Harmful Algae News An IOC Newsletter on Toxic Algae and Algal Blooms No. 84 September 2026 https://hab.ioc-unesco.org/ Unveiling biotic controls of Alexandrium minutum in the Ría de Vigo (NW Spain) Understanding biotic interactions that regulate the dynamics of harmful algal bloom (HAB) species remains a major challenge in aquatic microbial ecology with important implications for the management and conservation of marine ecosystems. Blooms of toxin-producing microalgae can cause substantial economic losses to the seafood industry when shellfish harvesting is suspended because toxin concentrations exceed regulatory limits. Improved understanding of how these interactions shape microbial communities could enhance predictions of global change impacts on microbial ecosystem services and support the management of marine resources, including aquaculture production. There is broad agreement regarding the abiotic conditions which favour the development of toxic dinoflagellates, including high nutrient concentrations, N:P ratios, riverine inputs and watercolumn stratification [1]. There is also growing evidence supporting the importance of biotic interactions, mostly mediated by molecular exchange, in regulating dinoflagellate growth and toxin production. Early evidence for the relevance of these interactions came from the difficulty of maintaining viable dinoflagellate cultures after the removal of their associated bacteria [2]. Twenty-five years later it was demonstrated that the presence of bacteria in cultures significantly affected gene expression in Alexandrium tamarense, influencing key metabolic processes such as photosynthesis [3]. Recently, the concept of the HAB holobiome, i.e., the assemblage of phycosphere-associated and free-living bacteria, was proposed as a potential framework for understanding microbial interactions that influence algal growth [4]. Although most biotic interactions are expected to occur within the phycosphere, several studies suggest that toxic dinoflagellate blooms may also be modulated by allelochemicals produced by benthic macrophytes [56]. Notably, Zostera marina can limit the abundance of planktonic dinoflagellates both within seagrass beds and up to 15 m away [7]. A recent review suggested that stable abiotic conditions favour the formation of toxic dinoflagellate blooms Content FEATURED ARTICLE Unveiling biotic controls of Alexandrium minutum in the Ría de Vigo (NW Spain) Eva Teira...................................................... Harmul Algal Blooms and management of their effects High biomass HABs during the austral summer in Atacama, northern Chile .......................................... 1 6 Monitoring, detection, and control programme for Alexandrium catenella in Southern Chile .................................... 9 HABs in a hypersaline mangrove lagoon, Seaflower Biosphere Reserve, Colombian Caribbean ........................... 11 Shelly-Ann Cox in memoriam.......... 13 Forthcoming Events IOC-FAO IPHAB ........................................ 14 ICHA 2027 in Aberdeen, Scotland (UK) ........................................... 14 ASLO 2027 in San Juan, Puerto Rico................................................. 14 II International Conference on Ostreopsis (ICOD) in the FrenchBasque Country ....................................... 15 NEW ! The IOC Toxins Database ...... 16 Fig. 1. A red tide of Alexandrium minutum in Campelo, Galician Rías Baixas, April 2023. Photo by Francisco Rodríguez. Harmful Algae News An IOC Newsletter on Toxic Algae and Algal Blooms No. 84 September 2026 https://hab.ioc-unesco.org/ Unveiling biotic controls of Alexandrium minutum in the Ría de Vigo (NW Spain) Understanding biotic interactions that regulate the dynamics of harmful algal bloom (HAB) species r Fig. 2. The study area in the Ría de Vigo (NW Spain). The red dots indicate the locations where the A. minutum strains were isolated. [1]. We propose that such stable abiotic conditions may facilitate the establishment and persistence of key biotic interactions between the microalgae and their micr Fig. 4. Abundance of Alexandrium minutum (Am) (A) and bacteria (B) in quasi-axenic monocultures (blue lines) or co-cultures with Alteromonas sp. (Alt) (red lines) with B-vitamins (w B-vit, solid lines) or without B-vitamins (wo B-vit, dashed lines). (C) Taxonomic composition of the bacterial microbi Fig. 5. Box and whisker plot showing the response ratios (RR) of Alexandrium minutum growth to the addition of different Zostera marina exudates at low (200300 μM DOC) and high (500600 μM DOC) concentrations. A total of 14 and 10 response ratios were included for the low and high concentration treat dinoflagellates. It has been suggested that phenolic compounds produced by marine angiosperms, e.g. [25], may play a significant role in the negative interaction between these plants and dinoflagellate growth [56, 21]. However, this hypothesis has not yet been experimentally tested. Acknowledgement Intense algal bloom during the austral summer in Inglesa and Calderilla Bays (Atacama, Chile): phytoplankton monitoring, environmental observations and implications for coastal aquaculture Fig. 1. Seawater discoloration in Inglesa Bay sector, Caldera during the first week of February 2026. (A) and Fig. 3. Dominant dinoflagellate species in the algal bloom. (A) Akashiwo sanguinea and (B) Tripos cf. furca. radically (2005, 2009, 2014, and 2016), including blooms of the toxic dinoflagellate species such as Protoceratium reticulatum, which produces yessotoxins (YTX) and has been detected along t Fig. 5. Abundance of other microplankton groups in February 2026 at the five sampling stations in the Caldera Bay System. 2. Rodríguez L 1978. Noticiario Mensual del Museo de Historia Natural de Chile 22(266):69 3. Rodríguez L 1985. Rev Biol Mar 21:173197. https://hdl.handle. net/20.500.14330/PER01 An overview of the 20182025 monitoring, detection, and control programme for Alexandrium catenella in Southern Chile The frequency, persistence, and geographical extent of harmful algal blooms (HABs) caused by the dinoflagellate Alexandrium catenella have increased in southern Chile over recent deca 3. 4. 5. 6. 7. 8. Fig. 2. Annual number of analysed samples and positive detections between 2018 and 2025. Bars represent the total number of samples collected each year. The red area indicates positive results (number shown at the base of each bar), whereas the blue area corresponds to negative res A mixed bloom of Prorocentrum lima species complex and Aphanocapsa sp. under hypersaline conditions in a mangrove lagoon of the Seaflower Biosphere Reserve, Colombian Caribbean Fig. 1. Old Point Regional Park, San Andrés Island, Seaflower Biosphere Reserve, showing the small mangrove lagoon where t Table 1. Measurements of physicochemical characteristics recorded during the mixed bloom of the Prorocentrum lima species complex and Aphanocapsa sp. Old Point, Colombia Lago delle Nazioni, Italy Paracas Bay, Peru 14 April 2026 December 2019 January 2020 2729 August 2017 4.5 106 4.7 107 2 morphological and molecular identification of benthic Prorocentrum and cyanobacteria with measurements of dissolved inorganic nutrients, dissolved and particulate organic carbon, phycotoxins, microbial community composition, sediment resuspension, and water-exchange and salinity dynamics. Acknowled Forthcoming IOC-FAO IPHAB The Eighteenth Session of the IOC-FAO Intergovernmental Panel on Harmful Algal Blooms (IPHAB-XVIII) will meet on 16-18 March 2027 at FAO Headquarters in Rome. The establishment of an intergovernmental panel on HABs in 1991 was a response to the need to improve the manageria The second International Conference on Ostreopsis Development ICOD 2, French Basque Country, June 1011, 2027 Over the last decades increasing attention has been paid to benthic dinoflagellates belonging to the genus Ostreopsis. The presence of these microalgae has been reported in many temperate re NEW! IOC Toxins database The IOC Toxins database is now on-line. It contains reference information about toxins, some of which are associated with Harmful Algal Events. The IOC Toxin Reference List is work in progress. Some sections are complete, some are partly complete and some are empty. The Lis