a simple design, low construction, maintenance, and repair costs, and does not have any special technology or energy demands [30]. Intriguingly, the mono-algal but non-axenic cultures performed herein proved resistant to external infections regarding protozoa, rotifers, etc., indicating well-adaptation of the microbial communities that were isolated from their natural habitats. The mASW used as a growth medium had a high salinity (i.e., 33%), simulating natural seawater (i.e., the salinity of 31-37.9%) of the Amvrakikos Gulf and the Gulf of Patras, two of the major gulfs of the Ionian Sea, from where most of the strains used herein were originally isolated [31; 32]. Also, mASW had an N-P ratio \(\cong\) 9, which was expected to be sufficient for biomassproduction and synthesis of high-added-value compounds in considerable amounts [33]. As shown in Table 2, nitrate uptake did not exceed 40%. Low nitrate uptake rates were also recorded when _Chlorella vulgaris_ CCAP 211/11B was cultured under different conditions on a nitrogen-rich medium [34]. Nevertheless, it must be taken into consideration that the photometric method used for the nitrate determination potentially poses a risk of inaccuracy in measurements since, at 220 nm, interference with several other substances may occur. Regarding orthophosphate uptake, rates in the range of 60-80% were observed (Table 2), which are similar to those reported by [PERSON] et al. (2012) [35]. These results suggest that a growth medium with lower initial nitrogen and phosphorus concentrations and a lower N-P ratio, compared with the growth medium used in this study, could be used without affecting biomass growth and bioproduct synthesis and accumulation. Microalgal growth, biomass production, and reserve material synthesis were examined after 10 and 19 days of culture (approximately 450 h) (Figure 2, Table 2). _N. pyriformis_ PAT2.7 and _P. costavermella_ VAS2.5 produced similar levels of biomass (i.e., 0.42 \(\pm\) 0.08 g/L and 0.41 \(\pm\) 0.00 g/L, respectively). Biomass production by _M. gaditana_ VON5.3 was slightly lower (0.36 \(\pm\) 0.04 g/L) compared with the above-mentioned strains, while _P. oklahome-nse_ SAG4.4 exhibited the lowest biomass yield (0.21 \(\pm\) 0.01 g/L). The aforementioned observations can be correlated with the cell size of each strain, which possibly affects the weight of total biomass. As shown in Figure 3, _Nephroselmis_ cells are almost twice the size and up to 1.8 times larger compared with _Picochlorum_ and _Microchloropsis_ cells, respectively, which might partially explain why _N. pyriformis_ PAT2.7 gained the highest final biomass production. _P. costavermella_ VAS2.5 exhibited lower biomass productivity rates compared with other strains of the genus, while even lower yields were recorded for _P. oklahomense_ SAG4.4. For example, the biomass productivity of _Picochlorum retroo_ NREL39-A8 and _Picochlorum celeri_ TG2-WT-CSM/EMRE cultivated in an ORWP of V\({}_{\mathrm{w}}\) = 100 L, having 0.2 m of culture depth, was 16.1 g/(m\({}^{2}\cdot\)d) and 22.1 g/(m\({}^{2}\cdot\)d), respectively [36].