Fig. 3. Light microscope images of living Fibrocapsa japonica cells. Scale bars = 20 μm. French waters. REPHY data [9] indicate that the previous maximum abundance occurred in 2013 in the Vilaine estuary, reaching 1.9 x 105 cells L1, approximately 25 times lower than values than maximum concentrations observed in 2025. Although associated with fish mortality events in Japan in the 1970s [7], no mortality events were reported during this bloom period in France. Some fishermen nevertheless reported unusually low fish abundance in Concarneau Bay and the Morbihan Gulf. An important mortality event involving Atlantic puffins (Fratercula arctica) was also recorded in southern Brittany in the weeks following the bloom (February). While the successive storms that hit the region during this period appear to have been the primary cause of mortality, a reduction in fish availability the puffins main prey linked to the bloom may have further weakened the birds, making them more vulnerable. Fig. 5. Viability percentage (expressed relative to solvent control L1 medium at 25%) of fish gill cells (RTgill-W1 cell line), after two-hour exposure to different concentrations of crude intracellular fractions from the French F. japonica strain IFR-CC 25-055. Different letters indicate significant differences (ANOVA followed by Tukey HSD post-hoc test, p<0.05, n = 6 replicate wells). HARMFUL ALGAE NEWS NO. 83 / 2026 Fig. 4. Temporal evolution of F. japonica abundances (cells L-1) recorded at REPHY stations during the bloom (November 2025 to February 2026). According to the Ligue pour la Protection des Oiseaux (LPO), 45,014 puffins were found beached along the French Atlantic coast between 19 December 2025 and 17 March 2026. This figure represents only a fraction of the total mortality, as experts estimate that only about one in ten birds reaches the shore [10]. In vitro bioassays were performed to assess the ichthyotoxic potential of a French strain (F. japonica IFR-CC 25055), isolated from a single-cell from the bloom. Intracellular content, obtained from the supernatant of lysed culture pellets, were tested on the RTgill-W1 fish gill cell line (CRL-2523, ATCC), following Beesoo et al. [11] and Rolton et al. [12]. No significant cytotoxicity was observed between 4.0 x 105 to 6.2 x 106 cells L1 (Fig. 5). A significant reduction in cell viability occurred only above 2.5 x 107 cells L1 (Fig. 5). The maximum environmental concentration observed (4.8 106 cells L1) therefore suggests limited cytotoxic potential under natural conditions. By contrast, Aguiar Juárez et al. [13] reported cytotoxicity on RTgill-W1 for Argentinian strains of F. japonica at much lower cell densities, (6.2 x 105 cells L1), highlighting straindependent toxicity modulated by environmental and physiological factors such as salinity, temperature, light intensity and nutrient availability. While the French strain appears weakly toxic under tested conditions, further comparative studies are needed to assess variability among strains and growth conditions. This event highlights the value of integrating fixed-station phytoplankton monitoring with citizen science observations for early detection of bloom events. It also demonstrates the importance of combining taxonomy, molecular biology, hydrology, chemistry and ecotoxicology for comprehensive characterisation of harmful algal events. References 1. Siano R et al. 2020. Mar Policy 117: 103039. https://doi.org/10.1016/j.marpol.2018.01.022 2. Anses 2021. Saisine n 2020-SA-0020. https://www.anses.fr/fr/system/ files?file=ERCA2020SA0020Ra-b.pdf 3. Kirkpatrick B et al. 2004. Harmful Algae 3(2):99115. https://doi.org/10.1016/j. hal.2003.08.005 4. Lassus P et al. 2016. Intergovernmental Oceanographic Commission of UNESCO. IOC Manuals and Guides, 68. https:// unesdoc.unesco.org/ark:/48223/ pf00002477675. 5. Amzil Z et al. 2026. Mar Drugs 24(2) : 67. doi.org/10.3390/md24020067. 