Abstract
Diatom–diazotroph associations (DDAs) are symbioses where trichome-forming cyanobacteria support the host diatom with fixed nitrogen through dinitrogen (N2) fixation. It is inferred that the growth of the trichomes is also supported by the host, but the support mechanism has not been fully quantified. Here, we develop a coarse-grained, cellular model of the symbiosis between Hemiaulus and Richelia (one of the major DDAs), which shows that carbon (C) transfer from the diatom enables a faster growth and N2 fixation rate by the trichomes. The model predicts that the rate of N2 fixation is 5.5 times that of the hypothetical case without nitrogen (N) transfer to the host diatom. The model estimates that 25% of fixed C from the host diatom is transferred to the symbiotic trichomes to support the high rate of N2 fixation. In turn, 82% of N fixed by the trichomes ends up in the host. Modeled C fixation from the vegetative cells in the trichomes supports only one-third of their total C needs. Even if we ignore the C cost for N2 fixation and for N transfer to the host, the total C cost of the trichomes is higher than the C supply by their own photosynthesis. Having more trichomes in a single host diatom decreases the demand for N2 fixation per trichome and thus decreases their cost of C. However, even with five trichomes, which is about the highest observed for Hemiaulus and Richelia symbiosis, the model still predicts a significant C transfer from the diatom host. These results help quantitatively explain the observed high rates of growth and N2 fixation in symbiotic trichomes relative to other aquatic diazotrophs.
Highlights
Assuming that the diatom’s N demand is fully covered by the N2 -fixing symbiont [1,6] and that both organisms grow at the same speed, the model predicts that the trichome must fix N2
With a simple mechanistic model of cellular metabolisms of Hemiaulus–Richelia symbiosis, we predict that the observed high rates of N2 fixation and growth of the trichomes [6,31] are supported by the C transfer from the host diatom, which is qualitatively consistent with the observations of plant–cyanobacteria symbiosis [23,24,28,29,30]
Our model explicitly accounts for the C cost for N2 fixation, which is a central factor in the competitive fitness of diazotrophs relative to other plankton
Summary
Diatom–diazotroph associations (DDAs) are symbioses where the diazotrophs (e.g., Richelia and Calothrix sp.) are associated with diatoms (e.g., Hemiaulus, Rhizosolenia, and Chaetoceros sp.) [1,2,3,4,5,6,7]. Since the trichomes form a heterocyst, a cell with a thick glycolipid layer to minimize O2 influx [45], we assume that intracellular O2 is managed with normal levels of respiration [37,46]. This simplification allows us to focus on the metabolisms of C and N as the basis of the symbiosis. The predicted balance of photosynthesis and metabolic demand for C suggests that a significant amount of C is transferred from the host to the trichome, sustaining its high rate of N2 fixation and enhanced growth.
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