The marine cyanobacterium Prochlorococcus is the numerically dominant 
photosynthetic organism in the oligotrophic oceans, and a model system in marine 
microbial ecology. Here we report 27 new whole genome sequences (2 complete and 
closed; 25 of draft quality) of cultured isolates, representing five major 
phylogenetic clades of Prochlorococcus. The sequenced strains were isolated from 
diverse regions of the oceans, facilitating studies of the drivers of microbial 
diversity-both in the lab and in the field. To improve the utility of these 
genomes for comparative genomics, we also define pre-computed clusters of 
orthologous groups of proteins (COGs), indicating how genes are distributed 
among these and other publicly available Prochlorococcus genomes. These data 
represent a significant expansion of Prochlorococcus reference genomes that are 
useful for numerous applications in microbial ecology, evolution and 
oceanography.