Harmful algal blooms (HAB) of various toxic microalgae negatively affect the Black Sea but there is limited data on the sources of phycotoxins in this area. During the PHYCOB cruise in September 2021, two strains of one diatom and thirty strains of ten dinoflagellate species of potentially toxigenic plankton were characterized in terms of morphology, phylogeny, toxin profiles, and toxin cell quotas. Twenty strains of potentially yessotoxins (YTXs) producing dinoflagellate species were investigated. All six strains of Protoceratium reticulatum contained yessotoxin (YTX), with cell quotas between 1.9 and 5.4 pg cell-1. Additionally, several YTX variants were detected in minor amounts in nine of twelve strains of Lingulaulax polyedra, whereas no YTXs were detected in two strains of other gonyaulacoids (Gonyaulax sp. and Sourniaea diacantha). All strains of Alexandrium spp. (four strains of A. tamutum, one strain each of A. andersonii, A. ostenfeldii, and A. pseudogonyaulax) were analyzed for all toxins known to be produced by the genus. None of the strains contained detectable levels of paralytic shellfish toxins (PSTs) or gymnodimines. A. pseudogonyaulax produced goniodomine A (GDA) and GDA-seco acid at cellular levels of 14.0 and 0.33 pg cell-1, respectively. Moreover, four previously unreported spirolide analogues (SPX) were detected in the A. ostenfeldii strain, with cell quotas between 1.0 and 1.6 pg cell-1. All strains of potentially ichthyotoxic species (Karlodinium sp., Polykrikos hartmanii) did not show extracellular lytic activity. In conclusion, these findings improve our understanding of the potential sources, diversity and dynamics of phycotoxins in the Black Sea.

As climate change raises global temperatures and increases the frequency of cyanobacterial blooms, understanding how rising mean temperatures affect cyanotoxin content is crucial. However, no clear consensus exists, and the use of different methodologies, including different units of measurement and experimental conditions could significantly alter the yield of the relationship between temperature and toxins content. In this study, we assessed free microcystin content and cell volume in Microcystis aeruginosa PCC 7806 acclimated to seven temperatures spanning its entire thermal niche. This experimental design firstly highlighted the significant reduction in cell volume with rising temperatures between 17 °C and 29 °C. As a result, when the microcystin concentration was normalized by its cell volume, its temperature response was transformed from a negative correlation to a bell-shaped curve, with higher free MC-LR content measured at an estimated optimum temperature of 25.5 °C, close to the thermal growth optimum of Microcystis aeruginosa. These findings provide new insights into the effects of climate warming on microcystin content.

To date, little key information about inorganic arsenic (iAs) and cadmium (Cd) has been promulgated to help manage the massive influxes of pelagic Sargassum that have inundated the Caribbean coasts since 2011 and are causing significant environmental, health and economic impacts. In particular, the levels of iAs and Cd, which often exceed regulatory levels and limit the seaweed valorization, have not been linked to an origin or route. By analyzing a large number of samples collected at different times in Guadeloupe and Martinique, we have highlighted for the first time a route that rapidly traces back through the Guiana, North-Brazil and equatorial currents, bringing Sargassum to the Lesser Antilles in <3 months from the equator. All the Sargassum morphotypes from this problematic fast track displayed high levels of iAs and Cd, very often exceeding 40 ppm and 1 ppm, respectively. The other routes, which bring Sargassum more slowly, either from the Cape Verde basin or from the North Brazil Retroflection area, have low or intermediate levels of iAs and Cd. Following an in-depth examination of the cadmium-phosphorus relationship in the tropical Atlantic Ocean, we provide here for the first time an overview of the spatial disparities in the Cd and P composition of Sargassum.

