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ABSTRACT The cyanobacterial genusMicrocystisis globally distributed and known for its ability to produce microcystins, a structurally diverse group of cyanotoxins. However, the biosynthetic capacity ofMicrocystisis vast; its diverse genomes contain a variety of biosynthetic gene clusters (BGCs) encoding the synthesis of metabolites that may be toxic, have important ecological function, or have applications for biotechnology or drug discovery. Recent studies illustrate that these BGCs vary significantly acrossMicrocystisstrains, can be highly expressed in environmental conditions, and may play key roles in cellular physiology, grazer deterrence, and microbial interactions. However, many of these BGCs and metabolites remain poorly characterized or completely uncharacterized, having been identified only through genome sequencing or mass spectrometry, respectively, leaving no knowledge of their structure, bioactivity, or physiological or ecological functions. Here, we synthesize the current body of knowledge regarding the secondary metabolism ofMicrocystisin terms of genetic and chemical diversity, potential drivers of synthesis, and physiological and ecological functions. This review highlights the need for further research to characterize the largely unexplored genetic and chemical diversity ofMicrocystisin communities in the environment and discusses the challenges and opportunities of integrating high-throughput multiomic approaches to link uncharacterized gene clusters with their corresponding metabolites.Microcystiswill continue to be a rich source for secondary metabolite research as its genetic and chemical potential likely plays a critical role in the persistence and observed dynamics of harmful algal blooms and may harbor uncharacterized toxins and metabolites. 

ABSTRACT Cyanobacterial harmful algal blooms (cyanoHABs) threaten human, animal, and ecosystem health and safety through production of toxic secondary metabolites.Microcystis,a cosmopolitan bloom‐forming cyanobacterial genus, is well‐known for producing hepatotoxic microcystins (MCs), but it can produce many other bioactive cyanopeptides, such as anabaenopeptins (APs), that occur at high levels in blooms. The toxicological and ecological impacts of such co‐occurring cyanopeptides in the natural environment remain understudied. Here we evaluated the effects of pure MCs and APs individually and in combination, as well as extracts ofMicrocystiscultures producing diverse suites of cyanopeptides, including strains with and without MCs and APs, on human lung (A549), kidney (HK2), and liver (Hep‐3B) cell viability. Individual MC and AP congeners exhibited a gradient of toxic effects across cell lines; MC‐LA caused the most toxic effects, MC‐LR had comparable effects to AP‐A and AP‐B, and MC‐RR caused the least toxicity. Combined exposure to MC‐LA and AP‐B produced dose‐dependent synergistic effects across all three cell lines.Microcystisculture extracts significantly reduced cell viability in dose‐ andMicrocystisstrain‐dependent patterns that could not be explained by microcystin or anabaenopeptin content alone, suggesting a role of other metabolites and their interactions within the mixtures. These findings demonstrate that mixtures of environmentally relevant cyanopeptides can have greater toxic threats than individual compounds and underscore the importance of considering metabolites beyond MCs and their potential interactions in public health, future risk assessments, and management strategies for cyanoHAB‐impacted waters. 

ABSTRACT Although evidence indicates that viruses are important in the ecology ofMicrocystisspp., many questions remain. For example, how doesMicrocystisexist at high, bloom-associated cell concentrations in the presence of viruses that infect it? The phenomenon of lysogeny and associated homoimmunity offer possible explanations for this question. Virtually nothing is known about lysogeny inMicrocystis, but a metatranscriptomic study suggests that widespread, transient lysogeny is active during blooms. These observations lead us to posit that lysogeny is important in modulatingMicrocystisblooms. Using a classic mitomycin C-based induction study, we tested for lysogeny in aMicrocystis-dominated community in Lake Erie in 2019. Treated communities were incubated with 1 mg L⁻¹mitomycin C for 48 h alongside unamended controls. We compared direct counts of virus-like particles (VLPs) and examined community transcription for active infection by cyanophage. Mitomycin C treatment did not increase VLP count. Mitomycin C effectively eliminated transcription in the cyanobacterial community, while we detected no evidence of induction. Metatranscriptomic analysis demonstrated that the standard protocol of 1 mg L⁻¹was highly toxic to the cyanobacterial population, which likely inhibited induction of any prophage present. Follow-up lab studies indicated that 0.1 mg L⁻¹may be more appropriate for use in freshwater cyanobacterial studies. These findings will guide future efforts to detect lysogeny inMicrocystisblooms.IMPORTANCEHarmful algal blooms dominated byMicrocystisspp. occur throughout the world’s freshwater ecosystems, leading to detrimental effects on ecosystem services that are well documented. After decades of research, the scientific community continues to struggle to understand the ecology ofMicrocystisblooms. The phenomenon of lysogeny offers an attractive potential explanation for several ecological questions surrounding blooms. However, almost nothing is known about lysogeny inMicrocystis. We attempted to investigate lysogeny in aMicrocystisbloom in Lake Erie and found that the standard protocols used to study lysogeny in aquatic communities are inappropriate for use inMicrocystisstudies, and perhaps freshwater cyanobacterial studies more broadly. This work can be used to design better methods to study the viral ecology ofMicrocystisblooms. 

