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What does it mean to be a global species? The marine cyanobacterium Prochlorococcus is ubiquitous and, arguably, the most abundant and productive of all living organisms. Although to our eyes the seas look uniform, to a bacterium the ocean's bulk is a plethora of microhabitats, and by large-scale single-cell genomic analysis of uncultured cells, Kashtan et al. (p. 416; see the Perspective by Bowler and Scanlan) reveal that Prochlorococcus has diversified to match. This “species” constitutes a mass of subpopulations—each with million-year ancestry—that vary seasonally in abundance. The subpopulations in turn have clades nested within that show covariation between sets of core alleles and variable gene content, indicating flexibility of responses to rapid environmental changes. Large sets of coexisting populations could be a general feature of other free-living bacterial species living in highly mixed habitats.
Extensive genomic diversity within coexisting members of a microbial species has been revealed through selected cultured isolates and metagenomic assemblies. Yet, the cell-by-cell genomic composition of wild uncultured populations of co-occurring cells is largely unknown. In this work, we applied large-scale single-cell genomics to study populations of the globally abundant marine cyanobacterium Prochlorococcus. We show that they are composed of hundreds of subpopulations with distinct “genomic backbones,” each backbone consisting of a different set of core gene alleles linked to a small distinctive set of flexible genes. These subpopulations are estimated to have diverged at least a few million years ago, suggesting ancient, stable niche partitioning. Such a large set of coexisting subpopulations may be a general feature of free-living bacterial species with huge populations in highly mixed habitats.