There have been sporadic reports of aquatic, benthic Microcoleus proliferations in freshwater rivers, lakes, and reservoirs for four decades, with reports increasing in frequency over the last twenty years, suggesting a possible rise in their global distribution, frequency, and intensity. Microcoleus can produce anatoxins which are neurotoxic, and ingestion of toxic mats has caused hundreds of dog fatalities and raised serious human and ecological health concerns. This review synthesizes and evaluates current knowledge on Microcoleus distribution, taxonomy, toxin production, toxicity, ecology, environmental drivers, and biotic interactions. Toxin-producing Microcoleus have been reported in at least 18 countries, though many regions have not conducted toxin testing, suggesting a broader but under-reported distribution. Proliferations occur across diverse habitats, including cobble-bedded streams, large sandy rivers, reservoirs, and lakes. Microcoleus proliferations also occur on macrophytes, both in lakes and rivers. Genomic analyses currently classify anatoxin-producing Microcoleus into distinct species, with all known anatoxin-producers isolated from freshwater ecosystems. Anatoxin concentrations vary widely over space and time, within and among waterbodies. While studies on environmental drivers remain limited, research in cobble-bedded rivers suggests that moderate enrichment of dissolved inorganic nitrogen and low dissolved reactive phosphorus concentrations in the water column promote proliferation. Metagenomic approaches have revealed unique nutrient acquisition and storage strategies used by Microcoleus. Key knowledge gaps remain around the environmental and ecological triggers of proliferation, toxin production, genomic diversity and microbial interactions. Addressing these gaps through coordinated, global studies using robust datasets and consistent methods is critical to improve prediction, monitoring, and mitigation of this increasingly widespread public and ecological health threat.

Microcoleus is a cosmopolitan, filamentous cyanobacterium and a key component of biological soil crusts-complex microbial communities essential for primary production in diverse terrestrial environments. Here, we performed a taxonomic revision of several species of Microcoleus based on a large population genomic dataset. The dataset was based on a Microcoleus speciation continuum characterized by variable levels of gene flow between the species. The putative species ranged from cryptic to distinctly morphologically defined lineages. We identify the type herbarium specimen and obtained a genome for the type species M. vaginatus and herein describe 10 novel species of Microcoleus. We provide epitypifications for the previously described species M. vaginatus and M. attenuatus. This research contributes to a more comprehensive understanding of terrestrial cyanobacterial biodiversity and cryptic species in cyanobacteria. It highlights the need for an extensive genomic and phenotypic dataset in the taxonomy of Cyanobacteria.

Freshwater ecosystems are increasingly impacted by anthropogenic pollutants, including the widely used herbicide metolachlor (MET) and cigarette butt (CB) litter. Parasites represent an additional biotic stressor that can modulate pollutant effects on their hosts. Cyanobacteria are key primary producers in aquatic ecosystems, and alterations in their metabolism can cascade through trophic webs, affecting ecosystem functioning. However, the metabolic responses of cyanobacteria to simultaneous exposure to pollutants and parasites remain poorly understood. In this study, we investigated the metabolic response of the toxigenic cyanobacterium Planktothrix agardhii using a multi-biomarker approach under simultaneous exposure to abiotic (MET and CB leachate) and biotic (the chytrid parasite Rhizophydium megarrhizum) stressors. Co-exposure to MET and chytrids led to a synergistic effect, inducing oxidative stress, primarily countered by non-enzymatic mechanisms, whereas MET alone had no measurable effect. In contrast, CB leachate alone induced oxidative stress, but this effect was mitigated when cyanobacteria were also infected by chytrids, indicating an antagonistic interaction. This study demonstrates the complexity of cyanobacterial metabolic responses to interacting biotic and abiotic stressors and highlights the importance of incorporating host-parasite interactions in ecotoxicological assessments. Metabolic alterations induced in toxigenic cyanobacteria by multiple stressors may influence bloom formation and trophic energy transfer, with broader ecological implications in freshwater ecosystems.

