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The factors and processes driving cyanobacterial blooms in eutrophic freshwater ecosystems have been extensively studied in the past decade. A growing number of these studies concern the direct or indirect interactions between cyanobacteria and heterotrophic bacteria. The presence of bacteria that are directly attached or immediately adjacent to cyanobacterial cells suggests that intense nutrient exchanges occur between these microorganisms. In order to determine if there is a specific association between cyanobacteria and bacteria, we compared the bacterial community composition during two cyanobacteria blooms of Anabaena (filamentous and N2-fixing) and Microcystis (colonial and non-N2 fixing) that occurred successively within the same lake. Using high-throughput sequencing, we revealed a clear distinction between associated and free-living communities and between cyanobacterial genera. The interactions between cyanobacteria and bacteria appeared to be based on dissolved organic matter degradation and on N recycling, both for N2-fixing and non N2-fixing cyanobacteria. Thus, the genus and potentially the species of cyanobacteria and its metabolic capacities appeared to select for the bacterial community in the phycosphere.

Microcystis aeruginosa is one of the most common bloom-forming cyanobacteria in freshwater ecosystems worldwide. This species produces numerous secondary metabolites, including microcystins, which are harmful to human health. We sequenced the genomes of ten strains of M. aeruginosa in order to explore the genomic basis of their ability to occupy varied environments and proliferate. Our findings show that M. aeruginosa genomes are characterized by having a large open pangenome, and that each genome contains similar proportions of core and flexible genes. By comparing the GC content of each gene to the mean value of the whole genome, we estimated that in each genome, around 11% of the genes seem to result from recent horizontal gene transfer events. Moreover, several large gene clusters resulting from HGT (up to 19 kb) have been found, illustrating the ability of this species to integrate such large DNA molecules. It appeared also that all M. aeruginosa displays a large genomic plasticity, which is characterized by a high proportion of repeat sequences and by low synteny values between the strains. Finally, we identified 13 secondary metabolite gene clusters, including three new putative clusters. When comparing the genomes of Microcystis and Prochlorococcus, one of the dominant picocyanobacteria living in marine ecosystems, our findings show that they are characterized by having almost opposite evolutionary strategies, both of which have led to ecological success in their respective environments.

Rising CO 2 levels may act as an important selective factor on the CO 2 -concentrating mechanism (CCM) of cyanobacteria. We investigated genetic diversity in the CCM of Microcystis aeruginosa , a species producing harmful cyanobacterial blooms in many lakes worldwide. All 20 investigated Microcystis strains contained complete genes for two CO 2 uptake systems, the ATP-dependent bicarbonate uptake system BCT1 and several carbonic anhydrases (CAs). However, 12 strains lacked either the high-flux bicarbonate transporter BicA or the high-affinity bicarbonate transporter SbtA. Both genes, bicA and sbtA , were located on the same operon, and the expression of this operon is most likely regulated by an additional LysR-type transcriptional regulator (CcmR2). Strains with only a small bicA fragment clustered together in the phylogenetic tree of sbtAB , and the bicA fragments were similar in strains isolated from different continents. This indicates that a common ancestor may first have lost most of its bicA gene and subsequently spread over the world. Growth experiments showed that strains with sbtA performed better at low inorganic carbon (C i ) conditions, whereas strains with bicA performed better at high C i conditions. This offers an alternative explanation of previous competition experiments, as our results reveal that the competition at low CO 2 levels was won by a specialist with only sbtA , whereas a generalist with both bicA and sbtA won at high CO 2 levels. Hence, genetic and phenotypic variation in C i uptake systems provide Microcystis with the potential for microevolutionary adaptation to changing CO 2 conditions, with a selective advantage for bicA -containing strains in a high-CO 2 world.

