Marine photosynthesis is largely driven by cyanobacteria, mainly Synechococcus and Prochlorococcus. Cyanophages, viruses that infect cyanobacteria, can alter cyanobacterial populations and therefore affect the global carbon cycle and oxygen supply. Genes encoding for photosystem II (PSII) and I (PSI) reaction center 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,D and psaD,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 the psaD,C,A,B gene organization is scarce and was obtained from only two Global ocean sampling (GOS) stations in the Pacific and Indian ocean. We conducted a more comprehensive bioinformatic search, including stations from the GOS and Tara Oceans project and detected more positive stations for viral-PSI genes in the Pacific, Atlantic and Indian Oceans. The stations containing the psaDCAB gene arrangement (as well as the longer gene arrangement) are confined to a strip along the equator (± 20o). In addition we closed the remaining gaps in psaA sequence using PCR. Based on protein sequence and structure modeling of PsaA proteins from the psaDCAB gene arrangement, we now predict that the principal host of phages containing this arrangement are marine Synechococcus.
