The relative influence of the photoperiod and of periodic ammonium pulses in entraining the cell division cycle in nitrogen-limited cyclostat cultures differs dramatically in
Hymenomonas carterae Braarud and Fagerl,
Amphidinium carteri Hulburt and
Thalassiosira weissflogii Grun. We examined how each species processes an NH
4+ pulse at various times during the cell cycle and the L/D cycle. Rates of NH
4+ uptake and changes in cellular concentrations of NH
4+, free amino acids, and protein were examined after the addition of an NH
4+ pulse. Depletion of NH
4+ from the medium occurred earlier when the pulse was given at the beginning of the light period than at the beginning of the dark period in
H. carterae and
A. carteri. Depletion took longer in the
T. weissflogii cultures and the kinetics were similar during both stages of the photocycle in this species. Similarly, the temporal phasing and maximum pool sizes varied with timing of the NH
4+ pulse in
H. carterae and
A. carteri but complete assimilation was relatively rapid. More persistent pools of NH
4+ and free amino acids accumulated in
T. weissflogii, and the patterns of assimilation varied little as a function of the timing of the pulse with respect to the photocycle. Although nitrogen metabolism occurred rapidly in nitrogen-limited
H. carterae and
A. carteri, the entrainment of the cell division cycle by the photoperiod resulted in a large degree of uncoupling between completion of nitrogen assimilation and cell division. It is hypothesized that the strong entrainment of the cell division cycle of
T. weissflogii by NH
4+ pulses results from a relatively slow rate of nitrogen metabolism.
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