ABSTRACT Here, we report on the raw and coassembled metatranscriptomes of 39 Lake Erie surface (1.0 m) water samples collected over a 2-day diel period encompassing episodic weather and bloom events. Preliminary taxonomic annotations and read mappings revealed thatMicrocystisspp. accounted for up to ~47% of the transcriptionally active community. 

ABSTRACT Cyanobacterial blooms pose environmental and health risks due to their production of toxic secondary metabolites. While current methods for assessing these risks have focused primarily on bloom frequency and intensity, the lack of comprehensive and comparable data on cyanotoxins makes it challenging to rigorously evaluate these health risks. In this study, we examined 750 metagenomic data sets collected from 103 lakes worldwide. Our analysis unveiled the diverse distributions of cyanobacterial communities and the genes responsible for cyanotoxin production across the globe. Our approach involved the integration of cyanobacterial biomass, the biosynthetic potential of cyanotoxin, and the potential effects of these toxins to establish potential cyanobacterial health risks. Our findings revealed that nearly half of the lakes assessed posed medium to high health risks associated with cyanobacteria. The regions of greatest concern were East Asia and South Asia, particularly in developing countries experiencing rapid industrialization and urbanization. Using machine learning techniques, we mapped potential cyanobacterial health risks in lakes worldwide. The model results revealed a positive correlation between potential cyanobacterial health risks and factors such as temperature, N₂O emissions, and the human influence index. These findings underscore the influence of these variables on the proliferation of cyanobacterial blooms and associated risks. By introducing a novel quantitative method for monitoring potential cyanobacterial health risks on a global scale, our study contributes to the assessment and management of one of the most pressing threats to both aquatic ecosystems and human health. IMPORTANCEOur research introduces a novel and comprehensive approach to potential cyanobacterial health risk assessment, offering insights into risk from a toxicity perspective. The distinct geographical variations in cyanobacterial communities coupled with the intricate interplay of environmental factors underscore the complexity of managing cyanobacterial blooms at a global scale. Our systematic and targeted cyanobacterial surveillance enables a worldwide assessment of cyanobacteria-based potential health risks, providing an early warning system. 

Abstract Algae, an important foundation of aquatic ecosystems, can become a nuisance or harmful when it grows in excess. Many government agencies have a role in monitoring, responding to, and confirming a harmful algal bloom (HAB). HAB scientists have important information to share, however, given the complexities of HABs, which often involve decoupled drivers from observed impacts, presents challenges to outreach and engagement. Understanding key audience information needs can help scientists prioritize key science communication and engagement opportunities to maximize the impact of such efforts. Scientists may need additional science communication training or support for scientist‐community partnerships. This will be evermore important into the future with the likely range expansion of HABs due to climate change. 

