Microcystin shapes the <i>Microcystis</i> phycosphere through community filtering and by influencing cross-feeding interactions

The cyanobacterium <i>Microcystis</i> causes harmful algal blooms that pose a major threat to human health and ecosystem services, particularly due to the prevalence of the potent hepatotoxin microcystin (MC). With their pronounced EPS layer, <i>Microcystis</i> colonies also serve as a hub for heterotrophic phycosphere bacteria. Here, we tested the hypothesis that the genotypic plasticity in its ability to produce MC influences the composition and assembly of the <i>Microcystis</i> phycosphere microbiome. In an analysis of individual colonies of a natural <i>Microcystis</i> bloom, we observed a significantly reduced richness of the community in the presence of MC biosynthesis genes. A subsequent synthetic community experiment with 21 heterotrophic bacterial strains in co-cultivation with either the wild-type strain <i>Microcystis aeruginosa</i> PCC 7806 or the MC-free mutant Δ<i>mcyB</i> revealed not only a tug-of-war between phototrophic and heterotrophic bacteria, but also a reciprocal dominance of two isolates of the genus <i>Sphingomonas</i> and <i>Flavobacterium</i>. In contrast, an <i>Agrobacterium</i> isolate thrived equally well in both consortia. In substrate utilization tests, <i>Sphingomonas</i> showed the strongest dependence on <i>Microcystis</i> exudates with a clear preference for the wild-type strain. Genome sequencing revealed a high potential for complementary cross-feeding, particularly for the <i>Agrobacterium</i> and <i>Sphingomonas</i> isolates but no potential for MC degradation. We postulate that strain-specific functional traits, such as the ability to perform glycolate oxidation, play a crucial role in the cross-feeding interactions, and that MC is one of the determining factors in the <i>Microcystis</i> phycosphere due to its interference with inorganic carbon metabolism.