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The effect of phosphorus (P) and nitrogen (N) additions on the Synechococcus cell cycle was tested with natural populations from the Mediterranean Sea in summer. In the absence of stimulation, the Synechococcus cell cycle was synchronized to the light-dark cycle. DNA synthesis began around 1600, a maximum of S-phase cells was observed at around dusk (2100), and a maximum of G(inf2)-phase cells was observed at around 2400. Addition of P (as PO(inf4)(sup3-)) caused, in all cases, a decrease in the fraction of cells in G(inf2) at around 1800, no change at around 2400, and an increase at around 1200 the next day, while addition of N (as NO(inf3)(sup-)) had no effect. We hypothesize that P addition induced a shortening of the G(inf1) phase, resulting in cells entering and leaving the S and G(inf2) phases earlier. These data suggest very strongly that the Synechococcus cells were P limited rather than N limited during this period of the year. In most cases, additions as low as 20 nM P induced a cell cycle response. From dose-response curves, we established that the P concentration inducing a 50% change in the percentage of cells in G(inf2) was low, close to 10 nM, at the beginning of the sampling period (30 June) and increased to about 50 nM by the end (9 July), suggesting a decrease in the severity of P limitation. This study extends recent observations that oligotrophic systems may be P rather than N limited at certain times of the year.

Effect of Phosphorus on the Synechococcus Cell Cycle in Surface Mediterranean Waters during Summer