Cyanobacterial blooms can be harmful to animals and humans exposed to their toxins; however, their environmental drivers and boundaries still need to be elucidated. Salinity has been demonstrated to be an important driver of community composition that sets boundaries of species migration and survival. The filamentous cyanobacteria Nodularia spumigena forms dense blooms in estuaries around the world, produces the hepatotoxin nodularin, and has been thought to not survive or fix nitrogen (N) in high salinities. From 2005-2009 we studied three bays of Great Salt Lake (USA), two of which are estuaries with salinities ranging from 0 to >90 g l-1 while the third, Gilbert Bay, had a salinity near 160 g l-1. Bear River Bay and the larger Gilbert Bay were meso‑eutrophic, while Farmington Bay, which receives direct inputs of secondary-treated sewage, was hypereutrophic with mean chlorophyll concentrations of 149 µg l-1 and dense blooms of N. spumigena. Cell densities were >500 times those of Nodularia studied in the Baltic Sea. In Farmington Bay blooms occur at salinities ranging from 8-50 g l-1, which are much higher than usually reported for this taxon. Concentrations of the cyanotoxin nodularin reached 660 µg l-1 (mean = 41 µg l-1), far above critical thresholds for contact recreation and above those causing bird mortalities elsewhere. The mean N2 fixation rate of Nodularia measured over a salinity range of 14 to 52 g l-1 was 47 mg N m-2d-1, which is among the highest reported values for freshwater and marine ecosystems. The local adaptation of Nodularia to the extreme salinity conditions in Great Salt Lake furthers our understanding of salinity adaptation and the potential spread of this species to new regions.

Mesozooplankton (200-2,000 µm) are an important link between primary producers and higher-level consumers and can influence phytoplankton biomass and community structure via grazing. Yet, during cyanobacteria harmful algal blooms (cHABs) we have a poor understanding of which food resources mesozooplankton use and their grazing pressure on phytoplankton. To investigate this, we conducted two gradient-grazer assays to measure mesozooplankton community grazing rates in western Lake Erie during the 2021 cHAB season. We measured mesozooplankton grazing on various food sources, including: total phytoplankton, phytoplankton major taxa (Cyanobacteria, Cryptophyta/Bacillariophyta, Chlorophyta), and autotrophic and heterotrophic picoplankton and nanoplankton. We used these data to estimate mesozooplankton feeding selectivity, the importance of each food source toward mesozooplankton carbon intake, as well as the percent standing stock and growth consumed by mesozooplankton. Mesozooplankton selected for autotrophic and heterotrophic nanoplankton, were neutrally selective for Cryptophyta/Bacillariophyta, and avoided Cyanobacteria and the total phytoplankton community. Mesozooplankton carbon intake was dominated by autotrophic nanoplankton and Cyanobacteria, indicating these food sources may be important in supporting zooplankton production during these cHAB events, although our understanding of the incorporation of Cyanobacteria carbon into mesozooplankton production is limited. While mesozooplankton grazed a small portion of total phytoplankton (1-12 % d-1) and Cyanobacteria (5-13 % d-1) standing stock biomass (<200 µm), they exerted a relatively stronger control on the growth of total phytoplankton (4-38 % d-1) and Cyanobacteria (64-106 % d-1), primarily due to the low measured growth rates of these food items during the study. Our results demonstrate that mesozooplankton graze on Cyanobacteria during cHAB events, and under the right conditions influence cHAB dynamics through direct consumption of Cyanobacteria and indirect effects on Cyanobacteria grazers and competitors.

Pelagic Sargassum, denoting the two species S. natans and S. fluitans, has become a focal point of both scientific and public interest due to recent superbloom events. As a result, investigation into the dynamics of Sargassum-associated microbiota, which was previously quite limited, has regained momentum in the past five years. This review collates the current state of knowledge relating to the Sargassum microbiome as well as synthesizes relevant biogeochemical findings which may aid future efforts in this field. We explore microbiome composition and functional roles within this holobiont, specifically in terms of taxonomic diversity, macronutrient cycling, and the fate of Sargassum-derived organic matter. Despite progress, significant gaps remain in our understanding of the dynamic interactions within these microbial communities and their broader ecological significance. Future research directions are identified alongside recommendations for standardized protocols, emphasizing the need for more in-depth studies utilizing advanced molecular and biogeochemical techniques to unravel the full complexity of the Sargassum microbiome.