In July 2022, a harmful algal bloom of Heterosigma akashiwo originated in a man-made lagoon and expanded to more than 4,500 km² of open-water habitat in the ...

YouTube video by Crystal Castles official

Am 23. gab die Präfektur Yamaguchi eine Rote-Flut-Warnung heraus, da die Zahl des schädlichen Planktons „Heterosigma akasio“ in der Mitajiri-Bucht vor der Küste der Stadt Hofu in der Präfektur den Standardwert überstieg.Präfekturbüro Yamaguchi Dies ist das erste Mal, dass dieses Jahr innerhalb der Präfektur ausgestellt wird. Derzeit liegen keine Berichte über Schäden an der Fischerei vor.

From the USGS: A new study by several scientists examines the environmental conditions that fueled a massive algal bloom in the San Francisco Bay. By analyzing water quality, circulation, and microscopic communities, the research helps explain how rare combinations of factors can drive major ecological events, including blooms of Heterosigma akashiwo, a species first identified in Japan. Tracking the Bloom Across the Bay Algal blooms are not unusual in the San Francisco Bay, where nutrient levels are high and seasonal […]

Marine microalgae play a crucial role in marine ecosystem by supplying dissolved organic matter to h

The structures of oceanic prokaryotic communities in particle-associated (PA) and free-living (FL) fractions trapped on filters with pore sizes of 3 and > 0.2-µm, respectively, have been investigated. However, prokaryotic interactions between the PA and FL fractions, which may ...

Marine algal blooms play a vital role in oceanic carbon cycling, yet the ecological consequences of algal organic matter released following their collapse via viral infection are poorly understood. Recent studies have shown that viral infection dramatically alters the host's intracellular metabolite composition, and the subsequent viral lysate selectively promotes the growth of specific prokaryotic populations. This study aimed to elucidate the effect of organic matter derived from healthy and virus-infected cells of the bloom-forming alga Heterosigma akashiwo on the growth of heterotrophic eukaryotes, specifically Labyrinthulomycetes. These marine protists are primarily saprotrophic or predatory and contribute to dissolved organic matter (DOM) decomposition and nutrient cycling. Our field monitoring in Osaka Bay over 12 months revealed that while the overall Labyrinthulomycetes community showed no clear seasonality, specific populations of the protists co-occurred with Heterosigma akashiwo . To mechanistically investigate the potential trophic linkage suggested by these field observations, a co-culture system comprising H. akashiwo , its specific virus (HaV53), and Aurantiochytrium sp. NBRC102614, used here as a model Labyrinthulomycete, was established. In the co-culture experiments, viral lysis of H. akashiwo led to a significant increase in the cell density of Aurantiochytrium sp., demonstrating that Aurantiochytrium can thrive on substrates derived from the virus-infected alga, such as viral-induced dissolved organic matter (vDOM). These findings highlight that heterotrophic Labyrinthulomycetes are one of key consumers of virus-modified organic matter, playing a pivotal role in carbon cycling following the collapse of harmful algal blooms and influencing carbon transfer in coastal microbial food webs.

Despite the ecological importance of viruses, our understanding of their evolutionary dynamics in natural environments remains limited. This gap is particularly pronounced for giant dsDNA viruses of the phyla Nucleocytoviricota and Mirusviricota . Most knowledge on their population genetic dynamics is derived from a small number of laboratory-based experiments, whereas patterns in nature are rarely observed. To overcome this limitation, we traced genetic structure and transcription status of Heterosigma akashiwo virus (HaV) using high-frequency, time-resolved sampling during a host bloom in a coastal area of Japan, by integrating cell counting, metabarcoding, metagenomic and metatranscriptomic sequencing. Our study revealed that HaV dominated the giant virus community in most samples, with the relative abundances of up to 56%. Despite the high abundances, the HaV population exhibited a relatively low level of microdiversity but with a high pN/pS ratio compared to other giant viruses in the study site. Microdiversity increased during the early sampling period, reached a maximum at mid-sampling, and decreased during the later period, consistent with rapid diversification during viral expansion, possibly driven by both in situ mutations and the succession of pre-existing minor variants. Several accessory genes, including a glycosyltransferase and an endonuclease, were highly expressed, providing functional evidence consistent with host interaction-driven selective pressure during the bloom. Together, these results indicate that HaV population dynamics during algal blooms are shaped by host-driven selection acting on standing genetic variation. ### Competing Interest Statement The authors have declared no competing interest. JSPS/KAKENHI, 18H02279, 19H05667, 17H03850, 21H05057, 19K15895, 23K23685 MEXT, 16H06429, 16K21723, 16H06437 JST PRESTO, JPMJPR23G3 Collaborative Research Program of the Institute for Chemical Research, Kyoto University, 2016-28, 2019-35

The primary production by marine microalgae and the consumption of the produced organic matter by prokaryotes significantly contribute to biogeochemical cycling. Microalgae are often infected by viruses, and in infected cells (virocells), the viruses modulate host metabolism for propagation and endo-metabolites. However, the impact of microalgal virocells on prokaryotic communities is not fully understood. This study investigates effects of lysates from virocells of Heterosigma akashiwo, a globally distributed harmful bloom-forming raphidophycean alga, on the prokaryotic community. Our data suggest that changes in biochemical properties in H. akashiwo virocells promoted the growth of specific bacterial populations that appeared to have metabolic capacity to utilize certain organic compounds enriched in the lysate. Additionally, as those populations included fish-pathogenic bacteria, we propose that viral infection to H. akashiwo can indirectly affect higher trophic-level consumers in marine ecosystems.

Okinawa University researchers unveil Heterosigma's mechanism of leveraging bacteria for nutrient uptake, shedding light on red tide proliferation.

In a groundbreaking study published in the April 2025 issue of the Journal of Hazardous Materials, researchers from the Korea Institute of Ocean Science & Technology (KIOST) have uncovered a critical mechanism by which marine microalgae contribute to the fate of buoyant microplastics (MPs) in oceanic environments. Led by Dr. Seung Ho Baek and Dr. Young Kyun Lim, the team focused on the role of…

From the USGS: A new study by several scientists examines the environmental conditions that fueled a massive algal bloom in the San Francisco Bay. By analyzing water quality, circulation, and microsco...