Low-Ammonium Environment Increases the Nutrient Exchange between Diatom–Diazotroph Association Cells and Facilitates Photosynthesis and N(2) Fixation—a Mechanistic Modeling Analysis

Diatom–diazotroph associations (DDAs) are one of the most important symbiotic dinitrogen (N(2)) fixing groups in the oligotrophic ocean. Despite their capability to fix N(2), ammonium (NH(4)(+)) remains a key nitrogen (N) source for DDAs, and the effect of NH(4)(+) on their metabolism remains elusive. Here, we developed a coarse-grained, cellular model of the DDA with NH(4)(+) uptake and quantified how the level of extracellular NH(4)(+) influences metabolism and nutrient exchange within the symbiosis. The model shows that, under a fixed growth rate, an increased NH(4)(+) concentration may lower the required level of N(2) fixation and photosynthesis, and decrease carbon (C) and N exchange. A low-NH(4)(+) environment leads to more C and N in nutrient exchange and more fixed N(2) to support a higher growth rate. With higher growth rates, nutrient exchange and metabolism increased. Our study shows a strong effect of NH(4)(+) on metabolic processes within DDAs, and thus highlights the importance of in situ measurement of NH(4)(+) concentrations.