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1. Metagenomic and Metatranscriptomic Insights into Population Diversity of Microcystis Blooms: Spatial and Temporal Dynamics of mcy Genotypes, Including a Partial Operon That Can Be Abundant and Expressed

   https://doi.org/10.1128/aem.02464-21  

   Yancey, Colleen E.; Smith, Derek J.; Den Uyl, Paul A.; Mohamed, Osama G.; Yu, Fengan; Ruberg, Steven A.; Chaffin, Justin D.; Goodwin, Kelly D.; Tripathi, Ashootosh; Sherman, David H.; et al (May 2022, Applied and Environmental Microbiology)

   Rudi, Knut (Ed.)

   ABSTRACT Cyanobacterial harmful algal blooms (cyanoHABs) degrade freshwater ecosystems globally. Microcystis aeruginosa often dominates cyanoHABs and produces microcystin (MC), a class of hepatotoxins that poses threats to human and animal health. Microcystin toxicity is influenced by distinct structural elements across a diversity of related molecules encoded by variant mcy operons. However, the composition and distribution of mcy operon variants in natural blooms remain poorly understood. Here, we characterized the variant composition of mcy genes in western Lake Erie Microcystis blooms from 2014 and 2018. Sampling was conducted across several spatial and temporal scales, including different bloom phases within 2014, extensive spatial coverage on the same day (2018), and frequent, autonomous sampling over a 2-week period (2018). Mapping of metagenomic and metatranscriptomic sequences to reference sequences revealed three Microcystis mcy genotypes: complete (all genes present [ mcyA–J ]), partial (truncated mcyA , complete mcyBC , and missing mcyD–J ), and absent (no mcy genes). We also detected two different variants of mcyB that may influence the production of microcystin congeners. The relative abundance of these genotypes was correlated with pH and nitrate concentrations. Metatranscriptomic analysis revealed that partial operons were, at times, the most abundant genotype and expressed in situ , suggesting the potential biosynthesis of truncated products. Quantification of genetic divergence between genotypes suggests that the observed strains are the result of preexisting heterogeneity rather than de novo mutation during the sampling period. Overall, our results show that natural Microcystis populations contain several cooccurring mcy genotypes that dynamically shift in abundance spatiotemporally via strain succession and likely influence the observed diversity of the produced congeners. IMPORTANCE Cyanobacteria are responsible for producing microcystins (MCs), a class of potent and structurally diverse toxins, in freshwater systems around the world. While microcystins have been studied for over 50 years, the diversity of their chemical forms and how this variation is encoded at the genetic level remain poorly understood, especially within natural populations of cyanobacterial harmful algal blooms (cyanoHABs). Here, we leverage community DNA and RNA sequences to track shifts in mcy genes responsible for producing microcystin, uncovering the relative abundance, expression, and variation of these genes. We studied this phenomenon in western Lake Erie, which suffers annually from cyanoHAB events, with impacts on drinking water, recreation, tourism, and commercial fishing. 

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2. Bacterial community and cyanotoxin gene distribution of the Winam Gulf, Lake Victoria, Kenya

   https://doi.org/10.1111/1758-2229.13297  

   Brown, Katelyn_M; Barker, Katelyn_B; Wagner, Ryan_S; Ward, Christopher_S; Sitoki, Lewis; Njiru, James; Omondi, Reuben; Achiya, James; Getabu, Albert; McKay, R_Michael; et al (June 2024, Environmental Microbiology Reports)

   Abstract The Winam Gulf (Kenya) is frequently impaired by cyanobacterial harmful algal blooms (cHABs) due to inadequate wastewater treatment and excess agricultural nutrient input. While phytoplankton in Lake Victoria have been characterized using morphological criteria, our aim is to identify potential toxin‐producing cyanobacteria using molecular approaches. The Gulf was sampled over two successive summer seasons, and 16S and 18S ribosomal RNA gene sequencing was performed. Additionally, key genes involved in production of cyanotoxins were examined by quantitative PCR. Bacterial communities were spatially variable, forming distinct clusters in line with regions of the Gulf. Taxa associated with diazotrophy were dominant near Homa Bay. On the eastern side, samples exhibited elevatedcyrAabundances, indicating genetic capability of cylindrospermopsin synthesis. Indeed, near the Nyando River mouth in 2022,cyrAexceeded 10 million copies L⁻¹where there were more than 6000Cylindrospermopsisspp. cells mL⁻¹. In contrast, the southwestern region had elevatedmcyEgene (microcystin synthesis) detections near Homa Bay whereMicrocystisandDolichospermumspp. were observed. These findings show that within a relatively small embayment, composition and toxin synthesis potential of cHABs can vary dramatically. This underscores the need for multifaceted management approaches and frequent cyanotoxin monitoring to reduce human health impacts. 

