Algae and microbial proteins for alternative proteins: Untapped molecular resources.

Algal and microbial proteins constitute a vast, still underexploited pool of sustainable biomolecules with growing relevance to the future of alternative proteins. This chapter critically reviews the taxonomic, biochemical, and structural diversity of microalgae, cyanobacteria, yeasts, filamentous fungi, and bacterial platforms currently used for single-cell protein (SCP) production. Particular emphasis is placed on recent advances in extraction and downstream processing, including mechanical, enzymatic, chemical, and membrane-based strategies, to improve protein recovery while preserving structural integrity and minimising environmental impact. Molecular characterisation strategies are discussed to clarify how amino acid composition, structural organisation, and post-translational modifications shape key techno-functional properties, including solubility, emulsification, foaming, gelation, and film-forming capacity. The nutritional quality and bioactive potential of algal and microbial proteins are illustrated by selected examples, including Spirulina, Chlorella, fungal mycoprotein, and bacterial biomass, highlighting essential amino acid balance, digestibility, and the generation of bioactive peptides with antioxidant and immunomodulatory activities. In addition, regulatory frameworks, safety and allergenicity issues, sensory optimisation, and life-cycle assessment are examined to situate these protein sources within a broader context of sustainability and consumer acceptance. Finally, emerging advances in synthetic biology, adaptive laboratory evolution, metabolic engineering, and multi-omics-guided strain optimisation are discussed as enabling tools to improve functionality, scalability, and economic viability. By integrating molecular insights with process engineering and circular bioeconomy principles, this chapter positions algal and microbial proteins as credible and versatile building blocks for next-generation sustainable food systems.