Cyanobacterial harmful algal blooms (cyanoHABs) are a primary source of water quality degradation in eutrophic lakes. The occurrence of cyanoHABs is ubiquitous and expected to increase with current climate and land use change scenarios. However, it is currently unknown what environmental parameters are important for indicating the presence of cyanoHAB toxins making them difficult to predict or even monitor on time-scales relevant to protecting public health. Using qPCR, we aimed to quantify genes within the microcystin operon (mcy) to determine which cyanobacterial taxa, and what percentage of the total cyanobacterial community, were responsible for microcystin production in four eutrophic lakes. We targeted Microcystis-16S, mcyA, and Microcystis, Planktothrix, and Anabaena-specific mcyE genes. We also measured microcystins and several biological, chemical, and physical parameters--such as temperature, lake stability, nutrients, pigments and cyanobacterial community composition (CCC)--to search for possible correlations to gene copy abundance and MC production. All four lakes contained Microcystis-mcyE genes and high percentages of toxic Microcystis, suggesting Microcystis was the dominant microcystin producer. However, all genes were highly variable temporally, and in few cases, correlated with increased temperature and nutrients as the summer progressed. Interestingly, toxin gene abundances (and biomass indicators) were anti-correlated with microcystin in all lakes except the largest lake, Lake Mendota. Similarly, gene abundance and microcystins differentially correlated to CCC in all lakes. Thus, we conclude that the presence of microcystin genes are not a useful tool for eliciting an ecological role for toxins in the environment, nor are microcystin genes (e.g. DNA) a good indicator of toxins in the environment.

A novel rod-shaped, Gram-stain-negative bacterial strain MS17 was obtained from a co-culture of Microcystis aeruginosa and Myriophyllum spicatum. The examination of the 16S ribosomal RNA gene sequence showed a significant degree of similarity between strain MS17 and Paucibacter sediminis S2-9 (98.4%), Roseateles violae PFR6 (98.1%), 'Roseateles cellulosilyticus' P8 (98.0%), Roseateles aquae APW11 (97.9%), Roseateles oligotrophus CHU3 (97.7%), Roseateles saccharophilus DSM 654 (97.6%), Kinneretia aquatilis CR182 (97.7%), Pelomonas aquatica CCUG 52575 (97.6%), and Roseateles toxinivorans 2C20 (97.0%). Between strain MS17 and the type strains of closely related species, digital DNA-DNA hybridization (dDDH) and average nucleotide identity (ANI) values were below 21.9% and 78.48%. The respiratory quinone was ubiquinone Q-8. The main fatty acids (> 10.0%) were C ω6c, C , C , and summed feature 3 (C ω7c and/or C ω6c). The polar lipids comprised phosphatidylethanolamine, unidentified phospholipid, phosphatidylglycerol, unidentified aminophospholipid, unidentified aminolipid, and three unidentified lipids. The genomic G + C content was 65.9%. According to phenotypic characteristics, phylogenetic relationships, and chemotaxonomic data, strain MS17 has been categorized as a newly discovered species belonging to the Roseateles genus, and the name Roseateles microcysteis sp. nov. is suggested. It exhibits distinct biochemical properties that differentiate it from closely related species. The type strain is MS17 (=KCTC 8001  = LMG 33142 ).

Climate change scenarios predict a doubling of the atmospheric CO 2 concentration by the end of this century. Yet, how rising CO 2 will affect the species composition of aquatic microbial communities is still largely an open question. In this study, we develop a resource competition model to investigate competition for dissolved inorganic carbon in dense algal blooms. The model predicts how dynamic changes in carbon chemistry, pH and light conditions during bloom development feed back on competing phytoplankton species. We test the model predictions in chemostat experiments with monocultures and mixtures of a toxic and non-toxic strain of the freshwater cyanobacterium Microcystis aeruginosa . The toxic strain was able to reduce dissolved CO 2 to lower concentrations than the non-toxic strain, and became dominant in competition at low CO 2 levels. Conversely, the non-toxic strain could grow at lower light levels, and became dominant in competition at high CO 2 levels but low light availability. The model captured the observed reversal in competitive dominance, and was quantitatively in good agreement with the results of the competition experiments. To assess whether microcystins might have a role in this reversal of competitive dominance, we performed further competition experiments with the wild-type strain M. aeruginosa PCC 7806 and its mcyB mutant impaired in microcystin production. The microcystin-producing wild type had a strong selective advantage at low CO 2 levels but not at high CO 2 levels. Our results thus demonstrate both in theory and experiment that rising CO 2 levels can alter the community composition and toxicity of harmful algal blooms.