Only recently has the cyanotoxin dihydroanatoxin-a (dhATX-a) been detected more frequently in different surface waters, some of which are used for supplying drinking water. As data about the fate of dhATX-a in drinking water treatment processes are still scarce, the present study investigated the behavior of dhATX-a in different water treatment steps: slow sand filtration, flocculation, adsorption onto activated carbon, ozonation and chlorination. The almost complete removal (>95%) of dhATX-a was observed in sand columns simulating slow sand filtration without showing a long adaptation phase. The results further indicate that dhATX-a can be removed using powdered activated carbon at dosages of 50 mg/L with removal rates between 75 and 93% and also by using ozonation with dosages above 1 mg/L at a concentration of ca. 4.5 mg/L background organic carbon. In contrast, no elimination of dhATX-a was observed in flocculation and chlorination experiments.

Freshwater communities are increasingly impacted by anthropogenic pollutants, such as the widely used herbicide metolachlor (MET) and chemicals leached from cigarette butts (CBs), one of the most common types of litter worldwide. Cyanobacteria often interact with their parasitic chytrid fungi, which play a role in controlling their growth. Parasites represent an additional biotic stressor that can alter the effects of pollutants on their hosts. However, the metabolic responses of cyanobacteria to simultaneous exposure to pollutants and parasites remain poorly understood. In this study, we investigated the combined effects of abiotic (MET and CB leachate) and biotic (chytrid parasite Rhizophydium megarrhizum ) stressors on the metabolic response of the toxigenic cyanobacterium Planktothrix agardhii . Co-exposure to MET and chytrids led to a synergistic effect, inducing oxidative stress, countered primarily by non-enzymatic mechanisms, whereas MET alone had no measurable effect. In contrast, CB leachate alone induced oxidative stress, but this effect was mitigated when cyanobacteria were also infected by chytrids, indicating an antagonistic interaction. This study demonstrates the complexity of cyanobacterial responses to interacting biotic and abiotic stressors and highlights the importance of considering host-parasite interactions in ecotoxicological assessments of cyanobacteria. A deeper understanding of how pollutants such as MET and CB leachate affect cyanobacteria-chytrid interactions at the metabolic level is crucial for evaluating their broader ecological impacts. ### Competing Interest Statement The authors have declared no competing interest. Deutsche Forschungsgemeinschaft, grant MA 9934/1-1

Cyanobacteria are favored by climate change and global warming; however, to date, most research and monitoring programs have focused on planktic cyanobacteria. Benthic cyanobacteria blooms also increase and pose a risk to animal and human health; however, there is limited knowledge of their occurrence, distribution and the toxins involved, especially in relation to their planktic conspecifics. Therefore, we analyzed the benthic and planktic life forms of cyanobacterial communities in 34 lakes in Germany, including a monitoring of cyanotoxins. Community analyses were based on microscopic examination and Illumina sequencing of the 16S rRNA gene. The analyses of cyanotoxins were carried out using LC-MS/MS and ELISA. Observed benthic mats containing cyanobacteria consisted mainly of Nostocales and Oscillatoriales, being present in 35% of the lakes. Anatoxin was the most abundant cyanotoxin in the benthic samples, reaching maximum concentrations of 45,000 µg/L, whereas microcystin was the predominate cyanotoxin in the open-water samples, reaching concentrations of up to 18,000 µg/L. Based on the results, specific lakes at risk of toxic cyanobacteria could be identified. Our findings suggest that monitoring of benthic cyanobacteria and their toxins should receive greater attention, ideally complementing existing open-water sampling programs with little additional effort.

Hellweger et al. (Reports, 27 May 2022, pp. 1001) predict that phosphorus limitation will increase concentrations of cyanobacterial toxins in lakes. However, several molecular, physiological, and ecological mechanisms assumed in their models are poorly supported or contradicted by other studies. We conclude that their take-home message that phosphorus load reduction will make Lake Erie more toxic is seriously flawed.

