## 3 Major Bioactive Compounds and Potential Applications
### Phenolic Compounds  
Phenolic compounds are a structurally diverse class of secondary metabolites characterised by aromatic rings bearing one or more hydroxyl groups. In marine microalgae, phenolics, including simple phenolic acids (e.g., gallic, chlorogenic), flavonoids (e.g., quercetin), and complex phlorotannins, are produced in response to environmental stressors such as UV exposure, salinity, or oxidative stress. These compounds have attracted increasing interest for their antioxidant, anti-inflammatory, anticancer, and antimicrobial properties, which make them highly relevant in pharmaceuticals, functional foods, and cosmetics [66].  
Microalgae such as _Nammochloropsis_ sp., _Tetraselmis_ sp., and _Phaeodactylum tricornutum_ have been shown to accumulate phenolic compounds, with contents ranging from 5 to 15 mg gallic acid equivalents (GAE)/g dry biomass, depending on cultivation and extraction methods [5]. Their antioxidant activity, often measured via DPPH, ABTS, or FRAP assays, correlates strongly with total phenolic content. For instance, _Desmodesmus perforatus_ acetone extracts demonstrated over 22% radical scavenging activity, while methanolic extracts inhibited MCF-7 breast cancer cells by up to 87% [67].  
The anti-inflammatory potential of marine phenolics is mediated through the inhibition of enzymes such as COX-2 and iNOS, and suppression of pro-inflammatory transcription factors like NF-kB. Studies have demonstrated that phenolic-rich extractsfrom _P. tricornutum_ and _Tetraselmis_ significantly downregulate cytokines, including IL-6 and TNF-\(\alpha\) in vitro [68]. Furthermore, these extracts exert neuroprotective effects by attenuating microglial activation in models of neuroinflammation, suggesting potential for the prevention of neurodegenerative diseases [69].  
Phenolic compounds have also shown anticancer potential, particularly through mechanisms involving oxidative stress regulation, apoptosis induction, and anti-proliferative activity. Phlorotannins, for example, modulate mitochondrial apoptosis pathways and interfere with cancer cell signalling. Extracts from _Acutodesimus_ obliquus and _Desmodesmus_ sp. induced apoptosis in breast cancer cell lines, with methanolic fractions achieving over 80% inhibition at concentrations below 100 \(\upmu\)g/mL [67].  
Beyond therapeutic potential, microalgal phenolics are increasingly used in cosmetic applications. Their antioxidant and anti-inflammatory actions help combat skin ageing, reduce UV-induced damage, and support barrier repair. This makes them suitable for incorporation into photoprotective and anti-ageing skincare formulations. In addition, they have been explored as active agents in formulations targeting acne and hyperpigmentation due to their antimicrobial and tyrosinase-inhibitory effects [70].  
In the functional food sector, phenolic compounds offer natural antioxidant protection and contribute to cardiovascular and metabolic health. Studies have reported that dietary intake of microalgal phenolics may reduce oxidative stress markers and improve lipid profiles in animal models. Despite promising results, further research is needed to validate bioavailability, optimise extraction, and standardise dosage levels for human health claims [71].  
In summary, phenolic compounds from marine microalgae offer a potent combination of antioxidant, anti-inflammatory, and anticancer properties, with high potential for use in nutraceuticals, pharmaceuticals, and cosmetics. Advances in cultivation, extraction, and encapsulation technologies will be pivotal in fully realising their biotechnological applications and commercial potential.