Bacteria play key roles in the function and diversity of aquatic systems, but aside from study of specific bloom systems, little is known about the diversity or biogeography of bacteria associated with harmful cyanobacterial blooms (cyanoHABs). CyanoHAB species are known to shape bacterial community composition and to rely on functions provided by the associated bacteria, leading to the hypothesized cyanoHAB interactome, a coevolved community of synergistic and interacting bacteria species, each necessary for the success of the others. Here, we surveyed the microbiome associated with Microcystis aeruginosa during blooms in 12 lakes spanning four continents as an initial test of the hypothesized Microcystis interactome. We predicted that microbiome composition and functional potential would be similar across blooms globally. Our results, as revealed by 16S rRNA sequence similarity, indicate that M. aeruginosa is cosmopolitan in lakes across a 280 longitudinal and 90 latitudinal gradient. The microbiome communities were represented by a wide range of operational taxonomic units and relative abundances. Highly abundant taxa were more related and shared across most sites and did not vary with geographic distance, thus, like Microcystis, revealing no evidence for dispersal limitation. High phylogenetic relatedness, both within and across lakes, indicates that microbiome bacteria with similar functional potential were associated with all blooms. While Microcystis and the microbiome bacteria shared many genes, whole-community metagenomic analysis revealed a suite of biochemical pathways that could be considered complementary. Our results demonstrate a high degree of similarity across global Microcystis blooms, thereby providing initial support for the hypothesized Microcystis interactome.

With the aim of explaining the variations in microcystin (MC) concentrations during cyanobacterial blooms, we studied several Microcystis aeruginosa populations blooming in different freshwater ecosystems located in the same geographical area. As assessed by real-time PCR, it appeared that the potentially MC-producing cells (mcyB ؉ ) were predominant (70 to 100%) in all of these M. aeruginosa populations, with the exception of one population in which non-MC-producing cells always dominated. Apart from the population in the Grangent Reservoir, we found that the proportions of potentially MC-producing and non-MC-producing cells varied little over time, which was consistent with the fact that according to a previous study of the same populations, the intergenic transcribed spacer (ITS) genotype composition did not change (38). In the Grangent Reservoir, the MC-RR variant was the dominant microcystin variant throughout the bloom season, despite changes in the ITS composition and in the proportions of mcyB Microcystins (MCs) are the most common cyanotoxins and have been involved in several animal and human poisoning episodes (8). These hepatotoxic cyclic heptapeptides are synthesized by a multifunctional enzyme complex (10, 40), and the discovery of the gene cluster encoding this complex has made it possible in recent years to develop molecular tools for studying the relative proportions of MC-producing and non-MCproducing cells in natural cyanobacterial populations. Potentially MC-producing and non-MC-producing cells can coexist in these populations, but the factors and processes governing the dynamics of these subpopulations remain unclear.Some recent papers on the Microcystis genus have shown that the proportions of potentially MC-producing cells can differ considerably from lake to lake. For example, in Lake Wannsee, Germany, this proportion was between 0 and 40% (28), as it was in Lake Oneida, United States (18), and in Lake Mikata, Japan (48). In contrast, large variations over time (6 to 93%) of potentially MC-producing cells were found in the Grangent Reservoir, France (4). Major variations (30 to 80%) were also found in a natural French population of Planktothrix agardhii (3), and the variations in the proportions of potentially MC-producing cells reflected those of the MC concentrations. However, only 54% of the variation in MC concentrations could be explained by changes in the proportion of MC-producing cells, suggesting that a considerable part of the MC concentrations was also due to variations in MC cell quotas. These findings suggest that the toxic risks during cyanobacterial proliferations are due to variations in both the proportion of MC-producing cells and the production of MC by the toxic cells.