BACKGROUND: Phages infecting marine picocyanobacteria often carry a psbA gene, which encodes a homolog to the photosynthetic reaction center protein, D1. Host encoded D1 decays during phage infection in the light. Phage encoded D1 may help to maintain photosynthesis during the lytic cycle, which in turn could bolster the production of deoxynucleoside triphosphates (dNTPs) for phage genome replication.METHODOLOGY/PRINCIPAL FINDINGS: To explore the consequences to a phage of encoding and expressing psbA, we derive a simple model of infection for a cyanophage/host pair--cyanophage P-SSP7 and Prochlorococcus MED4--for which pertinent laboratory data are available. We first use the model to describe phage genome replication and the kinetics of psbA expression by host and phage. We then examine the contribution of phage psbA expression to phage genome replication under constant low irradiance (25 microE m(-2) s(-1)). We predict that while phage psbA expression could lead to an increase in the number of phage genomes produced during a lytic cycle of between 2.5 and 4.5% (depending on parameter values), this advantage can be nearly negated by the cost of psbA in elongating the phage genome. Under higher irradiance conditions that promote D1 degradation, however, phage psbA confers a greater advantage to phage genome replication.CONCLUSIONS/SIGNIFICANCE: These analyses illustrate how psbA may benefit phage in the dynamic ocean surface mixed layer.