Abstract
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.