## 3 Major Bioactive Compounds and Potential Applications
### Phycolilibrioteins  
Phycolilibrioteins are water-soluble, highly pigmented proteins found predominantly in cyanobacteria and red algae, where they serve as light-harvesting antennae. Among marine microalgae, Spirulina is the richest and most studied source, producing three key phycobilibrioteins: C-phycocyanin (C-PC), allophycocyanin (APC), and phycoerythrin (PE). For example, Spirulina can yield up to 20% C-PC by dry weight, and _Porphyridium cruentum_ accumulates PE levels of \(\sim\)25-30%. These bioactive molecules not only contribute to photosynthesis but also exhibit diverse pharmacological and nutraceutical properties, making them attractive for applications in medicine, cosmetics, and functional foods [72].  
C-PC can constitute up to 12-20% of the dry weight of Spirulina biomass, with yields reaching over 120 mg/g under optimised extraction and purification conditions [73]. This pigment is especially valued for its antioxidant properties, scavenging reactive oxygen species (ROS) like nitric oxide and peroxyl radicals, and thus mitigating oxidative damage. Purified C-PC has demonstrated EC\({}_{50}\) values for NO scavenging as low as 4.5 \(\upmu\)g/mL, comparable to vitamin C [73].  
In anti-inflammatory and immunomodulatory applications, phycobiliproteins suppress the expression of COX-2, iNOS, and pro-inflammatory cytokines such as TNF-\(\alpha\) and IL-6, while enhancing macrophage and NK cell activity. These effects have been validated in models of metabolic syndrome, nephrotoxicity, and neuroinflammation, where C-PC modulates apoptosis regulators (e.g., Bcl-2, Bax) and reduces caspase-3/9 activity [74].  
In the cosmetics sector, C-PC and PE offer both bioactivity and pigmentation. Beyond their natural blue colour, popular in clean-label formulations, they provide UV-protection,anti-ageing, and skin-soothing effects. Studies demonstrate that C-PC downregulates oxidative and inflammatory mediators in keratinocytes, while stimulating fibroblast proliferation, enhancing collagen synthesis, and promoting skin regeneration [75]. Additionally, C-PC exhibits strong emulsifying and foaming properties, adding functional value in formulation development [76].  
As a nutraceutical ingredient, phycobiliproteins are often marketed for immune enhancement, fatigue reduction, and systemic antioxidant support. Oral supplementation of Spirulina or isolated C-PC has shown benefits in improving lipid profiles, glucose metabolism, and even cognitive function in animal and human studies [77]. Additionally, C-PC-derived peptides show promise as DPP-IV inhibitors, suggesting potential as adjuncts in managing type 2 diabetes [78].  
Extraction technologies for phycobiliproteins have also evolved. Ultrasonic-assisted, membrane, and green solvent extraction methods now yield higher purities and bioactivity retention. Supercritical CO\({}_{2}\) pre-treatment has been shown to enhance the purity and functionality of recovered C-PC [79].  
In conclusion, phycobiliproteins, especially C-PC from Spirulina, represent highly valuable multifunctional biomolecules. Their broad spectrum of antioxidant, anti-inflammatory, immunomodulatory, cosmetic, and nutritional properties makes them prime candidates for widespread biotechnological use across health, food, and cosmetic industries.