The cell cycle behavior of four marine strains of the unicellular cyan
obacterium Synechococcus sp. was analyzed by examining the DNA frequen
cy distributions of exponentially growing and dark-blocked populations
and by considering the patterns of change in these distributions duri
ng growth under a diel light-dark cycle. The two modes of cell cycle r
egulation previously identified in a freshwater and coastal marine Syn
echococcus isolate, respectively, were represented among the three ope
n-ocean strains we examined. The first of these modes of regulation is
consistent with the slow-growth case of the widely accepted prokaryot
ic cell cycle paradigm. The second appears to involve asynchronous ini
tiation 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 invo
lvement of a large probabilistic component in cell cycle regulation wh
ich could make the application of cell cycle-based estimators of in si
tu growth rate to Synechococcus populations problematic.