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3. Roles of Nutrient Limitation on Western Lake Erie CyanoHAB Toxin Production

   https://doi.org/10.3390/toxins13010047  

   Barnard, Malcolm A.; Chaffin, Justin D.; Plaas, Haley E.; Boyer, Gregory L.; Wei, Bofan; Wilhelm, Steven W.; Rossignol, Karen L.; Braddy, Jeremy S.; Bullerjahn, George S.; Bridgeman, Thomas B.; et al (January 2021, Toxins)

   null (Ed.)

   Cyanobacterial harmful algal bloom (CyanoHAB) proliferation is a global problem impacting ecosystem and human health. Western Lake Erie (WLE) typically endures two highly toxic CyanoHABs during summer: a Microcystis spp. bloom in Maumee Bay that extends throughout the western basin, and a Planktothrix spp. bloom in Sandusky Bay. Recently, the USA and Canada agreed to a 40% phosphorus (P) load reduction to lessen the severity of the WLE blooms. To investigate phosphorus and nitrogen (N) limitation of biomass and toxin production in WLE CyanoHABs, we conducted in situ nutrient addition and 40% dilution microcosm bioassays in June and August 2019. During the June Sandusky Bay bloom, biomass production as well as hepatotoxic microcystin and neurotoxic anatoxin production were N and P co-limited with microcystin production becoming nutrient deplete under 40% dilution. During August, the Maumee Bay bloom produced microcystin under nutrient repletion with slight induced P limitation under 40% dilution, and the Sandusky Bay bloom produced anatoxin under N limitation in both dilution treatments. The results demonstrate the importance of nutrient limitation effects on microcystin and anatoxin production. To properly combat cyanotoxin and cyanobacterial biomass production in WLE, both N and P reduction efforts should be implemented in its watershed. 

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4. Multi-year molecular quantification and ‘omics analysis of Planktothrix-specific cyanophage sequences from Sandusky Bay, Lake Erie

   https://doi.org/10.3389/fmicb.2023.1199641  

   McKindles, Katelyn M.; Manes, Makayla; Neudeck, Michelle; McKay, Robert Michael; Bullerjahn, George S. (June 2023, Frontiers in Microbiology)

   Introduction Planktothrix agardhii is a microcystin-producing cyanobacterium found in Sandusky Bay, a shallow and turbid embayment of Lake Erie. Previous work in other systems has indicated that cyanophages are an important natural control factor of harmful algal blooms. Currently, there are few cyanophages that are known to infect P. agardhii , with the best-known being PaV-LD, a tail-less cyanophage isolated from Lake Donghu, China. Presented here is a molecular characterization of Planktothrix specific cyanophages in Sandusky Bay. Methods and Results Putative Planktothrix -specific viral sequences from metagenomic data from the bay in 2013, 2018, and 2019 were identified by two approaches: homology to known phage PaV-LD, or through matching CRISPR spacer sequences with Planktothrix host genomes. Several contigs were identified as having viral signatures, either related to PaV-LD or potentially novel sequences. Transcriptomic data from 2015, 2018, and 2019 were also employed for the further identification of cyanophages, as well as gene expression of select viral sequences. Finally, viral quantification was tested using qPCR in 2015–2019 for PaV-LD like cyanophages to identify the relationship between presence and gene expression of these cyanophages. Notably, while PaV-LD like cyanophages were in high abundance over the course of multiple years (qPCR), transcriptomic analysis revealed only low levels of viral gene expression. Discussion This work aims to provide a broader understanding of Planktothrix cyanophage diversity with the goals of teasing apart the role of cyanophages in the control and regulation of harmful algal blooms and designing monitoring methodology for potential toxin-releasing lysis events. 

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5. Influence of light, temperature, and nutrients on microcystin concentration during a winter cyanobacteria-dominated bloom

   https://doi.org/10.1093/plankt/fbaf061  

   Andersen, Isabelle M; Rusche, Katherine C; Voyles, Maggie E; Bros, Alexandra J; Knoll, Lesley B (October 2025, Journal of Plankton Research)

   Beisner, Beatrix E (Ed.)

   Abstract Freshwater ecosystems are increasingly at risk of experiencing toxin-producing cyanobacterial blooms during the winter due to anthropogenic nutrient loading and climate change. However, understanding how increased light, temperature and nutrient levels impact cyanotoxin production during the winter is limited, as most research has historically focused on blooms during the summer and fall. We conducted 2 × 2 × 2 incubation experiments in February and March to test the individual and interactive effects of light intensity (50 and 150 μmol m−2 s−1 PAR), elevated temperature (+3°C), and nitrogen and phosphorus enrichment on microcystin concentrations in a Planktothrix agardhii-dominated community sampled from Grand Lake Saint Mary’s, a hypereutrophic Ohio reservoir. Microcystin concentration significantly increased with elevated temperature in both months. In February, low light also promoted higher microcystin concentrations, particularly when combined with elevated temperature and nutrient enrichment. In March, nutrient enrichment had individual and interactive effects with temperature that caused higher microcystin concentrations. These results demonstrate that toxin-producing cyanobacteria are active in winter and that climate-driven changes in environmental conditions can interactively increase total toxin concentrations in the water column, even in the non-growing season. 

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