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Articles

Effects of Environmental Stresses on the Cell Cycle of Two Marine Phytoplankton Species ¹

Ralph M. Parsons Laboratory, Massachusetts Institute of Technology 48-425, Cambridge, Massachusetts 02139

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 G₁ phase. In H. carterae light limitation caused only G₁ phase to expand, but in T. weissflogii both G₂ + M and G₁ 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 G₁ was a constant proportion of that of G₂ + 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.

³ Present address: Station biologique de Roscoff, Roscoff 29211, France.

¹ Supported in part by National Science Foundation grants OCE-8211525, OCE-8118475, OCE-8121270, OCE-8316616, CEE-8211525, by grant N00014-83-K-0661 from the Office of Naval Research, and by the Doherty Professorship of Ocean Utilization awarded to S. W. Chisholm.

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