6. Le Moigne M et al. 2025. Expertise Ifremer N 25-079 7. Toriumi S & Takano H 1973. Bull Tokai Reg Fish Reg Lab 76:2535. 8. Billard C 1992. Cryptogamie, Algologie 13(3):225231. 9. REPHY 2023. REPHY dataset SEANOE. doi.org/10.17882/47248. 10. France Info, 2026. Plus de 45 000 macareux échoués sur les plages de la côte atlantique cet hiver. Available at: https://www.franceinfo.fr/environnement/plus-de-45-000-macareuxechoues-sur-les-plages-de-la-coteatlantique-cet-hiver_7934996.html 11. Beesoo R et al. 2025. Harmful Algae 150:102980. doi.org/10.1016/j. hal.2025.102 980. 12. Rolton A et al. 2025. Harmful Algae 150:102991. doi.org/10.1016/j. hal.2025.102980 13. Aguiar Juárez D et al. 2025. Toxins 17(8):386. doi.org/10.3390/toxins17080386 Authors Ewenn Cochet, Audrey Duval, Amélie Derrien, Malwenn Lassudrie, Nicolas Chomérat, & Gwenael Bilien, Ifremer, COAST, F-29900 Concarneau, France Michael Retho, Ifremer, COAST, F-56100 Lorient, France Email corresponding author: ewenn.cochet@ifremer.fr https://doi.org/10.5281/zenodo.20583225 9 Harmful Algae News An IOC Newsletter on Toxic Algae and Algal Blooms No. 83 June 2026 https://hab.ioc-unesco.org/ Long and Winding Sea-lanes for Fish-Killing Algal Events An ancient idiom dead fish rot (or stink) from the head down possibly attributable to Turkish or Persian fishers but the orig Fig. 2. Programme for the Advanced International Colloquium and Technical Workshop on fish killing marine algae and their effects. blooms. The WG also decided to revise the classic but outdated Cooperative Research Report [2] on HAB effects on mariculture and marine fisheries published in 1992 for Chilean government, through CORFO and cooperation of CREAN-IFOP (reported in HAN 63 [3]) (Fig. 2). The colloquium convenors invited international experts to Puerto Varas, Chile in 2019 to review disciplinary knowledge on all aspects of fish-killing algae and associated mortality events (Fig. 3). A p ins were added during the IPHAB XVII Intersessional (2025-2026), but the total meagre reported ichthyotoxins score (by March 2025) (zero goniodomins, zero prymnesins, one karlotoxin [sterolysin]) has increased dramatically (by February 2026): seven goniodomins, four prymnesins, one karlotoxin, and m (admittedly controversial) explain how toxigenic blooms may directly kill fish in aquaculture operations [e.g., 11]. Access to comprehensive time-series databases on HAB events (HAEDAT, HAIS/ OBIS) has allowed for interpretation of fish-killing events over decades on a regional geographical basis, e Catastrophic marine mass mortalities, shellfish toxicity and human respiratory problems from a Karenia cristata dinoflagellate bloom in South Australia, 20252026 Fig. 1. Satellite chlorophyll image from March 2024 showing widespread offshore diatom blooms in response to a massive upwelling event du from which the species name cristata is derived), and a longer hypocone with the right lobe slightly longer than the left. Scanning electron microscopy revealed that the crest was formed by a slight elevation of the right side of the apical groove. On the dorsal side, the apical groove extended to o Unprecedented bloom of Fibrocapsa japonica on French coasts Fig. 1. Map of the different bays in southern Brittany monitored as part of the REPHY program. Since 1987, the REPHY (French Phytoplankton and Hydrology Monitoring Network in Coastal Waters), operated by IFREMER, has conducted long-term m Fig. 3. Light microscope images of living Fibrocapsa japonica cells. Scale bars = 20 μm. French waters. REPHY data [9] indicate that the previous maximum abundance occurred in 2013 in the Vilaine estuary, reaching 1.9 x 105 cells L1, approximately 25 times lower than values than maximum concentrati First Record of Fukuyoa sp. (Gambierdiscoideae) in the Northeastern Region of Términos Lagoon, Campeche, Mexico Fig. 1. Map of sampling stations at Términos Lagoon, Campeche, Mexico. The station where Fukuyoa sp. was found is circled in red. Términos