## 1 Introduction  
The oceans, covering more than 70% of the Earth's surface, are a vast and underutilised reservoir of biodiversity, holding enormous potential for discovering novel bioactive compounds [1]. Among the most promising marine resources are marine microalgae, microscopic photosynthetic organisms fundamental to freshwater and marine ecosystems [2]. Major taxonomic groups include Chlorophyta (green algae), Rhodophyta (red algae), Haptophyta, Cryptophyta, Bacillariophyta (diatoms), Dinophyta (dinoflagellates), and cyanobacteria (blue-green algae) [3, 4]. Marine microalgae are not only a vital component of marine food webs but also act as prolific producers of secondary metabolites. These metabolites include polyunsaturated fatty acids (PUFAs) such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), pigments like fucoxanthin and astaxanthin, polysaccharides, sterols, peptides, and phenolic compounds, each demonstrating diverse biological properties such as antioxidant, anti-inflammatory, antimicrobial, and anticancer activities [3; 5]. Recent years have witnessed a surge in biotechnological interest in marine microalgae, largely due to their rapid growth rates, adaptability to diverse environmental conditions, and ability to be cultivated in controlled systems such as photobioreactors. These features make them a sustainable alternative to terrestrial plants and synthetic sources for producing high-value compounds with health-promoting attributes [6]. Additionally, microalgal cultivation does not compete with agricultural land or freshwater resources, aligning with global goals of environmental sustainability and climate change mitigation [7]. The potential applications of microalgal compounds extend across several high-growth sectors. In human health and pharmaceuticals, n-3 PUFAs derived from microalgae have demonstrated positive effects in reducing cardiovascular disease risk, neurodegeneration, and inflammation [8; 9]. Pigments such as astaxanthin are potent antioxidants shown to enhance immune function and protect against oxidative stress-related conditions, including ageing and cancer [10]. Similarly, microalgal polysaccharides and sterols have displayed immunomodulatory and cholesterol-lowering effects, offering potential as functional ingredients in nutraceuticalal products [11]. In the cosmetics sector, the demand for naturally derived ingredients with anti-ageing, moisturising, and photoprotective properties is growing rapidly. Microalgae-derived compounds like fucoxanthin and mycosporine-like amino acids (MAAs) have demonstrated photoprotective effects by absorbing ultraviolet radiation and reducing oxidative damage in skin cells [12]. Their antioxidant, anti-inflammatory, and collagen-stimulating properties further support their use in anti-ageing skincare formulations. In functional foods, microalgal ingredients are incorporated to enhance nutritional profiles and offer added health benefits. Algal biomass or extracts can be integrated into beverages, supplements, dairy alternatives, and energy bars. Species such as _Chlorella_ and Spirulina (formerly _Arthrospira platensis_, and currently reclassified as _Limnospira platensis_) are widely approved and utilised in food-grade applications due to their established safety and health-promoting profiles [13]. Despite the growing body of evidence supporting microalgae's bioactive potential, the transition from laboratory to market faces several challenges. These include strain optimisation, scalable cultivation systems, cost-effective downstream processing, and regulatory hurdles related to food and cosmetic safety. Advances in metabolic engineering, synthetic biology, and bioreactor design are gradually addressing these limitations, enabling higher yields and greater consistency in compound production [14]. Moreover, omics technologies are being leveraged to uncover biosynthetic pathways and enhance the discovery of novel compounds.