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.
Trichodesmiumis one of the dominant dinitrogen(N2)-fixers in the ocean, influencing global carbon and nitrogen cycles through its biochemical reactions. Although the photosynthetic activity ofTrichodesmiumfluctuates rapidly at the cellular level, the physiological or ecological advantage of this characteristic is not clear. Here we develop a metabolic model ofTrichodesmiumthat can perform dinitrogen (N2) fixation during the daytime. We examined (1) the effect of the duration of switches between photosynthetic and non-photosynthetic cellular states and (2) the effect of the presence and absence of N2fixation in photosynthetic states. Our results show that a rapid switch between photosynthetic and non-photosynthetic states increasesTrichodesmiumgrowth rates by improving metabolic efficiencies due to optimized C and N metabolism. Our results show the possibility and advantage of the rapid switch, providing a strategy for previous observations that allTrichodesmiumcells can contain nitrogenase, which was previously considered to be a paradox. This study reveals the importance of fluctuating photosynthetic activity and provides a mechanism for daytime N2fixation that allowsTrichodesmiumto fix N2aerobically in the global ocean.
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