Modelling the dynamics of carbon-nitrogen metabolism in the unicellular diazotrophic cyanobacterium Crocosphaera watsonii WH8501, under variable light regimes

A dynamical model is proposed that describes the daily dynamics of diazotrophy in a unicellular cyanobacterium, Crocosphaera watsonii WH8501, in regard to light limitation and obligate diazotrophy. In this model, intracellular carbon and nitrogen are both divided into a functional pool and a storage pool. An internal pool that explicitly describes the nitrogenase enzyme is also added. The various intracellular carbon and nitrogen flows between these pools lead to a complex dynamics driven by the light regime. The model is successfully validated with continuous cultures experiments of C. watsonii under three light regimes, indicating that the proposed mechanisms accurately reproduce the growth dynamics of this organism under various light environments. Then, a series of model simulations is run for a range of light regimes with different photoperiods and daily light doses. Results reveal how nitrogen and carbon are coupled, through the diel cycle, along with nitrogenase dynamics whose activity is constrained by the light regime. In an ecological perspective, we picture the effect of such irradiance condition on growth and on the carbon to nitrogen stoichiometry on cells. This model could prove useful to understand the latitudinal distribution of this cyanobacterium in the global ocean.