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 microbiome at day 10. modulate A. minutum dynamics in the Ría de Vigo, we isolated several strains and conducted a set of experiments to explore the specific role of the associated holobiome and the effects of substances released by common eelgrass (Zostera marina). We observed facilitative associations between A. minutum and bacteria as well as a negative impact of Z. marina exudates on A. minutum growth. These findings contribute to our understanding of the complex biological interactions that influence HAB occurrence. Facilitative interaction between the holobiome and Alexandrium minutum Like many other HAB-forming species, A. minutum is auxotrophic for B vitamins, particularly vitamin B12, suggesting that interactions with prototrophic bacteria may be essential for bloom development and persistence [14]. Previous analyses of the bacterial microbiome of the A. minutum V1 strain, which was isolated from the Ría de Vigo showed a dominance of the order Flavobacteriales, particularly Winogradskyella, followed by members of the Rhodobacterales [15]. Further analyses of other strains revealed a similar microbiome structure. Figure 3 shows the mean microbiome composition of the A. minutum VGO1607 strain, which was isolated from Baiona Bay and maintained in culture for 1.5 years. Here, three genera dominated the A. minutum microbiome: Winogradskyella (Flavobacteriales), Sulfitobacter (Rhodobacterales), and HARMFUL ALGAE NEWS NO. 84 / 2026 Alteromonas (Alteromonadales). Winogradskyella contributed up to 4 % to the particle-attached bacterial community in field samples collected during A. minutum blooms in Baiona Bay. Sulfitobacter and Alteromonas were also well represented in field samples collected during A. minutum blooms, contributing up to 7 % to the particle-attached fraction in the case of Alteromonas, and up to 10% to the free-living fraction in the case of Sulfitobacter (data not shown). This suggests that the microbiome present in the isolated strains may be representative of the natural microbiome. The genus Winogradskyella also dominates the phycosphere of other toxic dinoflagellates, such as Prorocentrum minimum [16]. However, it was not identified in the core microbiome of A. minutum strains isolated from the Mediterranean Sea [17]. This suggests that different strains recruit different bacterial taxa depending on environmental conditions. The co-culture approach is a wellestablished method for probing the interactions between algae and bacteria. It requires axenic strains to evaluate algal growth in monoculture or in coculture with bacterial isolates. Although we failed to obtain axenic A. minutum cultures using a broad-spectrum antibiotic mixture containing penicillin, neomycin and streptomycin, a 99 % reduction in the bacterial load was achieved, resulting in a quasi-axenic culture. We also tested the procedure described in [18], but axenization was not feasible for the VGO1607 strain. After antibiotic treatment, we noticed that Winograd- skyella was the dominant genus (data not shown), suggesting that its resistance to the antibiotic prevented effective axenization of the algal culture. We successfully isolated several bacterial species from A. minutum VGO1607, including members of the genus Alteromonas, and tested their effects on A. minutum growth in co-culture (Fig. 4). Co-culturing with Alteromonas sp. immediately enhanced A. minutum growth relative to the quasi-axenic control. Interestingly, this facilitative effect does not appear to be related to vitamin provision, as the same response was observed in culture media lacking B vitamins (Fig. 4A). After 10 days, the microbiome composition revealed the proliferation of Winogradskyella sp. in the quasi-axenic controls, beginning after 6 days, and the coexistence of Alteromonas sp. and Winogradskyella sp. in the co-cultures (Fig. 4C). A similar facilitative interaction between Alteromonas sp. and Alexandrium catenella was described [19, as A. fundyense], although several studies have demonstrated antagonistic interactions between Alteromonas spp. and Alexandrium spp. [20]. We have recently isolated additional bacterial strains from the VGO1607 culture, including members of the genera Alteromonas, Winogradskyella, Cellulophaga, and Sulfitobacter. These will allow us to test additional strains and strain combinations in co-culture. Such experiments will expand our knowledge of the role of bacteria associated with A. minutum and may ultimately improve our ability to predict HAB dynamics by incorporating biotic controls. 3 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