EFFECT OF PHOSPHORUS STARVATION ON THE CELL-CYCLE OF THE PHOTOSYNTHETIC PROKARYOTE PROCHLOROCOCCUS SPP

The influence of phosphorus (P) starvation on the cell cycle of Prochl orococcus was studied for several strains from the Mediterranean Sea, the Atlantic and Pacific Oceans. P starvation could only be induced at high N/P ratio (greater than or equal to 150). In P-starved stationar y phase, the fraction of cells in G(1) dropped markedly, while cells a ccumulated not only in G(2), but also in the DNA synthesis phase S, in dicating that cells were not arrested at a precise cell cycle location but in all cell cycle phases. The percentage of P-starved cells in ea ch phase varied slightly between strains. For the Mediterranean Sea st rain CCMP 1378, cell cycle arrest in G(1) and G(2) could be reversed b y addition of phosphates, while cells arrested in S could not resume t heir cycle and died. Such irreversible arrest in S had never been repo rted previously in phytoplankton. This suggests that cell cycle contro ls are not very tight in Prochlorococcus, since cells are not prevente d from entering the critical S phase in the absence of phosphorus.