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  1. Summary

    Interactions between bacteria and phytoplankton in the phycosphere have impacts at the scale of whole ecosystems, including the development of harmful algal blooms. The cyanobacteriumMicrocystiscauses toxic blooms that threaten freshwater ecosystems and human health globally.Microcystisgrows in colonies that harbour dense assemblages of other bacteria, yet the taxonomic composition of these phycosphere communities and the nature of their interactions withMicrocystisare not well characterized. To identify the taxa and compositional variance withinMicrocystisphycosphere communities, we performed 16S rRNA V4 region amplicon sequencing on individualMicrocystiscolonies collected biweekly via high‐throughput droplet encapsulation during a western Lake Erie cyanobacterial bloom. TheMicrocystisphycosphere communities were distinct from microbial communities in whole water and bulk phytoplankton seston in western Lake Erie but lacked ‘core’ taxa found across all colonies. However, dissimilarity in phycosphere community composition correlated with sampling date and theMicrocystis16S rRNA oligotype. Several taxa in the phycosphere were specific to and conserved withMicrocystisof a single oligotype or sampling date. Together, this suggests that physiological differences betweenMicrocystisstrains, temporal changes in strain phenotypes, and the composition of seeding communities may impact community composition of theMicrocystisphycosphere.

     
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  2. Summary

    Microcystisis a cyanobacterium that forms toxic blooms in freshwater ecosystems around the world. Biological variation among taxa within the genus is apparent through genetic and phenotypic differences between strains and via the spatial and temporal distribution of strains in the environment, and this fine‐scale diversity exerts strong influence over bloom toxicity. Yet we do not know how varying traits ofMicrocystisstrains govern their environmental distribution, the tradeoffs and links between these traits, or how they are encoded at the genomic level. Here we synthesize current knowledge on the importance of diversity withinMicrocystisand on the genes and traits that likely underpin ecological differentiation of taxa. We briefly review spatial and environmental patterns ofMicrocystisdiversity in the field and genetic evidence for cohesive groups withinMicrocystis. We then compile data on strain‐level diversity regarding growth responses to environmental conditions and explore evidence for variation of community interactions acrossMicrocystisstrains. Potential links and tradeoffs between traits are identified and discussed. The resulting picture, while incomplete, highlights key knowledge gaps that need to be filled to enable new models for predicting strain‐level dynamics, which influence the development, toxicity and cosmopolitan nature ofMicrocystisblooms.

