## 3 Major Bioactive Compounds and Potential Applications
### Pigments (Carotenoids)  
Marine microalgae are prolific producers of pigments, especially carotenoids, which include both carotenes and xanthophylls such as astaxanthin, fucoxanthin, lutein, zeaxanthin, and \(\beta\)-carotene. These pigments not only serve essential roles in photosynthesis and light protection but also exhibit potent antioxidant, anti-inflammatory, anticancer, and neuroprotective activities, which support their widespread use in pharmaceuticals, cosmetics, and functional foods [80; 81].  
Astaxanthin, primarily synthesised by _Haematococcus pluvialis_, reaching up to 3.8% of its dry weight, is considered one of the most potent natural antioxidants, being 10-100 times more effective than vitamin E in quenching free radicals. It has demonstrated significant neuroprotective, cardiovascular, and anti-ageing effects, while also modulating inflammatory cytokines such as IL-6 and TNF-\(\alpha\)[82]. Emerging delivery technologies like nanoencapsulation are improving their bioavailability and efficacy in topical and oral formulations [83].  
Fucoxanthin, derived from diatoms like _Phaeodactylum tricornutum_ and brown algae, has gained attention for its anti-obesity, anticancer, and anti-diabetic properties. It promotes thermogenesis by inducing UCP1 in white adipose tissue and suppresses tumour growth via caspase-mediated apoptosis. In diabetic models, it improves glucose metabolism and reduces insulin resistance [84]. Fucoxanthin also contributes to neuroprotection by inhibiting pro-inflammatory mediators in microglial cells [85].  
Lutein and zeaxanthin, abundant in _Chlorella zofingiensis_ and _Scenedesmus_ spp., are well known for protecting the retina from oxidative stress and harmful blue light. These xanthophylls accumulate in the macula and are essential for preventing age-related macular degeneration. Microalgae-derived lutein has been shown to reach productivity levels 100-200\(\times\) higher than marigold crops per square metre, offering a more sustainable industrial source [86].  
\(\beta\)-Carotene, especially from _Dunaliella salina_, serves as a provitamin A and is widely used as a colourant and antioxidant in the food and cosmetic industries. Under stress conditions, D. salina can accumulate \(\beta\)-carotene up to 12.6% of dry weight, and its carotenoid profile also includes zeaxanthin, lutein, and cryptaxanthin [87].  
New microalgal strains like _Bracteacoccus aggregatus_ have been found to co-produce astaxanthin and \(\beta\)-carotene simultaneously, making them dual-source platforms for natural pigments. This strain can yield up to 48% astaxanthin and 13% \(\beta\)-carotene of total pigment mass, showcasing high industrial potential [88].  
Applications of these pigments extend into cosmetic formulations where they serve not only as natural colourants but also as bioactive ingredients with UV-protective, skin-brightening, and anti-wrinkle effects. They help in reducing melanin synthesis, stimulating collagen production, and improving skin hydration. In functional foods, algal pigments are included in supplements, fortified beverages, and health snacks to enhance antioxidant intake and support vision, cardiovascular, and immune health [89; 90].  
In conclusion, pigments like astaxanthin, fucoxanthin, lutein, and \(\beta\)-carotene from marine microalgae represent high-value, multifunctional compounds with broad applications across health, nutrition, and skincare sectors. Their increasing market demand is driving innovation in biotechnological production systems, including genetic engineering, stress-induced cultivation, and photobioreactor optimisation, to enhance yields and cost-effectiveness [91].