Abstract
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.
Highlights
Cyanobacteria play an important role in aquatic ecosystems (e.g. [1,2]) but they can disrupt the functioning of these ecosystems and their use by humans, because of the ability of several species to proliferate and to produce harmful toxins (e.g. [3])
Population genetic studies have revealed that M. aeruginosa populations are characterized by wide genetic diversity and the lack of biogeographical patterns of genetic differentiation, suggesting that M. aeruginosa strains are able to proliferate in a wide range of ecosystems [8,9]
By performing a PCA analysis on the main characteristics of the twelve M. aeruginosa genomes described in Table 1, the NIES-843 genome was clearly distinguishable from all the other genomes
Summary
Cyanobacteria play an important role in aquatic ecosystems (e.g. [1,2]) but they can disrupt the functioning of these ecosystems and their use by humans, because of the ability of several species to proliferate and to produce harmful toxins (e.g. [3]). Several studies have shown that sharply contrasting ecological strategies have allowed cyanobacteria to live in a large range of habitats in the euphotic zone of aquatic ecosystems. One exciting scientific challenge is to elucidate the genomic basis of the different ecological strategies developed by the Prochlorococcus and Microcystis genera. With this goal, the genomes of many P. marinus strains have recently been sequenced (for a review, see [10]). The genomes of many P. marinus strains have recently been sequenced (for a review, see [10]) These studies have indicated that almost all sequenced P. marinus genomes are characterized by a small size that seems to result from a genome reduction process. The combination of a niche specialization process involving genome reduction [16,17], and a high adaptive potential with gene acquisition by HGT, might explain the ecological success of this species
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