Examining the Function of Metabolites Generated from Cyanobacteria as Antimicrobial Agents from a 21st-Century Perspective
Cyanobacteria are among the oldest oxygenic photosynthetic organisms on Earth and occupy an extraordinarily broad range of ecological niches, from marine and freshwater systems to extreme terrestrial habitats. Their capacity to withstand intense ultraviolet radiation, desiccation, nutrient scarcity and microbial competition is underpinned by a rich biosynthetic repertoire of secondary metabolites, many of which display potent antibacterial, antifungal and antiviral activities. This review examines the chemical diversity, biosynthetic origin and mechanisms of action of cyanobacterial antimicrobial metabolites, situating the discussion within the contemporary crisis of antimicrobial resistance and the stagnation of conventional antibiotic discovery pipelines. Cyclic and linear peptides, alkaloids, polyketides, macrolides, lipids and lectins isolated predominantly from filamentous genera such as Nostoc, Lyngbya, Moorea, Fischerella, Calothrix and Anabaena are discussed with reference to their molecular targets, which include bacterial RNA polymerase, fungal ergosterol-rich membranes, viral envelope glycoproteins and components of the bacterial cell wall and electron transport chain. The contribution of non-ribosomal peptide synthetase and polyketide synthase gene clusters to this chemical diversity is examined alongside genome-mining approaches that have accelerated the discovery of cryptic biosynthetic pathways. Biotechnological obstacles to translating laboratory findings into clinically usable agents, including low and inconsistent yields, cultivation scale-up difficulties, structural complexity and the toxicological profile of certain cyanobacterial metabolites, are critically appraised, together with emerging nanoformulation strategies intended to overcome bioavailability constraints. The review concludes that cyanobacteria represent a comparatively underexploited reservoir of structurally novel antimicrobial chemotypes that merit sustained, mechanistically grounded investigation, while acknowledging that translational progress remains constrained by supply, safety and regulatory hurdles that have yet to be systematically resolved.