mardi 21 avril 2009
par   G. Grégori

Olson RJ, Vaulot D, Chisholm SW (1986) Effects of Environmental Stresses on the Cell Cycle of Two Marine Phytoplankton Species. Plant Physiol 80 :918-925


Cell cycle phase durations of cultures of Hymenomonas carterae Braarud and Fagerl, a coccolithophore, and Thalassiosira weissflogii Grun., a centric diatom, in temperature-, light- or nitrogen-limited balanced growth were determined using flow cytometry. Suboptimal temperature caused increases in the duration of all phases of the cell cycle (though not equally) in both species, and the increased generation time of nitrogen-limited cells of both species was due almost wholly to expansion of G(1) phase. In H. carterae light limitation caused only G(1) phase to expand, but in T. weissflogii both G(2) + M and G(1) were affected. These results are discussed in relation to cell division phasing patterns of these two species and to models of phytoplankton growth. Simultaneous measurements of protein and DNA on individual cells indicated that under all conditions, the protein content of cells in G(1) was a constant proportion of that of G(2) + M cells. Simultaneous measurements of RNA and protein on each cell indicated that the amounts of these two cell constituents were always tightly correlated. Under conditions of nitrogen limitation both protein and RNA per cell decreased to less than one-third of the levels found in nonlimited cells. This indicates, at least for nitrogen-replete cells, that neither protein nor RNA levels are likely to act as the trigger for cell cycle progression. Strict control by cell size is also unlikely since mean cell volume decreased as growth rates were limited by light and nitrogen supply, but increased with decreasing temperature.

Vaulot D, Olson RJ, Chisholm SW (1986) Light and dark control of the cell cycle in two marine phytoplankton species. Exp Cell Res 167 :38-52


The effect of light and dark on growth, DNA replication and cell division of two marine phytoplankters Thalassiosira weissflogii (a diatom) and Hymenomonas carterae (a coccolithophorid) was investigated using flow cytometry. The two species displayed very differing behavior. When transferred from light to prolonged darkness, all coccolithophorid cells were arrested at the beginning of the G1 stage of the cell cycle. When shifted back into light, they resumed cycling at a rate slightly slower than prior to arrest. In contrast, diatom cells were arrested either in the G1 or G2 stage of the cell cycle in the dark. Upon re-exposure to light, cells which had been dark-arrested in G1 resumed cycling at the same rate as prior to arrest, while cells arrested in G2 cycled much more slowly. These results suggest that in both species, light control of cell cycle progression is effective only over a restricted part of the cell cycle, as has been hypothesized by Spudich & Sager (J cell biol 83 (1980) 136) [38] for Chlamydomonas. In the coccolithophorid there is a single light-dependent segment located at the beginning of G1, whereas the diatom appears to have two such segments, one in G1 and the other in G2, corresponding to two different light requiring processes.