Lagoon is a region of substantial economic rele Fig. 2. (AB). Fukuyoa sp. in ventral view. (C). Dorsal view. (D). Antapical view showing plates S. d. p., 1, 1. (EF). Ventral view showing plates 1, 1,2, 7, S. d. p., 1,1. (GH). Antapical view showing plates 1p,2,3, 1, 2. Scale bars = 20 μm. in Australia [6] recorded F. paulensis at temperatures of Trophic interactions shape ciguatera risk in a warming ocean Ciguatera is one of the most widespread marine poisonings worldwide, caused by the consumption of fish that bioaccumulate ciguatoxins (CTXs) produced by dinoflagellate species belonging to the genus Gambierdiscus. Its expansion into non-en drivers such as temperature variability and habitat disturbance. Calibration and validation with empirical data on cell densities, toxin concentrations, and ecosystem dynamics will be essential to generate robust predictions. The inclusion of spatial structure and coupling with human health risk mod Red and green waters in southern B rittany (France) in March 2026 linked to a bloom of Mesodinium spp. Fig. 1. Map of reported water discoloration events in southern Brittany (1012 March 2026) based on PHENOMER observations and direct reports. Locations include the Bay of Audierne and the Bay of C Fig. 3. Sentinel-2 satellite image (11 March 2026) showing the spatial extent of the bloom in southern Finistère, with burgundy-red patches indicative of high Mesodinium surface concentrations. are more stable and impart a green coloration. This transformation can occur within minutes, explaining t Fig. 6. Cells of Mesodinium major. 13. Different views of living cells. 46. Representation of pigment dynamics during Mesodinium cell degradation: transition from phycoerythrin-dominated red coloration to chlorophyll-dominated green coloration following cell lysis. All images to scale. accompanied Who turned on the light? First report of extensive bioluminescent blooms of the heterotrophic dinoflagellate Noctiluca scintillans with low abundance of bioluminescent bacteria in the Gulf of Nicoya, Costa Rica Historically, the Gulf of Nicoya has experienced recurrent algal blooms, including events Fig. 2. Dominant dinoflagellate of the Noctiluca scintillans algal bloom around Cedros Island. (A) Bloom of N. scintillans. (B) Ciliate fed on by Noctiluca, Strombidium sp. (C) Culture of bioluminescent bacteria. (D, E) bioluminescence observed at night on Cedros Island and surrounding areas (photog estuaries supporting fisheries, tourism, and biodiversity. Monitoring these processes allows differentiation between benign and harmful blooms and supports the preservation of ecosystem integrity. Ultimately, such phenomena not only expand scientific understanding but also underscore the resilience International Conference on Molluscan Shellfish Safety (ICMSS) Dear colleagues, We wanted to draw your attention to the upcoming International Conference on Molluscan Shellfish Safety (ICMSS), taking place 611 September 2026 at the University of Exeter, UK. Further details, including registration (w DART in action: Scientists launch regional effort against toxic diatoms Scientists from across Asia have come together to tackle the growing threat of toxic diatoms that produce neurotoxin Domoic Acid (DA). On 1718 March 2026, researchers from China, Malaysia, and Singapore gathered in Qingdao, Chin Fig. 2. Kick-off meeting and research presentations. (A) Nancheng Chen, the lead PI, delivers the opening address. (BD) PIs from participating countries presenting their national research plans. (EF) Selected presentations delivered during the workshop. molecular tools can greatly improve our abilit IOC-FAO Intergovernmental Panel on Harmful Algal Blooms (IPHAB) Extraordinary online Session 27 October 2026 and 18th Session (IPHAB-XVIII), 1820 March 2027, FAO, Rome The Intergovernmental Panel on Harmful Algal Blooms (IPHAB) was established in 1992 to strengthen the scientific, managerial, and fi