Lysogen formation governs colonies while lytic infection is more prevalent in single cells of the bloom-forming cyanobacterium, Microcystis

While the bloom-forming cyanobacterium Microcystis can exist as free-living single cells or within dense mucilaginous colonies, the drivers and consequences of colony formation remain unclear. Here, we integrated metatranscriptomic datasets from two Microcystis bloom events in Lake Taihu, China, to analyze and validate the functional differences between colonial and single-cell Microcystis. Our results confirmed colony expression profiles were disproportionately enriched in Microcystis transcripts (and functions) compared to other prokaryotic taxa. Concomitantly, viral infection strategies diverged by Microcystis community morphology: colony-associated cells expressed lysogeny-associated genes, while single cells exhibited increased signatures of lytic infection. These data are consistent with the hypothesis that Microcystis colonies foster conditions favorable to lysogen formation--likely due to local high cell densities and the resulting advantage of superinfection immunity--whereas solitary cells experience stronger lytic pressure. On a broader scale, our findings refine the understanding of bloom dynamics by identifying how community morphological states coincide with distinct host-virus interactions. Cumulatively, this work underscores the importance of colony formation in shaping Microcystis ecology and highlights the need for mechanistic studies that disentangle the interplay between phage infection modes, colony formation, and microbial community structure.