Modelling light-dark regime influence on the carbon-nitrogen metabolism in a unicellular diazotrophic cyanobacterium

We propose a dynamical model depicting nitrogen (N2) fixation (diazotrophy) in a unicellular cyanobacterium, Crocosphaera watsonii, grown under 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 model is successfully validated with continuous culture experiments of C. watsonii under three light regimes, indicating that 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, with nitrogenase dynamics, whose activity is constrained by the light regime. We finally identify optimal productivity conditions.