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

The cyanobacterium Microcystis causes harmful algal blooms (cyanoHABs) that pose a major threat to human health and ecosystem services, particularly due to the prevalence of the potent hepatotoxin microcystin. With their pronounced EPS layer, Microcystis colonies also serve as a hub for heterotrophic phycosphere bacteria. Here, we tested the hypothesis that the genotypic plasticity in its ability to produce microcystin influences the composition and assembly of the Microcystis phycosphere microbiome. In an analysis of individual colonies of a natural Microcystis bloom, we observed a significantly reduced richness of the community in the presence of microcystin biosynthesis genes. A subsequent synthetic community experiment with 21 heterotrophic strains in co-cultivation with either the wild-type strain M. aeruginosa PCC 7806 or the microcystin-free mutant ΔmcyB revealed not only a tug-of-war between phototrophic and heterotrophic bacteria, but also a reciprocal dominance of two isolates of the genus Sphingomonas and Flavobacterium. In contrast, an Agrobacterium isolate thrived equally well in both consortia. In substrate utilization tests, Sphingomonas showed the strongest dependence on Microcystis exudates with a clear preference for the wild-type strain. Genome sequencing revealed a high potential for complementary cross-feeding, particularly for the Agrobacterium and Sphingomonas isolates but no potential for microcystin degradation. We postulate that strain-specific functional traits, such as the ability to perform photorespiration and to produce vitamin B12, play a crucial role in the cross-feeding interactions, and that microcystin is one of the determining factors in the Microcystis phycosphere due to its interference with inorganic carbon metabolism.