In this study, the effect of media composition, N/P ratio and cultivation strategy on the formation of carotenoids in a Coelastrella sp. isolate was investigated. A two-stage process utilizing different media in the vegetative stage, with subsequent re-suspension in medium without nitrate, was employed to enhance the formation of carotenoids. The optimal growth and carotenoid content (β-carotene and lutein) in the vegetative phase were obtained by cultivation in M-8 and BG11 media. Use of a N/P ratio of 37.5 and low light intensity of 40 μmol m⁻² s⁻¹ (control conditions) led to optimal biomass production of up to 1.31 g L⁻¹. Low concentrations of astaxanthin (maximum of 0.31 wt. %) were accumulated under stress conditions (nitrogen-deficient medium containing 1.5 % of NaCl and light intensity of 500 μmol m⁻² s⁻¹), while β-carotene and lutein (combined maximum of 2.12 wt. %) were produced under non-stress conditions. Lipid analysis revealed that palmitic (C16:0) and oleic (C18:1) constituted the main algal fatty acid chains (50.2 ± 2.1% of the total fatty acids), while esterifiable lipids constituted 17.2 ± 0.5% of the biomass by weight. These results suggest that Coelastrella sp. could also be a promising feedstock for biodiesel production.

Ten different microalgal species were screened for their antioxidant activity using 2,2 ́-diphenyl-1-picrylhydrazyl (DPPH) scavenging assay, both total phenolic and carotenoid contents were determined. The ethanolic extract of Oscillatoria sp. showed the highest antioxidant activity (69.1 %), while the green Chlorella sp. showed higher phenolic content (39.1 mg GAE g^-1 dry wt.) and Scenedesmus obliquus recorded higher carotenoid content (3.73 mg L^-1), than cyanobacterial species Microalgal biomass plays a great role as a potential source of natural antioxidants, not only due to the carotenoids but also the phenolic compounds.

Spirulina platenisis was screened for antioxidant activity, water soluble pigments (phycobiliproteins) and secondary metabolites. Successive extraction by hexane, chloroform, ethyl acetate, ethanol (70%) and water was carried out. Ethyl acetate extract demostrated high antioxidant activity with both 2,2 ́-diphenyl-1-picrylhydrazyl (DPPH) scavenging (47±0.32 %) and β-Carotene-linoleic acid bleaching assays (93.64±0.25%),while, it showed very low antioxidant activity with 2, 2 ́-azino-bis (3-ethylbenzthiazoline-6-sulfonic acid) (ABTS) radical scavenging method. Relatively high phycobiliproteins (0.16±0.01 mg ml^-1), total phenolic content (21.88±1.67 mg GAE g^-1 dry wt.), total alkaloids (3.02±0.06 %) and total terpenoids (0.14±0.00%) were determined.