We investigated possibility of predicting whether blooms, if they occur, would be formed of microcystin-producing cyanobacteria. DGGE analysis of 16S-ITS and mcy A genes revealed that only Planktothrix and Microcystis possessed mcy -genes and Planktothrix was the main microcystin producer. qPCR analysis revealed that the proportion of cells with mcy -genes in Planktothrix populations was almost 100%. Microcystin concentration correlated with the number of potentially toxic and total Planktothrix cells and the proportion of Planktothrix within all cyanobacteria, but not with the proportion of cells with mcy -genes in total Planktothrix . The share of Microcystis cells with mcy -genes was low and variable in time. Neither the number of mcy -possessing cells, nor the proportion of these cells in total Microcystis , correlated with the concentration of microcystins. This suggests that it is possible to predict whether the bloom in the Masurian Lakes will be toxic based on Planktothrix occurrence. Two species of toxin producing Planktothrix , P. agardhii and P. rubescens , were identified by phylogenetic analysis of 16S-ITS. Based on morphological and ecological features, the toxic Planktothrix was identified as P. agardhii . However, the very high proportion of cells with mcy -genes suggests P. rubescens . Our study reveals the need of universal primers for mcy A genes from environment.

The Carpathian Basin is a lowland plain located mainly in Hungary. Due to the nature of the bedrock, alluvial deposits, and a bowl shape, many lakes and ponds of the area are characterized by high alkalinity. In this study, we characterized temporal changes in eukaryal and bacterial community dynamics with high throughput sequencing and relate the changes to environmental conditions in Lake Velence located in Fejér county, Hungary. The sampled Lake Velence microbial populations (algal and bacterial) were analyzed to identify potential correlations with other community members and environmental parameters at six timepoints over 6 weeks in the Spring of 2012. Correlations between community members suggest a positive relationship between certain algal and bacterial populations (e.g. Chlamydomondaceae with Actinobacteria and Acidobacteria), while other correlations allude to changes in these relationships over time. During the study, high nitrogen availability may have favored non-nitrogen fixing cyanobacteria, such as the toxin-producing Microcystis aeruginosa, and the eutrophic effect may have been exacerbated by high phosphorus availability as well as the high calcium and magnesium content of the Carpathian Basin bedrock, potentially fostering exopolymer production and cell aggregation. Cyanobacterial bloom formation could have a negative environmental impact on other community members and potentially affect overall water quality as well as recreational activities. To our knowledge, this is the first prediction for relationships between photoautotrophic eukaryotes and bacteria from an alkaline, Hungarian lake.

Cyanobacterial blooms are characterized by intense growth of one or few species that will dominate the phytoplankton community for periods of few months to an entire year or more. However, even during persistent blooms, important seasonal changes among dominant species can be observed. Pampulha reservoir is a tropical eutrophic reservoir presenting permanent blooms. To identify the main species in this environment, a closer analysis performed by microscopy and 16S-rRNA DGGE revealed Cylindrospermopsis raciborskii as highly dominant throughout the year. The second most abundant group comprised species belonging to the Microcystis genus. They followed a well-defined seasonal pattern described by interesting species-specific ecological trends. During thermal stratification in the rainy/warmer season, C. raciborskii dominated in the water column, while Microcystis spp. were abundant at the end of the dry season, a period characterized by higher total phosphorus concentrations. Phylogenetic analyses confirmed the two dominant taxa and their seasonal trends. The results showed that cyanobacteria major controlling factors were strongly species dependent, shifting from physical/climate related (stratification) to more chemical driven (nutrients/eutrophication). Identifying these drivers is therefore essential for the understanding of the bloom dynamics and the real risks associated with each species, and to eventually adopt the most appropriate and effective management strategies.