## 3 Major Bioactive Compounds and Potential Applications
### Vitamins  
Marine microalgae are increasingly recognised as natural biofactories for essential vitamins, offering a sustainable and bioavailable alternative to synthetic supplements. They produce a wide range of both fat- and water-soluble vitamins, including \(\beta\)-carotene (provitamin A), tocopherols (vitamin E), phylloquinone (vitamin K1), cobalamin (vitamin B12), and various B-complex forms. These micronutrients are critical to metabolic regulation, immune balance, cognitive integrity, and skin health, which positions microalgae at the intersection of nutrition, medicine, and cosmetics [121]. \(\beta\)-Carotene is one of the most commercially valuable microalgal vitamins. _Dunaliella salina_ accumulates it up to 10-14% of dry biomass, particularly under stress conditions like high light and salinity. \(\beta\)-Carotene acts as a precursor to vitamin A, supporting vision, immune defence, and epithelial integrity. In cosmetic formulations, it serves as an antioxidant that combats oxidative stress and UV damage, helping maintain skin elasticity and reducing photoaging [122]. Vitamin B12 (cobalamin) is typically absent in plant-based foods, making _Chlorella vulgaris_ and _Nannochloropsis oceanica_ valuable vegan sources. These species contain bioactive B12 at levels between 1.5 and 3.5 \(\upmu\)g/g dry weight, sufficient to meet daily nutritional requirements with modest consumption. B12 is vital for hematologic and neurologic function, and its deficiency is linked to anaemia, neuropathy, and cognitive decline, conditions prevalent among older adults and vegans [71]. Vitamin K1 (phylloquinone), known for its role in blood clotting and bone metabolism, is present in high concentrations in species like _Anabaena cylindrica_ and _Scenedesmus obliquus_, with values exceeding 200 \(\upmu\)g/g dry weight, vastly surpassing common terrestrial vegetables. Vitamin K1 from microalgae is now being studied for insulin sensitivity modulation and arterial calcification prevention [121]. Vitamin E (\(\alpha\)-tocopherol), a lipid-soluble antioxidant, plays a crucial role in protecting PUFA in cell membranes and regulating immune responses. Microalgae such as _Chlorella, Tetraselmis_, and _Isochrysis_ synthesise tocopherols at levels of 10-80 mg/100 g dry biomass, contributing to their therapeutic potential in oxidative stress disorders and cardiovascular health [123]. In the cosmetic sector, vitamins A, C, and E derived from microalgae are included in formulations targeting wrinkle reduction, skin brightening, and UV defence. Their ability to neutralise reactive oxygen species and promote collagen synthesis enhances the skin barrier and slows ageing processes [70]. In functional foods, microalgae are used as natural vitamin fortifiers in beverages, bars, and dairy analogues. Fortified food products with Spirulina or _Chlorella_ are widely available, offering a full spectrum of B-complex vitamins and antioxidant activity. These products appeal to consumers seeking clean-label, plant-based nutrition [124]. From an industrial perspective, microalgal vitamin production offers advantages including low land use, renewability, and control over quality and yield through biotechnological and environmental modulation. Techniques such as two-phase photobioreactor cultivation, UV and salinity stress, and strain engineering have successfully increased vitamin yields, especially \(\beta\)-carotene and tocopherols [125]. In summary, marine microalgae serve as eco-friendly, potent sources of essential vitamins, enabling sustainable innovation in functional foods, dietary supplements, and cosmetics. Ongoing research continues to enhance their bioavailability, yield, and industrial scalability, strengthening their role in preventive nutrition and health. Table 1 provides a comprehensive overview of the principal classes of bioactive compounds synthesised by marine microalgae, encompassing a diverse array of chemical groups such as carotenoids, phycobiliproteins, polysaccharides, peptides, PUFAs, sterols, phenolics, MAAs, alkaloids, and vitamins.