Chytrid parasites are increasingly recognized as ubiquitous and potent control agents of phytoplankton, including bloom-forming toxigenic cyanobacteria. In order to explore the fate of the cyanobacterial toxin microcystins (MCs) and assess potential upregulation of their production under parasite attack, a laboratory experiment was conducted to evaluate short- and long-term variation in extracellular and intracellular MC in the cyanobacteria Planktothrix agardhii and P. rubescens, both under chytrid infection and in the presence of lysates of previously infected cyanobacteria. MCs release under parasite infection was limited and not different to uninfected cyanobacteria, with extracellular toxin shares never exceeding 10%, substantially below those caused by mechanical lysis induced by a cold-shock. Intracellular MC contents in P. rubescens under infection were not significantly different from uninfected controls, whereas infected P. agardhii showed a 1.5-fold increase in intracellular MC concentrations, but this was detected within the first 48 hours after parasite inoculation and not later, indicating no substantial MC upregulation in cells being infected. The presence of lysates of previously infected cyanobacteria did not elicit higher intracellular MC contents in exposed cyanobacteria, speaking against a putative upregulation of toxin production induced via quorum sensing in response to parasite attack. These results indicate that chytrid epidemics can constitute a bloom decay mechanism that is not accompanied by massive release of toxins into the medium.

Fatal dog poisoning after uptake of neurotoxic cyanobacteria associated with aquatic macrophytes in Tegeler See (Berlin, Germany) raised concerns about critical exposure of humans, especially children, to cyanotoxins produced by macrophyte associated cyanobacteria during recreational activity. From 2017 to 2021 a total of 398 samples of macrophytes washed ashore at bathing sites located at 19 Berlin lakes were analysed for anatoxins, microcystins, and cylindrospermopsins, as were 463 water samples taken in direct proximity to macrophyte accumulations. Cyanotoxins were detected in 66 % of macrophyte samples and 50 % of water samples, with anatoxins being the most frequently detected toxin group in macrophyte samples (58 %) and cylindrospermopsins in water samples (41 %). Microcoleus sp. associated with the water moss Fontinalis antipyretica was identified as anatoxin producing cyanobacterium in isolated strains as well as in field samples from Tegeler See. Anatoxin contents in macrophyte samples rarely exceeded 1 μg/g macrophyte fresh weight and peaked at 9. 2 μg/g f.w. Based on established toxicological points of departure, a critical anatoxin content of macrophyte samples of 3 μg/g f.w. is proposed. Five samples, all taken in Tegeler See and all associated with the water moss Fontinalis antipyretica, exceeded this value. Contents and concentrations of microcystins and cylindrospermopsins did not reach critical levels. The potential exposure risks to anatoxins for children and dogs are assessed and recommendations are given.

Concern is widely being published that the occurrence of toxic cyanobacteria is increasing in consequence of climate change and eutrophication, substantially threatening human health. Here, we review evidence and pertinent publications to explore in which types of waterbodies climate change is likely to exacerbate cyanobacterial blooms; whether controlling blooms and toxin concentrations requires a balanced approach of reducing not only the concentrations of phosphorus (P) but also those of nitrogen (N); how trophic and climatic changes affect health risks caused by toxic cyanobacteria. We propose the following for further discussion: (i) Climate change is likely to promote blooms in some waterbodies-not in those with low concentrations of P or N stringently limiting biomass, and more so in shallow than in stratified waterbodies. Particularly in the latter, it can work both ways-rendering conditions for cyanobacterial proliferation more favourable or less favourable. (ii) While N emissions to the environment need to be reduced for a number of reasons, controlling blooms can definitely be successful by reducing only P, provided concentrations of P can be brought down to levels sufficiently low to stringently limit biomass. Not the N:P ratio, but the absolute concentration of the limiting nutrient determines the maximum possible biomass of phytoplankton and thus of cyanobacteria. The absolute concentrations of N or P show which of the two nutrients is currently limiting biomass. N can be the nutrient of choice to reduce if achieving sufficiently low concentrations has chances of success. (iii) Where trophic and climate change cause longer, stronger and more frequent blooms, they increase risks of exposure, and health risks depend on the amount by which concentrations exceed those of current WHO cyanotoxin guideline values for the respective exposure situation. Where trophic change reduces phytoplankton biomass in the epilimnion, thus increasing transparency, cyanobacterial species composition may shift to those that reside on benthic surfaces or in the metalimnion, changing risks of exposure. We conclude that studying how environmental changes affect the genotype composition of cyanobacterial populations is a relatively new and exciting research field, holding promises for understanding the biological function of the wide range of metabolites found in cyanobacteria, of which only a small fraction is toxic to humans. Overall, management needs case-by-case assessments focusing on the impacts of environmental change on the respective waterbody, rather than generalisations.

