## 3 Major Bioactive Compounds and Potential Applications
### Peptides (Bioactive Peptides)  
Bioactive peptides are small protein fragments typically composed of 2 to 20 amino acids, produced through the enzymatic hydrolysis of larger protein molecules. Marine microalgae, especially species like _Chlorella vulgaris_, Spirulina, and various marine diatoms, are emerging as rich sources of these peptides. Owing to their high protein content, Spirulina and _Chlorella vulgaris_ provide between 50 and 70% as well as 40-60% protein by dry weight, respectively, representing promising platforms for generating bioactive peptides with therapeutic value [54]. These peptides demonstrate a broad spectrum of biological activities, including antioxidant, antihypertensive, antimicrobial, anti-inflammatory, immunomodulatory, and anticancer effects. The presence of specific amino acid sequences, such as those rich in proline, arginine, or histidine, enhances their activity and specificity for biological targets [55; 56]. Peptides derived from microalgae are particularly attractive due to their bioavailability, digestibility, and minimal allergenic potential [57; 58]. A key health-promoting function of microalgal peptides is their antihypertensive activity via inhibition of angiotensin-converting enzyme (ACE). Enzymatic hydrolysates from marine diatoms like _Bellerochea malleus_, prepared using proteases such as papain or trypsin, have demonstrated significant ACE-inhibitory activity and in vivo reduction in systolic blood pressure by up to 17 mmHg in hypertensive rat models [59]. This positions these peptides as candidates for natural blood pressure management in functional foods. Antioxidant activity is another well-documented property of microalgal peptides. They scavenge reactive oxygen species, chelate transition metals, and inhibit lipid peroxidation in biological membranes. Peptide-rich hydrolysates from Spirulina and _Chlorella_ have shown high DPPH and ABTS radical scavenging capacity, with IC\({}_{50}\) values as low as 45 ug/mL. Such peptides are also capable of protecting DNA from oxidative damage and modulating antioxidant defence enzymes, making them relevant to anti-ageing skincare and metabolic health formulations [60]. Bioactive peptides from microalgae have also shown anticancer potential. Peptides obtained from Spirulina and _Chlorella_ can inhibit proliferation and induce apoptosis in various human cancer cell lines, including MCF-7 (breast), HepG2 (liver), and AGS (gastric), at concentrations ranging from 25 to 100 \(\upmu\)g/mL. Mechanistically, these peptides may activate mitochondrial apoptotic pathways, suppress NF-kB signalling, or inhibit angiogenesis, depending on their structure and sequence [61]. In cosmetics, microalgal peptides offer multifunctional benefits: they reduce inflammation, promote collagen synthesis, enhance wound healing, and protect against UV damage. Several studies have shown that bioactive peptides from Spirulina suppress cytokines like IL-6 and TNF-\(\upalpha\) in keratinocyte models, while also promoting fibroblast proliferation. This suggests utility in anti-ageing, soothing, and regenerative skincare applications [62]. Functionally, peptides also serve as natural preservatives or bio-stabilisers in food and personal care products due to their antimicrobial properties. Peptides from marine diatoms and cyanobacteria have shown inhibitory effects against both Gram-positive and Gram-negative bacteria, including _Listeria monocytogenes_ and _E. coli_. This positions them as sustainable, clean-label preservatives in formulations aiming to reduce synthetic additives [63]. From an industrial standpoint, the production of microalgal peptides involves a multi-step process: biomass harvesting, protein extraction, enzymatic hydrolysis, and peptide purification. Techniques such as membrane filtration, ultrasonication, and high-pressure homogenization are being employed to improve yield and maintain activity.