The role of inter-generation memory in diel phytoplankton division patterns

Abstract

Phytoplankton populations are strongly influenced by light and therefore exhibit diel rhythms in many ways, including biomass growth and division. Cells typically divide at about the same time of day, but when the average generation time is longer than 24 h, only a fraction of the population divides each day. This means that two given cells born at the same time and exposed to the same environment can have different generation times (e.g. 24, 48 h). Existing explanations attribute this variability to stochastic internal variability, and several cell cycle models have been proposed. This paper proposes that a deterministic inter-generation memory mechanism, specifically biomass inheritance, is the source for the generation time variability. To explore this hypothesis, the continuum model (an alternative to the cell cycle model that accounts for inter-generation memory) was modified for phytoplankton. Two models with the traditional stochastic and the new deterministic mechanisms were constructed and implemented in an agent-based simulation model. An autonomous endogenous circadian clock is included using a modified van der Pol oscillator. The two models were tested against cell density data from light/dark experiments with average generation times ≈24 h and >24 h from the literature. The applications demonstrate that the deterministic model can reproduce the data without introducing stochastic internal variability, and that it is applicable to a wider range of conditions. Under realistic field conditions, with a light intensity time series from a Lagrangian particle tracking model, the two approaches produce significantly different results. Additional model-data comparisons for cell densities from free-running experiments, cell biomass, and DNA content from direct measurement and flow cytometric analysis are presented. The results suggest that the inter-generation memory mechanism is responsible for the generation time variability underlying diel phytoplankton division patterns and that the difference is of practical significance.