Raphidiopsis raciborskii is a freshwater, potentially toxigenic cyanobacterium, originally described as a tropical species that is spreading to northern regions over several decades. The ability of R. raciborskii to produce cyanotoxins - in particular the alkaloid cylindrospermopsin (CYN), which is toxic to humans and animals - is of serious concern. The first appearance of R. raciborskii in Russia was noted in Lake Nero in the summer of 2010. This is the northernmost (57 degrees N) recorded case of the simultaneous presence of R. raciborskii and detection of CYN. In this study, the data from long-term monitoring of the R. raciborskii population, temperature and light conditions in Lake Nero were explored. CYN and cyr/aoa genes present in environmental samples were examined using HPLC/MS-MS and PCR analysis. A R. raciborskii strain (R104) was isolated and its morphology, toxigenicity and phylogeography were studied. It is supposed that the trigger factor for the strong development of R. raciborskii in Lake Nero in summer 2010 may have been the relatively high water temperature, reaching 29-30 degrees C. Strain R. raciborskii R104 has straight trichomes and can produce akinetes, making it morphologically similar to European strains. Phylogeographic analysis based on nifH gene and 16S-23S rRNA ITS1 sequences showed that the Russian strain R104 grouped together with R. raciborskii strains isolated from Portugal, France, Germany and Hungary. The Russian strain R104 does not contain cyrA and cyrB genes, meaning that it - like all European strains - cannot produce CYN. Thus, while recent invasion of R. raciborskii into Lake Nero has occurred, morphological, genetic, and toxicological data supported the spreading of this cyanobacterium from other European lakes. Detection of CYN and cyr/aoa genes in environmental samples indicated the cyanobacterium Aphanizomenon gracile as a likely producer of CYN in Lake Nero. The article also discusses data on the global biogeography of R. raciborskii. Genetic similarity between R. raciborskii strains isolated from very remote continents might be related to the ancient origin of the cyanobacterium inhabiting the united continents of Laurasia and Gondwana, rather than comparably recent transoceanic exchange between R. raciborskii populations.

Lake Tegel is an extreme case of restoration: inflow treatment reduced its main external phosphorus (TP) load 40-fold, sharply focused in time, and low-P water flushed the lake volume ≈ 4 times per year. We analysed 35 years of data for the time TP concentrations took to decline from ≈ 700 to 20–30 µg/l, biota to respond and cyanobacteria to become negligible. The internal load proved of minor relevance. After 10 years, TP reached 35–40 µg/l, phytoplankton biomass abruptly declined by 50% and cyanobacteria no longer dominated; yet 10 years later at TP < 20–30 µg/l they were below quantifiable levels. 20–25 years after load reduction, the lake was stably mesotrophic, macrophytes had returned down to 6–8 m, and vivianite now forms, binding P insolubly in the sediment. Bottom-up control of phytoplankton through TP proved decisive. Five intermittent years with a higher external P load caused some ‘re-eutrophication’, delaying recovery by 5 years. While some restoration responses required undercutting thresholds, particularly that of phytoplankton biomass to TP, resilience and hysteresis proved irrelevant. Future research needs to focus on the littoral zone, and for predicting time spans for recovery more generally, meta-analyses should address P load reduction in combination with flushing rates.

