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.