## 2 Marine Microalgae Diversity and Biotechnological Relevance  
Marine microalgae are among the most promising and multifaceted groups of organisms in modern biotechnology. These microscopic, photosynthetic organisms form the base of the marine food web and are responsible for nearly half of global oxygen production and carbon fixation. Their exceptional biodiversity, metabolic versatility, and environmental resilience render them critical to ocean ecosystems and industrial innovation across health, food, energy, and environmental sectors. As biotechnology increasingly moves toward sustainable and renewable resources, microalgae have emerged as ideal candidates for scalable, eco-friendly bioproduction platforms [18]. Microalgae encompass diverse organisms spanning multiple taxonomic groups (Chlorophyta, Rhodophyta, Hartopophyta, Cryptophyta, Bacillariophyta, Dinophyta, and cyanobacteria) that have arisen through billions of years of evolution and adaptation to various aquatic environments, from sunlit surface waters to nutrient-rich deep-sea zones. Each taxonomic group exhibits distinct physiological traits and biochemical profiles that influence their suitability for different biotechnological applications. For example, diatoms possess unique silica shells and are known for their rich lipid content, while green algae such as _Chlorella_ are renowned for their protein and chlorophyll content [19]. Beyond taxonomic differences, microalgae are ecologically versatile, thriving in environments ranging from tropical coastal zones to hypersaline lakes and polar regions. This adaptability underpins their use in diverse industrial settings, including open-pond cultivation in tropical climates and controlled photobioreactors in temperate zones [20]. One of the most compelling features of microalgae is their ability to biosynthesise a wide variety of primary and secondary metabolites. These include PUFAs like EPA and DHA, carotenoids such as astaxanthin and fucoxanthin, antioxidants, vitamins, polysaccharides, and bioactive peptides. Many of these compounds have demonstrated pharmaceutical and nutraceutical properties, including anti-inflammatory, antioxidant, antimicrobial, and anticancer effects [21]. Microalgae can be cultivated using open ponds, closed photobioreactors, or hybrid systems. Open systems are cost-effective and suitable for robust species like Spirulina, whereas photobioreactors provide better control over environmental variables and are favoured for producing high-value compounds. Environmental factors such as light intensity, CO\({}_{2}\) enrichment, nitrogen levels, and pH are carefully modulated to optimise growth and metabolite yield [22]. Furthermore, microalgae exhibit remarkable metabolic plasticity under stress conditions. Nutrient deprivation, light stress, and salinity fluctuations often trigger elevated synthesis of valuable secondary metabolites. This metabolic responsiveness can be harnessed for controlled production of target compounds in industrial settings [23]. Recent innovations include the integration of wastewater as a nutrient source, which not only cuts costs but also serves environmental goals by reducing effluent pollution. Cultivating microalgae on industrial CO2 emissions is also gaining momentum as a carbon capture strategy [24]. Emerging technologies such as synthetic biology and omics integration are playing a pivotal role in optimising microalgae bioactive production. Synthetic biology tools, including CRISPR/Cas9, TALENs, and modular pathway engineering, are increasingly being used to enhance yields of targeted metabolites. For instance, genetic modifications in _Nannochloropsis_ and _Pavlovon_ strains have successfully increased EPA and DHA output. Transcriptomic and metabolomic profiling have revealed critical gene clusters associated with lipid biosynthesis and stress-induced metabolite accumulation [25; 26]. Furthermore, multi-omics integration (genomics, transcriptomics, proteomics, metabolomics) enables systems-level insights into microalgae biology.