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Marine photosynthesis is largely driven by cyanobacteria, namelySynechococcusandProchlorococcus. Genes encoding for photosystem (PS) I and II reaction centre proteins are found in cyanophages and are believed to increase their fitness. Two viral PSI gene arrangements are known,psaJF→C→A→B→K→E→DandpsaD→C→A→B. The shared genes between these gene cassettes and their encoded proteins are distinguished by %G + C and protein sequence respectively. The data on thepsaD→C→A→Bgene organization were reported from only two partial gene cassettes coming from Global Ocean Sampling stations in the Pacific and Indian oceans. Now we have extended our search to 370 marine stations from six metagenomic projects. Genes corresponding to both PSI gene arrangements were detected in the Pacific, Indian and Atlantic oceans, confined to a strip along the equator (30°N and 30°S). In addition, we found that the predicted structure of the viral PsaA protein from thepsaD→C→A→Borganization contains a lumenal loop conserved in PsaA proteins fromSynechococcus, but is completely absent in viral PsaA proteins from thepsaJF→C→A→B→K→E→Dgene organization and mostProchlorococcusstrains. This may indicate a co-evolutionary scenario where cyanophages containing either of these gene organizations infect cyanobacterial ecotypes biogeographically restricted to the 30°N and 30°S equatorial strip.