Microcystin shapes the phycosphere through community filtering and by influencing cross-feeding interactions.

The cyanobacterium 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, 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 phycosphere microbiome. In an analysis of individual colonies of a natural 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 PCC 7806 or the MC-free mutant Δ revealed not only a tug-of-war between phototrophic and heterotrophic bacteria, but also a reciprocal dominance of two isolates of the genus and . In contrast, an isolate thrived equally well in both consortia. In substrate utilization tests, showed the strongest dependence on exudates with a clear preference for the wild-type strain. Genome sequencing revealed a high potential for complementary cross-feeding, particularly for the and 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 phycosphere due to its interference with inorganic carbon metabolism.