Swimming marine Synechococcus strains with widely different photosyntheticpigment ratios form a monophyletic group

Unicellular marine cyanobacteria are ubiquitous in both coastal and oligotr ophic regimes. The contribution of these organisms to primary production an d nutrient cycling is substantial on a global scale. Natural populations of marine Synechococcus strains include multiple genetic lineages, but the li nk, if any, between unique phenotypic traits and specific genetic groups is still not understood. We studied the genetic diversity (as determined by t he DNA-dependent RNA polymerase rpoC1 gene sequence) of a set of marine Syn echococcus isolates that are able to swim, Our results show that these isol ates form a monophyletic group. This finding represents the first example o f correspondence between a physiological trait and a phylogenetic group in marine Synechococcus. In contrast, the phycourobilin (PUB)/phycoerythrobili n (PEB) pigment ratios of members of the motile clade varied considerably. An isolate obtained from the California Current (strain CC9703) displayed a pigment signature identical to that of nonmotile strain WH7803, which is c onsidered a model for low-PUB/PEB-ratio strains, whereas several motile str ains had higher PUB/PEB ratios than strain WH8103, which is considered a mo del for high-PUB/PEB-ratio strains. These findings indicate that the PUB/FE B pigment ratio is not a useful characteristic for defining phylogenetic gr oups of marine Synechococcus strains.