ABSTRACT: The influence of phosphorus (P) starvation on the cell cycle of Prochlorococcus 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 (>=150). In P-starved stationary phase, the fraction of cells in G₁ dropped markedly, while cells accumulated not only in G₂, but also in the DNA synthesis phase S, indicating that cells were not arrested at a precise cell cycle location but in all cell cycle phases. The percentage of P-starved cells in each phase varied slightly between strains. For the Mediterranean Sea strain CCMP 1378, cell cycle arrest in G₁ and G₂ could be reversed by addition of phosphates, while cells arrested in S could not resume their cycle and died. Such irreversible arrest in S had never been reported previously in phytoplankton. This suggests that cell cycle controls are not very tight in Prochlorococcus, since cells are not prevented from entering the critical S phase in the absence of phosphorus.

KEY WORDS: Prochlorococcus . Phosphate starvation . Cell cycle . Flow cytometry . Growth rate

Published in MEPS Vol. 132 (1996) on February 29
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 1996