A new Bio-GO-SHIP study was just published today in ISME Journal. Here, Ustick and colleagues use the global metagenome collection to describe the microdiversity of Prochlorococcus – the most abundant phytoplankton on the planet. We find a clear biogeography among closely related lineages including a subclade adapted to low phosphorus conditions. This is the first time we identify a phytoplankton lineage adapted to cope with the limitation of particular nutrient. Once again, Prochlorococcus proves to be an excellent system for understanding eco-evolutionary processes in the ocean.