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 treatments, respectively. Point colours indicate the month in which the exudates were prepared. Filled symbols correspond to exudates obtained from plants collected at the low intertidal level, whereas open symbols represent exudates from the high intertidal level. The horizontal red line (RR = 1) indicates no effect relative to the control. Allelopathic interaction between Zostera marina and Alexandrium minutum The negative effect of Zostera marina extracts on dinoflagellate growth has been known since the early 1980s. Harrison and Chan [5] exposed cultures of Lingulaulax polyedra (as Gonyaulax) and Alexandrium tamarense (as Protogonyaulax) to the water-soluble fraction of dead eelgrass leaves showing either a complete absence of viable cells in cultures treated with the Z. marina extract or a loss of motility within 24 h of extract addition. Since then, several studies have explored the allelopathic interactions between bloom-forming dinoflagellate species and marine angiosperms including the potential role of the latters microbiome. Experiments performed on cultures of Alexandrium catenella demonstrated the inhibitory effect of crude methanolic and aqueous extracts of two eelgrass species: Zostera marina and Nanozostera noltei on the growth rate of this neurotoxic dinoflagellate [6]. Further research explored the effect of allelochemicals released by fresh leaves, rather than by crude extracts, of the macrophytes N. noltei and Cymodocea nodosa on several dinoflagellate species [21]. The researchers reported that Nanozostera noltei had a 4 negative effect on the benthic dinoflagellate Prorocentrum lima whereas exudates derived from Cymodocea nodosa reduced the growth rates of both P. lima and Alexandrium pacificum. We investigated the effect of dissolved matter released from Z. marina leaves on A. minutum while also examining the potential influence of the associated prokaryotic community on the observed response. Our results confirmed the negative impact of the products released by the leaves of this seagrass species on A. minutum, with complete inhibition of dinoflagellate growth occurring at high concentrations of the seagrass exudate [22]. The results also showed that growth inhibition was independent of the presence of an active prokaryotic community in the culture, as a reduction in the growth rate of the dinoflagellates was also measurable in parallel samples treated with a mixture of antibiotics. We also investigated whether the allelopathic interaction observed between Z. marina and A. minutum persisted across seasons and intertidal levels in the Ría de Vigo. We conducted 24-hour bioassays in which cultured A. minutum cells were exposed to exudates derived from Z. marina leaves collected from high and low intertidal zones during different seasons. A. minutum abundance was estimated by flow cytometry over the following 72 h. The average abundances measured in cultures amended with different amounts of Z. marina exudates were compared with those in the unamended control treatments. Figure 5 shows the response ratio, i.e., the ratio of the A. minutum abundance in the treated samples to that in the controls, estimated for all the experiments performed. Values below 1 therefore indicate a negative effect of Z. marina exudates on A. minutum. Our results show that the inhibitory effect of Z. marina exudates on A. minutum was observed in the majority of experiments (96%). However, the magnitude of this effect varied considerably over time and space. Exudates from plants collected in the upper intertidal zone were more inhibitory than those from the lower zone. The strongest inhibitory effects were measured for exudates derived from plants collected in autumn. These results were interpreted in terms of the effect of environmental stress associated with emersion, and variations in chemical composition linked to the plants seasonal growth cycle [23]. Several research questions remain unexplored. Firstly, the negative interaction between seagrasses and dinoflagellates observed in laboratory cultures has not yet been confirmed through experimentation with natural populations. Nevertheless, indirect evidence from environmental DNA analysis along transects extending from seagrass meadows to nearby areas without seagrass revealed dramatic reductions in dinoflagellate abundance in the vicinity of Z. marina [7]. This observation is consistent with the allelopathic effect observed in cultures. A second relevant aspect that needs to be elucidated is the significance of, and the relative contribution made by the microbiome inhabiting seagrasses to the inhibitory effect. In this regard, experimental evidence suggests that bacteria isolated from the Z. marina biofilm can inhibit the growth of Alexandrium tamarense and Heterosigma akashiwo [24]. Finally, further research is needed to identify the specific allelopathic compounds responsible for the observed effect of seagrasses on HAB-forming HARMFUL ALGAE NEWS NO. 84 / 2026 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