Cell Cycle Regulation in Marine Synechococcus sp. Strains

The cell cycle behavior of four marine strains of the unicellular cyanobacterium Synechococcus sp. was analyzed by examining the DNA frequency distributions of exponentially growing and dark-blocked populations and by considering the patterns of change in these distributions during growth under a diel light-dark cycle. The two modes of cell cycle regulation previously identified in a freshwater and coastal marine Synechococcus isolate, respectively, were represented among the three open-ocean strains we examined. The first of these modes of regulation is consistent with the slow-growth case of the widely accepted prokaryotic cell cycle paradigm. The second appears to involve asynchronous initiation of chromosome replication, the presence of multiple chromosome copies at low growth rates, and variability in chromosome copy number among cells in the population. These characteristics suggest the involvement of a large probabilistic component in cell cycle regulation which could make the application of cell cycle-based estimators of in situ growth rate to Synechococcus populations problematic.