Stage structure and synchronization cause single-species oscillations in phytoplankton chemostat populations

We show experimentally that the physiological structure of phytoplankton populations can crucially affect single-species population dynamics. In our chemostat experiments, synchronization of the individuals’ cell division cycle led to oscillatory dynamics in separate cultures of three different unicellular species of phytoplankton. A stage-structured model by Pascual & Caswell (1997) suggests that nutrient dependence of certain phases of the cell cycle may ultimately cause single-population oscillations. In line with this work, we propose that nitrogen-dependence in the G₁-phase and the fact that the individual development  temporarily ceases when this essential nutrient is deficient provided a mechanism by which the cell cycles within algal populations became synchronized. Synchronized experimental populations exhibited moderate-amplitude oscillations around a steady-state. The period lengths of the oscillations ranged from 25 h (Chlamydomonas reinhardtii) to 33 h (Monoraphidium minutum) and 44 h (Chlorella vulgaris). Hence, the oscillations cannot be explained by circadian rhythms, at least in the two latter cases. Our study provides strong experimental evidence for intrinsic oscillations in single-species algal populations, i.e. oscillatory dynamics that occur in the absence of multi-species interaction or external forcing.