## 3 Major Bioactive Compounds and Potential Applications
### Polyunsaturated Fatty Acids  
Long-chain n-3 PUFAs, namely EPA (20:5n-3) and DHA (22:6n-3), are among the most therapeutically valuable metabolites derived from marine microalgae. These essential fatty acids support cardiovascular, neurological, immune, and metabolic health, with well-documented effects on reducing inflammation, lowering blood triglycerides, and protecting cognitive function [8,100]. Species such as _Nammochloropsis gaditana_, _Schizochytrium_ sp., _Isochrysis galbana_, and _Phaocadactylum tricornutum_ are widely cultivated for their lipid-rich biomass. For instance, EPA content in _Nammochloropsis_ may reach 30% of total lipids, while _Schizochytrium_ sp. can accumulate DHA levels of up to 45% under optimised conditions [101]. _Pavlova lutheri_ accumulates up to 30-40% EPA and 7-12% DHA. Likewise, co-cultivation strategies combining DHA-rich species like _Isochrysis galbana_ and EPA-rich species like _Nammochloropsis oceanica_ have achieved balanced production with a DHA:EPA ratio of 1:1 and increased biomass yields [102]. EPA and DHA functionally integrate into cellular phospholipid bilayers and are enzymatically converted into specialised pro-resolving mediators (SPMs), such as resolvins, protectins, and maresins. These bioactive lipids actively modulate inflammatory signalling, reduce leukocyte infiltration, and facilitate the resolution phase of inflammation, thereby offering therapeutic potential for chronic inflammatory diseases [100,103]. Mechanistically, n-3 PUFAs reduce the availability of arachidonic acid in membrane lipids and suppress the production of pro-inflammatory eicosanoids, including prostaglandandins and leukotrienes. In parallel, they inhibit key intracellular pathways such as nuclear factor-kappa B (NF-kB) and mitogen-activated protein kinases, while promoting anti-inflammatory cytokines like interleukin-10 [104,105]. The cardioprotective role of EPA and DHA has been widely documented in clinical and epidemiological studies. Supplementation with these fatty acids is known to reduce plasma triglycerides, lower blood pressure, and improve endothelial function, while also decreasing the risk of thrombosis through anti-aggregatory effects on platelets. These effects collectively contribute to the reduction in cardiovascular events such as myocardial infarction and stroke [106,107]. In the nervous system, DHA plays a critical structural role, particularly in the grey matter of the brain and in retinal photoreceptor cells. It enhances synaptic plasticity, supports neuronal membrane stability, and protects against neuroinflammation and oxidative stress. Several preclinical and clinical studies suggest that sufficient DHA intake may delay the onset of cognitive decline and mitigate symptoms of neurodegenerative disorders, including Alzheimer's disease and Parkinson's disease [107]. In clinical settings, EPA/DHA supplementation has demonstrated 25-30% reductions in serum triglycerides and improvements in cognitive function, visual acuity, and neuroprotection in ageing populations [108]. From a cosmetic perspective, PUFAs enhance skin elasticity, barrier repair, and UV protection by modulating eicosanoid profiles and preventing oxidative damage. This supports their use in dermatocosmetics targeting inflammation, ageing, and dry skin [109]. Sustainable production is now possible even in tropical regions, thanks to heat-tolerant strains like _Tetraselmis_ sp. DS3, which can maintain growth at 40 \({}^{\circ}\)C and produce up to 33% n-3 fatty acids of total lipids under nitrogen stress [110]. In colder environments, Antarcticstrains like _Chaetoceros brevis_ can yield EPA up to 174 ug L\({}^{-1}\) day\({}^{-1}\), while cold-water _Emiliania luxleyi_ produces comparable DHA levels [111]. Beyond health benefits, PUFA-rich microalgae are increasingly incorporated into fortified beverages, capsules, infant formulas, and functional foods to meet rising consumer demand for plant-based, mercury-free alternatives to fish oil [112].