Single-Cell Genomics Reveals Hundreds of Coexisting Subpopulations in Wild Prochlorococcus

Abstract

Cyanobacterial Diversity 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.