Prochlorococcus in the lab and in silico: The importance of representing exudation

In this study, we test the applicability of the Droop/Caperon internal stores model to describe the growth and decline of the globally abundant marine cyanobacterium Prochlorococcus in batch culture as a function of internal and external inorganic and organic carbon and nitrogen. A rigorous parameter fitting exercise, constrained by the measured cell density, ammonium and inorganic carbon concentrations, revealed many different combinations of parameter values that provided equally good model-data fit. Introducing data on C : N ratio from the published literature provided additional constraints which could only be satisfied with a structural change to the model. The simplest addition that satisfied these constraints and improved quantitative overall model-data agreement was to explicitly represent excretion, which was predicted to account for 0.03–8.5% of the daily primary productivity; somewhat lower than previous estimates based on radiotracer incorporation. We argue that this significant and biologically meaningful process should not be neglected in models of marine phytoplankton.