ABSTRACT <p>Winter is a relatively under-studied season in freshwater ecology. The paucity of wintertime surveys has led to a lack of knowledge regarding microbial community activity during the winter in Lake Erie, a North American Great Lake. Viruses shape microbial communities and regulate biogeochemical cycles by acting as top-down controls, yet very few efforts have been made to examine active virus populations during the winter in Lake Erie. Furthermore, climate change-driven declines in seasonal ice cover have been shown to influence microbial community structure, but no studies have compared viral community activity between different ice cover conditions. We surveyed surface water metatranscriptomes for viral hallmark genes as a proxy for active virus populations and compared activity metrics between ice-covered and ice-free conditions from two sampled winters. Transcriptionally active viral communities were detected in both winters, spanning diverse phylogenetic clades of putative bacteriophage (<italic>Caudoviricetes</italic>), giant viruses (<italic>Nucleocytoviricota</italic>, or NCLDV), and RNA viruses (<italic>Orthornavirae</italic>). However, viral community activity metrics revealed pronounced differences between the ice-covered and ice-free winters. Viral community composition was distinct between winters and viral hallmark gene richness was reduced in the ice-covered relative to the ice-free conditions. In addition, the observed differences in viral communities correlated with microbial community activity metrics. Overall, these findings contribute to our understanding of the viral populations that are active during the winter in Lake Erie and suggest that viral community activity may be associated with ice cover extent.</p><sec><title>IMPORTANCEAs seasonal ice cover is projected to become increasingly rare on large temperate lakes, there is a need to understand how microbial communities might respond to changing ice conditions. Although it is widely recognized that viruses impact microbial community structure and function, there is little known regarding wintertime viral activity or the relationship between viral activity and ice cover extent. Our metatranscriptomic analyses indicated that viruses were transcriptionally active in the winter surface waters of Lake Erie. These findings also expanded the known diversity of viral lineages in the Great Lakes. Notably, viral community activity metrics were significantly different between the two sampled winters. The pronounced differences we observed in active viral communities between the ice-covered and ice-free samples merit further research regarding how viral communities will function in future, potentially ice-free, freshwater systems. 

Abstract Global expansion of cyanobacterial harmful algal blooms, driven by anthropogenic nutrient loading and climate change, poses escalating threats to water security. While altered precipitation regimes are recognized as critical climate multipliers, their hierarchical controls on cyanobacterial harmful algal bloom dynamics remain poorly quantified, particularly in stratified reservoirs. Here, we integrate a 60‐yr (1961–2020) trend analysis of rainfall extremes with 4 yr (2017–2020) of high‐frequency monitoring of hydrology, nutrients, and phytoplankton through manual sampling and autonomous buoys in Lake Qiandaohu, a deep subtropical reservoir vital for drinking water in eastern China. Using Mann–Kendall trend analysis, random forest, and partial least squares path modeling, we elucidate the hierarchical mechanisms linking altered rainfall regimes to phytoplankton community shifts, with a focus on cyanobacterial harmful algal blooms. Our results reveal significant increases in both annual rainfall and rainstorm frequency over the past six decades, with peak events coinciding notably with early summer agricultural fertilization. Intensified early summer rainstorms triggered disproportionate phosphorus loading, significantly lowering N/P ratios and transiently suppressing cyanobacterial dominance. However, a critical approximately 20‐d lag period followed these events, wherein rapid thermal stratification recovery, combined with persistent legacy nutrients, selectively favored cyanobacterial proliferation. Furthermore, subsequent continuous dry‐hot spells amplified cyanobacterial dominance by enhancing water column stability and elevating temperatures. These findings reveal a paradoxical post‐storm vulnerability window that facilitates bloom formation in climate‐sensitive reservoirs. Given projected intensification of rainfall extremes under climate change, this study highlights the urgent need for integrated management strategies combining watershed nutrient control, meteorological forecasting, and adaptive reservoir operation to mitigate cyanobacterial harmful algal bloom risks. 

ABSTRACT We report 40 metagenomic libraries collected from the Winam Gulf of Lake Victoria during May–July of 2022–2023 and an additional eight opportunistic libraries from adjacent Lakes Simbi, Naivasha, and regional river systems. The sampling period captured cyanobacterial bloom events – shedding insight onto community composition and genomic potential. 

Abstract While resequencing Microcystis aeruginosa (PCC7806) and its nontoxigenic mutant (PCC7806 ΔmcyB), we discovered identical unreported plasmids in both strains. These strains were separated in culture over 25 years ago, resulting in sequence divergence among their chromosomes. RNA-seq data demonstrated these plasmids were transcriptionally active during chemostat growth. Moreover, in situ metatranscriptomes from Lake Erie revealed genes like those on the PCC7806 plasmid were expressed in the environment. As we investigated plasmids in Microcystis, we found that M. aeruginosa NIES-298 also had a putatively conserved plasmid, but with phage-like features. To gain an understanding of the ecological relevance of these plasmids, we examined Lake Erie metatranscriptomes and found that transcript abundance for predicted plasmid-like contigs was significantly higher than predicted virus-like contigs across the microbial community: this trend was also present when metatranscriptomic reads were mapped to Microcystis-infecting phage and Microcystis-specific plasmid genomes. Our observations demonstrate a potential ecological importance and stability of these extrachromosomal elements in Microcystis. Additionally, this work draws attention to the potential overlap between Microcystis plasmid and phage genomes, and how this may complicate molecular investigations.