IOC-UNESCO Taxonomic Reference List of Harmful Microalgae: Cyanobacteria Updates

The IOC-UNESCO Taxonomic Reference List of Harmful Microalgae is a globally recognised catalogue that provides an authoritative and curated list of microalgal species known to produce toxins or cause harmful effects in aquatic environments. Maintained under the Intergovernmental Oceanographic Commission, the list supports monitoring, research, and management by promoting taxonomic stability and up-to-date nomenclature. As of December 2025, the database includes 268 taxa, among which 99 cyanobacteria represent an increasingly important component due to their ecological and public-health impacts. Recent updates to the cyanobacterial section incorporate newly described taxa, revised classifications, and improved documentation of toxin production across both freshwater and marine environments. These updates reflect rapid advances in cyanobacterial systematics driven by genomic and phylogenomic approaches, as well as growing recognition of the ecological importance of strain-level diversity. Cyanobacteria exhibit considerable genetic and functional microdiversity, meaning that toxic and non-toxic strains may coexist within a single species. Consequently, attributing toxicity to dominant species in environmental samples requires careful interpretation unless supported by characterised isolates. The updated list highlights the expanding diversity of toxic cyanobacteria, including newly described taxa and newly recognised toxins and ecological effects. Of the currently recognised cyanobacterial orders, ten contain toxic species, with the highest representation in Nostocales, Oscillatoriales, and Chroococcales. Microcystins remain the most widely reported toxins, followed by saxitoxins, anatoxins, and cylindrospermopsins, among others. The continuing discovery of new taxa, toxins, and strain-level variability demonstrates that the diversity and ecological roles of toxic cyanobacteria remain incompletely understood. Ongoing updates to the IOC-UNESCO reference list therefore provide an essential resource for the harmful algal bloom research community.