     
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  3. In Lake Erie, toxin-forming harmful algal blooms (HABs) occur following high concentrations of hydrogen peroxide (H 2 O 2 ). Correlation between H 2 O 2 concentrations and HABs revealed knowledge gaps on the controls of H 2 O 2 production in Lake Erie. One way H 2 O 2 is produced is upon absorption of sunlight by the chromophoric fraction of dissolved organic matter (CDOM). Rates of this photochemical production of H 2 O 2 may increase in proportion to the apparent quantum yield of H 2 O 2 ( Φ H 2 O 2 ,λ ) from CDOM. However, the Φ H 2 O 2 ,λ for H 2 O 2 production from CDOM remains too poorly constrained to predict the magnitude and range of photochemically produced H 2 O 2 , particularly in freshwaters like Lake Erie. To address this knowledge gap, the Φ H 2 O 2 ,λ was measured approximately biweekly from June–September 2019 in the western basin of Lake Erie along with supporting analyses ( e.g. , CDOM concentration and composition). The average Φ H 2 O 2 ,λ in Lake Erie was within previously reported ranges. However, the Φ H 2 O 2 ,λ varied 5-fold in space and time. The highest Φ H 2 O 2 ,λ was observed in the Maumee River, a tributary of Lake Erie. In nearshore waters of Lake Erie, the Φ H 2 O 2 ,λ decreased about five-fold from June through September. Integration of the controls of photochemical production of H 2 O 2 in Lake Erie show that the variability in rates of photochemical H 2 O 2 production was predominantly due to the Φ H 2 O 2 ,λ . In offshore waters, CDOM concentration also strongly influenced photochemical H 2 O 2 production. Together, the results confirm prior work suggesting that photochemical production of H 2 O 2 contributes but likely cannot account for all the H 2 O 2 associated with HABs in Lake Erie. 
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  4. Glass, Jennifer B. (Ed.)
    ABSTRACT Interactions between bacteria and phytoplankton can influence primary production, community composition, and algal bloom development. However, these interactions are poorly described for many consortia, particularly for freshwater bloom-forming cyanobacteria. Here, we assessed the gene content and expression of two uncultivated Acidobacteria from Lake Erie Microcystis blooms. These organisms were targeted because they were previously identified as important catalase producers in Microcystis blooms, suggesting that they protect Microcystis from H 2 O 2 . Metatranscriptomics revealed that both Acidobacteria transcribed genes for uptake of organic compounds that are known cyanobacterial products and exudates, including lactate, glycolate, amino acids, peptides, and cobalamins. Expressed genes for amino acid metabolism and peptide transport and degradation suggest that use of amino acids and peptides by Acidobacteria may regenerate nitrogen for cyanobacteria and other organisms. The Acidobacteria genomes lacked genes for biosynthesis of cobalamins but expressed genes for its transport and remodeling. This indicates that the Acidobacteria obtained cobalamins externally, potentially from Microcystis , which has a complete gene repertoire for pseudocobalamin biosynthesis; expressed them in field samples; and produced pseudocobalamin in axenic culture. Both Acidobacteria were detected in Microcystis blooms worldwide. Together, the data support the hypotheses that uncultured and previously unidentified Acidobacteria taxa exchange metabolites with phytoplankton during harmful cyanobacterial blooms and influence nitrogen available to phytoplankton. Thus, novel Acidobacteria may play a role in cyanobacterial physiology and bloom development. IMPORTANCE Interactions between heterotrophic bacteria and phytoplankton influence competition and successions between phytoplankton taxa, thereby influencing ecosystem-wide processes such as carbon cycling and algal bloom development. The cyanobacterium Microcystis forms harmful blooms in freshwaters worldwide and grows in buoyant colonies that harbor other bacteria in their phycospheres. Bacteria in the phycosphere and in the surrounding community likely influence Microcystis physiology and ecology and thus the development of freshwater harmful cyanobacterial blooms. However, the impacts and mechanisms of interaction between bacteria and Microcystis are not fully understood. This study explores the mechanisms of interaction between Microcystis and uncultured members of its phycosphere in situ with population genome resolution to investigate the cooccurrence of Microcystis and freshwater Acidobacteria in blooms worldwide. 
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  5. Nojiri, Hideaki (Ed.)
    ABSTRACT In the oligotrophic oceans, key autotrophs depend on “helper” bacteria to reduce oxidative stress from hydrogen peroxide (H 2 O 2 ) in the extracellular environment. H 2 O 2 is also a ubiquitous stressor in freshwaters, but the effects of H 2 O 2 on autotrophs and their interactions with bacteria are less well understood in freshwaters. Naturally occurring H 2 O 2 in freshwater systems is proposed to impact the proportion of microcystin-producing (toxic) and non-microcystin-producing (nontoxic) Microcystis in blooms, which influences toxin concentrations and human health impacts. However, how different strains of Microcystis respond to naturally occurring H 2 O 2 concentrations and the microbes responsible for H 2 O 2 decomposition in freshwater cyanobacterial blooms are unknown. To address these knowledge gaps, we used metagenomics and metatranscriptomics to track the presence and expression of genes for H 2 O 2 decomposition by microbes during a cyanobacterial bloom in western Lake Erie in the summer of 2014. katG encodes the key enzyme for decomposing extracellular H 2 O 2 but was absent in most Microcystis cells. katG transcript relative abundance was dominated by heterotrophic bacteria. In axenic Microcystis cultures, an H 2 O 2 scavenger (pyruvate) significantly improved growth rates of one toxic strain while other toxic and nontoxic strains were unaffected. These results indicate that heterotrophic bacteria play a key role in H 2 O 2 decomposition in Microcystis blooms and suggest that their activity may affect the fitness of some Microcystis strains and thus the strain composition of Microcystis blooms but not along a toxic versus nontoxic dichotomy. IMPORTANCE Cyanobacterial harmful algal blooms (CHABs) threaten freshwater ecosystems globally through the production of toxins. Toxin production by cyanobacterial species and strains during CHABs varies widely over time and space, but the ecological drivers of the succession of toxin-producing species remain unclear. Hydrogen peroxide (H 2 O 2 ) is ubiquitous in natural waters, inhibits microbial growth, and may determine the relative proportions of Microcystis strains during blooms. However, the mechanisms and organismal interactions involved in H 2 O 2 decomposition are unexplored in CHABs. This study shows that some strains of bloom-forming freshwater cyanobacteria benefit from detoxification of H 2 O 2 by associated heterotrophic bacteria, which may impact bloom development. 
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  6. A mechanistic, molecular-level model of a toxin-producing cyanobacterium explains ecology and informs management. 
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  7. null (Ed.)