In August 2019, three dogs died after bathing in or drinking from Mandichosee, a mesotrophic reservoir of the River Lech (Germany). The dogs showed symptoms of neurotoxic poisoning and intoxication with cyanotoxins was considered. Surface blooms were not visible at the time of the incidents. Benthic Tychonema sp., a potential anatoxin-a (ATX)-producing cyanobacterium, was detected in mats growing on the banks, as biofilm on macrophytes and later as aggregations floating on the lake surface. The dogs’ pathological examinations showed lung and liver lesions. ATX and dihydroanatoxin-a (dhATX) were detected by LC-MS/MS in the stomachs of two dogs and reached concentrations of 563 and 1207 µg/L, respectively. Anatoxins (sum of ATX and dhATX, ATXs) concentrations in field samples from Mandichosee ranged from 0.1 µg/L in the open water to 68,000 µg/L in samples containing a large amount of mat material. Other (neuro)toxic substances were not found. A molecular approach was used to detect toxin genes by PCR and to reveal the cyanobacterial community composition by sequencing. Upstream of Mandichosee, random samples were taken from other Lech reservoirs, uncovering Tychonema and ATXs at several sampling sites. Similar recent findings emphasize the importance of focusing on the investigation of benthic toxic cyanobacteria and applying appropriate monitoring strategies in the future.

The cyanotoxin cylindrospermopsin was discovered during a drinking water-related outbreak of human poisoning in 1979. Knowledge about the degradation of cylindrospermopsin in waterbodies is limited. So far, only few cylindrospermopsin-removing bacteria have been described. Manganese-oxidizing bacteria remove a variety of organic compounds. However, this has not been assessed for cyanotoxins yet. We investigated cylindrospermopsin removal by manganese-oxidizing bacteria, isolated from natural and technical systems. Cylindrospermopsin removal was evaluated under different conditions. We analysed the correlation between the amount of oxidized manganese and the cylindrospermopsin removal, as well as the removal of cylindrospermopsin by sterile biogenic oxides. Removal rates in the range of 0.4-37.0 μg L-1 day-1 were observed. When MnCO3 was in the media Pseudomonas sp. OF001 removed ∼100% of cylindrospermopsin in 3 days, Comamonadaceae bacterium A210 removed ∼100% within 14 days, and Ideonella sp. A288 and A226 removed 65% and 80% within 28 days, respectively. In the absence of Mn2+, strain A288 did not remove cylindrospermopsin, while the other strains removed 5-16%. The amount of manganese oxidized by the strains during the experiment did not correlate with the amount of cylindrospermopsin removed. However, the mere oxidation of Mn2+ was indispensable for cylindrospermopsin removal. Cylindrospermopsin removal ranging from 0 to 24% by sterile biogenic oxides was observed. Considering the efficient removal of cylindrospermopsin by the tested strains, manganese-oxidizing bacteria might play an important role in cylindrospermopsin removal in the environment. Besides, manganese-oxidizing bacteria could be promising candidates for biotechnological applications for cylindrospermopsin removal in water treatment plants.

Insight into how environmental change determines the production and distribution of cyanobacterial toxins is necessary for risk assessment. Management guidelines currently focus on hepatotoxins (microcystins). Increasing attention is given to other classes, such as neurotoxins (e.g., anatoxin-a) and cytotoxins (e.g., cylindrospermopsin) due to their potency. Most studies examine the relationship between individual toxin variants and environmental factors, such as nutrients, temperature and light. In summer 2015, we collected samples across Europe to investigate the effect of nutrient and temperature gradients on the variability of toxin production at a continental scale. Direct and indirect effects of temperature were the main drivers of the spatial distribution in the toxins produced by the cyanobacterial community, the toxin concentrations and toxin quota. Generalized linear models showed that a Toxin Diversity Index (TDI) increased with latitude, while it decreased with water stability. Increases in TDI were explained through a significant increase in toxin variants such as MC-YR, anatoxin and cylindrospermopsin, accompanied by a decreasing presence of MC-LR. While global warming continues, the direct and indirect effects of increased lake temperatures will drive changes in the distribution of cyanobacterial toxins in Europe, potentially promoting selection of a few highly toxic species or strains.

