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 with the potential to affect marine organisms and human activities [1]. This gulf receives freshwater discharges from the Tempisque and Grande de Tárcoles rivers, together with several smaller tributaries, whose inputs strongly influence circulation patterns, salinity gradients, and the transport of nutrients and organic matter within the system [2]. These processes, combined with the gulfs morphology and water residence time, regulate the mixing of continental and marine waters and modulate the distribution and dispersion of phytoplankton and other planktonic organisms, ultimately influencing their composition, abundance, and temporal dynamics [34]. The coastal waters surrounding sites as Paquera, Isla Cedros, Punta Cuchillo, and adjacent areas have exhibited recurrent discoloration events associated with algal blooms for several decades [1,5]. In February 2026, an extensive bloom characterized by intense orange discoloration visible to the naked eye occurred in the region of the Gulf of Nicoya. The bloom formed with continuous surface aggregations with an oily appearance and texture, likely associated with exceptionally high cell densities observed during the event. These patches were distributed across the water surface and were transported by local currents (Fig. 1). At night-time, strong and intense blue bioluminescence was observed in the Paquera area following mechanical disturbance caused by waves and boat movement (Fig. 2). During this event, surface water samples were collected both day and night from several locations near Cedros Island (Fig. 1). The samples were analyzed using optical microscopy to identify the bloom-forming microalgae and associated phytoplankton species. In addition, the abundance of total cultivable heterotrophic and bioluminescent bacteria (CFU mL1) was determined by filtering 5, 1, and 0.1 mL aliquots through 0.22 μm cellulose nitrate membrane filters. These filters were subsequently inoculated onto LSW-70 agar and incubated at 28 C for 24 h (Fig. 2). Water temperature during the event ranged from 27 to 28 C, with a salinity of 32. According to the National Meteorological Institute of Costa Rica (IMN), January and February were characterized by warm and predominantly dry conditions typical of the North Pacific dry season. Persistent northeasterly winds were recorded, with moderate to strong intensity (3040 km h-1), to- Fig. 1. Extensive orange-colored patches produced by the dinoflagellate Noctiluca scintillans around Cedros Island and the Gulf of Nicoya. (A) Map of the Gulf of Nicoya (94909 N and 845556 W). (B, C) Extensive patches with orange coloration (photographs by Marisol Hidalgo Prado). HARMFUL ALGAE NEWS NO. 83 / 2026 gether with mostly clear skies. Several cold front events occurred in February, which further enhanced wind activity in the region. Sample analyses revealed that the bloom was produced almost exclusively by the heterotrophic dinoflagellate Noctiluca scintillans, whose size ranged from 180 to 250 μm (Fig. 2). Minor amounts of the dinoflagellate Prorocentrum micans and the central diatom Stephanocyclus meneghinianus (=Cyclotella meneghiniana) were observed. As the water samples were highly concentrated with N. scintillans, accurate cell counting was difficult; therefore, several counts were performed using Utermöhl chambers, yielding concentrations of 1.7 103 to 1 106 cells L1. Noctiluca scintillans is a heterotrophic dinoflagellate that feeds on diatoms and bacteria, among other organisms. However, both in the water samples and inside Noctiluca cells, a high abundance of the planktonic ciliate Strombidium sp. was observed. This oligotrich ciliate is approximately 20 μm in diameter, and functions as a microzooplankton grazer feeding on bacteria and pico- to nanophytoplankton, and is commonly abundant in coastal and oceanic waters (Fig. 2). Although not previously reported in the Gulf of Nicoya, it is a widespread component of marine microzooplankton and plays a key role in microbial food webs. Strombidium spp. consume small dinoflagellates and diatoms and can exert grazing pressure on phytoplankton blooms. However, this effect is limited when blooms are dominated by larger dinoflagellates such as N. scintillans, which is itself a predator of smaller plankton. Consequently, in systems such as the Gulf of Nicoya, Strombidium may contribute to the regulation of small phytoplankton, but its effect diminishes under blooms dominated by N. scintillans. Strombidium is associated with relatively oligotrophic environments, thriving in nutrient-poor tropical or subtropical waters. It is widely distributed in tropical, subtropical, and temperate 17 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