Taxonomic, Physiological, and Biochemical Characterization of Asterarcys quadricellularis AQYS21 as a Promising Sustainable Feedstock for Biofuels and ω-3 Fatty Acids

<i>Asterarcys quadricellularis</i> strain AQYS21, a green microalga isolated from the brackish waters near Manseong-ri Black Sand Beach in Korea, shows considerable potential as a source of bioactive compounds and biofuels. Therefore, this study analyzed the morphological, molecular, and biochemical characteristics of this strain; optimized its cultivation conditions; and evaluated its suitability for biodiesel production. Morphological analysis revealed characteristics typical of the <i>Asterarcys</i> genus: spherical to ellipsoidal cells with pyrenoid starch plates and mucilage-embedded coenobia. Additionally, features not previously reported in other <i>A. quadricellularis</i> strains were observed. These included young cells with meridional ribs and an asymmetric spindle-shaped form with one or two pointed ends. Molecular analysis using small-subunit rDNA and <i>tuf</i>A sequences confirmed the identification of the strain AQYS21. This strain showed robust growth across a wide temperature range, with optimal conditions at 24 °C and 88 µmol m^-2s^-1 photon flux density. It was particularly rich in ω-3 α-linolenic acid and palmitic acid. Furthermore, its biodiesel properties indicated its suitability for biodiesel formulations. The biomass of this microalga may serve as a viable feedstock for biodiesel production and a valuable source of ω-3 fatty acids. These findings reveal new morphological characteristics of <i>A. quadricellularis</i>, enhancing our understanding of the species.