The marine cyanobacterium Prochlorococcus is the smallest and most abundant 
photosynthetic organism on Earth. In this Review, we summarize our understanding 
of the diversity of this remarkable phototroph and describe its role in ocean 
ecosystems. We discuss the importance of interactions of Prochlorococcus with 
the physical environment, with phages and with heterotrophs in shaping the 
ecology and evolution of this group. In light of recent studies, we have come to 
view Prochlorococcus as a 'federation' of diverse cells that sustains its broad 
distribution, stability and abundance in the oceans via extensive genomic and 
phenotypic diversity. Thus, it is proving to be a useful model system for 
elucidating the forces that shape microbial populations and ecosystems.