The Metabolic Tradeoffs of Diazotrophy in a Flexible Phytoplankton Cell Allocation Model

While diazotrophy can infer a large competitive advantage in oligotrophic waters, diazotrophs are saddled with a range of tradeoffs in order to escape a nitrogen limitation. These include the direct energetic demand by nitrogenase to reduce N₂, higher cellular iron quotas and the removal of oxygen from the immediate vicinity of the nitrogenase complex. Here, we present a cell allocation model that explicitly represents a number of cell metabolic processes and functional pools with the goal of evaluating the competitive balance of these tradeoffs as a function of the environmental parameters of light, nitrate, phosphate, iron, oxygen and temperature. We applied the cell model using environmental data from GEOTRACES Atlantic sections ga-02 and ga-03. Results indicate that where nitrate is low, iron, light, phosphate and iron each have spatially varying importance in dictating the favorability of diazotrophic growth. Additionally, the model predicts surprisingly favorable conditions for diazotrophy for the South Atlantic portion of the GEOTRACES ga-02 meridional section.