Changes in coordination of growth and the cell cycle in green algae through CDK activity at different temperatures.

Progression of the cell cycle in green algae dividing by multiple fission is, under otherwise unlimited conditions, affected by growth rate set by a combination of light intensity and temperature. We compared cell cycle characteristics of Desmodesmus quadricauda at 20°C or 30°C and shifts between them. The duration of the cell cycle in cells grown under continuous illumination was more than doubled at 20°C compared to 30°C, suggesting it was set directly by growth rate. Similarly, DNA, RNA and bulk protein content per cell at 20°C were about double that of cells grown at the higher temperature. For shift experiments, cells grown at either temperature were transferred to darkness to prevent further growth, and cultivated at the same or the other temperature. Upon transfer to the lower temperature, fewer nuclei and daughter cells were produced and not all cells were able to finish the cell cycle by division, remaining multinuclear. Correspondingly, cells put into the dark at the higher temperature divided faster into more daughter cells than control cells. These differences correlated with shifts in the preceding CDK activity, suggesting that cell cycle progression was not related to growth rate or cell biomass but correlates with CDK activity.