From the cell cycle to population cycles in phytoplankton-nutrient interactions

The internal demographic structure of a population influences its dynamics and its response to the environment. Most models for phytoplankton ignore internal structure and group all cells in a single variable such as total biomass or density. However, a cell does have a life history, the cell division cycle. We investigate the significance of the cell cycle to phytoplankton population dynamics in a variable nutrient environment, using chemostat models. Following the transition point hypothesis, nutrient uptake affects cell development only within a limited segment of the cell cycle. Simulation results demonstrate oscillations in cell numbers and population structure generated by this interaction. When nutrient input is varied periodically, the population displays an aperiodic response with frequencies different from that of the forcing. These results also hold for a model that includes nutrient storage by the cells. These dynamics differ from those of traditional chemostat models and from cell cycle models driven by light cycles. Resource control of cell cycle progression may explain the time delays previously postulated to explain oscillatory transients in chemostat experiments.