The cyanobacterium Prochlorococcus is the numerically dominant phototroph in oligotrophic parts of the oceans. Recently, it was shown that the distribution of phosphate acquisition genes did not match the 16S rRNA phylogeny among isolates from this group but rather appeared related to phosphate availability where the strains had been isolated. To further understand adaptation to phosphate limitation in Prochlorococcus, the distribution of phosphate acquisition genes was investigated in different ocean regions and related to local ortho-phosphate concentration. In regions characterized by less than 0.1 microM phosphate, most Prochlorococcus cells contain genes involved in phosphate uptake, regulation and utilization of organic phosphates. In contrast, most of these genes are absent in regions with more than 0.1 microM phosphate with the exception of genes involved in transport of phosphate (phoE and pstABCS) and three genes of unknown function. This pattern of phosphate acquisition genes showed no significant correspondence to the distribution of rRNA phylotypes. In addition, it was demonstrated that several genes in a separate genomic island were commonly present in low-P sites while absent in high-P sites. Overall, this study further demonstrates a linkage between environmental conditions in the ocean and genome content of Prochlorococcus.