Cyanobacteria are known to produce a wide array of metabolites, including various classes of toxins. Among these, hepatotoxins (Microcystins), neurotoxins (Anatoxin-A and PSP toxins) or cytotoxins (Cylindrospermopsins) have been subjected to numerous, individual studies during the past twenty years. Reports of toxins co-occurrences, however, remain scarce in the literature. The present work is an inventory of cyanobacteria with a particular focus on Nostocales and their associated toxin classes from 2007 to 2010 in ten lakes used for drinking water production in France. The results show that potential multiple toxin producing species are commonly encountered in cyanobacteria populations. Individual toxin classes were detected in 75% of all samples. Toxin co-occurrences appeared in 40% of samples as two- or three-toxin combinations (with 35% for the microcystins–anatoxin combination), whereas four-toxin class combinations only appeared in 1% of samples. Toxin co-occurrences could be partially correlated to species composition and water temperature. Peak concentrations however could never be observed simultaneously and followed distinct, asymmetrical distribution patterns. As observations are the key for preventive management and risk assessment, these results indicate that water monitoring should search for all four toxin classes simultaneously instead of focusing on the most frequent toxins, i.e., microcystins.

Under ongoing climate change and increasing anthropogenic activity, which continuously challenge ecosystem resilience, an in-depth understanding of ecological processes is urgently needed. Lakes, as providers of numerous ecosystem services, face multiple stressors that threaten their functioning. Harmful cyanobacterial blooms are a persistent problem resulting from nutrient pollution and climate-change induced stressors, like poor transparency, increased water temperature and enhanced stratification. Consistency in data collection and analysis methods is necessary to achieve fully comparable datasets and for statistical validity, avoiding issues linked to disparate data sources. The European Multi Lake Survey (EMLS) in summer 2015 was an initiative among scientists from 27 countries to collect and analyse lake physical, chemical and biological variables in a fully standardized manner. This database includes in-situ lake variables along with nutrient, pigment and cyanotoxin data of 369 lakes in Europe, which were centrally analysed in dedicated laboratories. Publishing the EMLS methods and dataset might inspire similar initiatives to study across large geographic areas that will contribute to better understanding lake responses in a changing environment.

In May 2017, at least 12 dogs showed signs of acute neurotoxicosis after swimming in or drinking from Lake Tegel, a mesotrophic lake in Berlin, Germany, and several of the affected dogs died shortly afterwards despite intensive veterinary treatment. Cyanobacterial blooms were not visible at the water surface or the shorelines. However, detached and floating water moss (Fontinalis antipyretica) with high amounts of Tychonema sp., a potential anatoxin-a (ATX) producing cyanobacterium, was found near the beaches where the dogs had been swimming and playing. Necropsies of two of the dogs revealed no specific lesions beside the anamnestic neurotoxicosis. ATX was detected in concentrations up to 8700 µg L−1 in the stomach contents, while other (neuro)toxic substances were not found. In the aqueous fraction of Fontinalis/Tychonema clumps sampled after the casualties, ATX was found in concentrations up to 1870 µg L−1. This is the first report of a dense population of Tychonema sp. in stands of Fontinalis resulting in high ATX contents. This case emphasizes the need for further investigation of potentially toxic, non-bloom forming cyanobacteria in less eutrophic water bodies and underlines the novel challenge of developing appropriate surveillance schemes for respective bathing sites.