## 3 Results & Discussion
### Comparison of MAE, SLE & SFE
Carotenoids are non-polar components consisting of non-polar hydrocarbons and more polar xanthophylls [73]. Among the abundance of carotenoids, the hydrocarbon \(\beta\)-carotene, as well as the xanthophylls, lutein, astaxanthin, cathaxanthin, violaxanthin and zeaxanthin have been identified in _C. vulgaris_ biomass [8; 74]. Utilizing a polar solvent during conventional SLE favored the extraction of lutein and other more polar carotenoids over the non-polar \(\beta\)-carotene. Providing microwave power through MAE using the same solvent simply enhanced the solubility of more polar carotenoids. Applying SFE with a non-polar solvent increased both hydrocarbons and xanthophylls, while the  
Figure 7: Comparison between the conventional solid-liquid extraction (SLE), microwave-assisted extraction (MAE), supercritical fluid extraction (SFE) and supercritical fluid extraction with 10% \(w/w\) cosolvent addition (SFE + 10% Ethanol) of _Chlorella vulgaris_, regarding the (**a**) extraction yield, total (**b**) phenolic and (**c**) chlorophyll content, (**d**) selected and (**e**) total carotenoid content, and (**f**) antioxidant activity. addition of a polar cosolvent during SFE-10% ethanol enhanced the coextraction of more polar carotenoids (e.g., lutein) and led to the extract with the highest carotenoid content. Moreover, IC\({}_{50}\) presented a decreasing trend with the following order: SLE (43.51 mg\({}_{\text{extr}}\)/mg\({}_{\text{DPPH}}\)) > MAE (40.49 mg\({}_{\text{extr}}\)/mg\({}_{\text{DPPH}}\)) > SFE (23.17 mg\({}_{\text{extr}}\)/mg\({}_{\text{DPPH}}\)) > SFE-10% ethanol (18.66 mg\({}_{\text{extr}}\)/mg\({}_{\text{DPPH}}\); Figure 7f), which is inversely proportional to antioxidant power. Carotenoids are valuable bioactive components that present a notable antioxidant activity [59]. Moreover, chlorophyll content can positively affect antioxidant power in case of high concentration [61]. Consequently, the chlorophyll and carotenoid richest extract emerging from SFE-10% ethanol presented the strongest antioxidant activity. Finally, the extracts of SLE and MAE presented a dark green color and a characteristic fishy odor, whereas the dark brown-green extract of SFE-10% ethanol and the dark yellow SFE extract presented no unpleasant smell. The fishy odor of the extract was avoided during SFE due to the abrupt depressurization of CO\({}_{2}\) and the subsequent removal of the VOCs, the volatile organic compounds responsible for the odor of microalgae [75]. Additionally, the evaluation of the biocomponent recovery per biomass was attempted for each extraction method. The recovery of carotenoids is presented indicatively in Figure 8, considering their significantly high prices (250-2000 USD/kg) [76]. The SLE presented the highest selected (1.08 mg/g\({}_{\text{bion}}\)) and total carotenoid (3.20 mg/g\({}_{\text{bion}}\)) recovery. However, MAE results (selected: 0.79 and total carotenoids: 2.77 mg/g\({}_{\text{bion}}\)) rival those of SLE and can support its application against the conventional technique. The rich selected carotenoid content of SFE compensates for the noticeably lower yield (0.65 mg/g\({}_{\text{bion}}\)); however, this is not the case for total carotenoid content, leading to a significantly lower recovery (1.15 mg/g\({}_{\text{bion}}\)). Nevertheless, cosolvent addition gives SFE a comparative advantage over the other methods regarding the selected carotenoids (2.52 mg/g\({}_{\text{bion}}\)) and is considered competitive with the conventional SLE and the proposed MAE referring to total carotenoids (2.52 mg/g\({}_{\text{bion}}\)). In conclusion, SLE provided the highest yield yet the most inferior extract in terms of the examined bioactive compounds and antioxidant activity. On the one hand, MAE offered the advantage of a satisfactory yield in an exceptionally reduced extraction time and with lower solvent consumption while providing relatively improved extract quality compared to SLE.