Effects of Photocycles and Periodic Ammonium Supply on Three Marine Phytoplankton Species. I. Cell Division Patterns1

Cell division patterns in Thalassiosira weissflogii (Grun.), Hymenomonas carterae (Braarud and Fagerl), and Amphidinium carteri (Hulburl) grown in cyclostat culture were analyzed as functions of the periodic supply of light and the limiting nutrient (ammonium) and of combinations of these two factors. In all three species, division patterns were phased by light/dark cycles in N–limited as well as N–replte conditions, and also to ammonium pulses in N–limited growth in continuous light. Both the degree and timing of the cell cycle phasing varied among species. When both stimuli were present, the influence of the photocycle overrode the N–pulse stimulus in H. carterae and A. carteri. while in T. weissflogii, division was always phased by the timing of the N–pulse regardless of the phase angle between the photocycle and the pulse.