Low-Ammonium Environment Increases the Nutrient Exchange between Diatom–Diazotroph Association Cells and Facilitates Photosynthesis and N2 Fixation—a Mechanistic Modeling Analysis

Abstract

Diatom–diazotroph associations (DDAs) are one of the most important symbiotic dinitrogen (N2) fixing groups in the oligotrophic ocean. Despite their capability to fix N2, ammonium (NH4+) remains a key nitrogen (N) source for DDAs, and the effect of NH4+ on their metabolism remains elusive. Here, we developed a coarse-grained, cellular model of the DDA with NH4+ uptake and quantified how the level of extracellular NH4+ influences metabolism and nutrient exchange within the symbiosis. The model shows that, under a fixed growth rate, an increased NH4+ concentration may lower the required level of N2 fixation and photosynthesis, and decrease carbon (C) and N exchange. A low-NH4+ environment leads to more C and N in nutrient exchange and more fixed N2 to support a higher growth rate. With higher growth rates, nutrient exchange and metabolism increased. Our study shows a strong effect of NH4+ on metabolic processes within DDAs, and thus highlights the importance of in situ measurement of NH4+ concentrations.

Microalgal and Nitrogen-Fixing Bacterial Consortia: From Interaction to Biotechnological Potential

“…Diatoms are an ecologically and morphologically diverse group of microalgae that have a cell wall that incorporates silica as a main component. These interactions are called diatom-diazotroph associations (DDAs) and provide an essential contribution to oceanic CO 2 fixation (primary productivity) supported by N fixation [85]. In DDAs, symbiotic diazotrophs form trichomes where one cell is specialized in N fixation, the heterocyst, while the remaining cells, the vegetative cells, are phototrophic and divide, whereas heterocysts cannot [86].…”

Diatom-Diazotroph Associations in hydrographically defined habitats of the South China Sea

“…The factors that promote multiple infection of DDA hosts are not fully understood, though our data clearly indicate that at least some of the variation in infection rate is hostspecific and likely reflects specific differences in the interaction between host and symbiont. The number of symbionts in a host cell will affect the carbon and nitrogen budgets of both partners in the symbiosis (Inomura et al, 2020), and the external nutrient environment can modulate the interaction between host and symbiont (Gao et al, 2022). While an increase in symbiont numbers will increase the total capacity for N 2 -fixation in the system, it will also reduce the host carbon subsidy available to support each symbiont's metabolic activity, including N 2 -fixation (Inomura et al, 2020).…”