Genetic diversity of marine Synechococcus and co‐occurring cyanophage communities: evidence for viral control of phytoplankton

<jats:title>Summary</jats:title><jats:p>Unicellular cyanobacteria of the genus <jats:italic>Synechococcus</jats:italic> are a major component of the picophytoplankton and make a substantial contribution to primary productivity in the oceans. Here we provide evidence that supports the hypothesis that virus infection can play an important role in determining the success of different <jats:italic>Synechococcus</jats:italic> genotypes and hence of seasonal succession. In a study of the oligotrophic Gulf of Aqaba, Red Sea, we show a succession of <jats:italic>Synechococcus</jats:italic> genotypes over an annual cycle. There were large changes in the genetic diversity of <jats:italic>Synechococcus</jats:italic>, as determined by restriction fragment length polymorphism analysis of a 403‐ bp <jats:italic>rpoC1</jats:italic> gene fragment, which was reduced to one dominant genotype in July. The abundance of co‐occurring cyanophage capable of infecting marine <jats:italic>Synechococcus</jats:italic> was determined by plaque assays and their genetic diversity was determined by denaturing gradient gel electrophoresis analysis of a 118‐bp <jats:italic>g20</jats:italic> gene fragment. The results indicate that both abundance and genetic diversity of cyanophage covaried with that of <jats:italic>Synechococcus</jats:italic>. Multivariate statistical analyses show a significant relationship between cyanophage assemblage structure and that of <jats:italic>Synechococcus</jats:italic>. These observations are consistent with cyanophage infection being a major controlling factor in picophytoplankton succession.</jats:p>