Microalgae represent an untapped resource of mixed polysaccharides with potential applications in biotechnology, therapeutics, pharmaceuticals, and drug delivery. There are presently ~160, 000 described species of algae and approximately 80% of these are classified as microalgae, including the cyanobacteria. Numerous reviews and research articles can be found on the structural characterization and bioactivity of a variety of polysaccharides derived from seaweeds and kelps, i.e., macroalgae, and many of these compounds are widely used in industry and clinical applications. By comparison, there are relatively few studies on the bioactivity of polysaccharides from microalgae or their potential applications. This chapter will provide an overview of structural and bioactive polysaccharides and related case studies for major groups of microalgae, including the Cyanophyta (cyanobacteria), Chlorophyta (green algae), Rhodophyta (red algae), and selected golden microalgae in the Chromista, e.g., Bacillariophyta, Haptophyta, and Ochrophyta. Many groups of microalgae produce bioactive polysaccharides, including extracellular polymeric substances (EPS) and sulfated polysaccharides (SPS) that have demonstrated antibacterial, antiviral, antioxidant, and anticancer potential. Microalgal glycans are composed predominantly of pentose, hexose, and deoxyhexose monosaccharide subunits with various glycosidic linkages and degrees of branching. Their bioactivity can often be attributed to the monosaccharide composition, content of uronic acids, molecular weight fraction, and/or degree of modification, e.g., sulfation, amination, acylation, phosphorylation, and methylation. While current studies have been limited to a few species, there are many more microalgae to be explored, providing immense opportunities for metabolite discovery in the field of microalgal polysaccharides and natural products.
