Nutrient Competition as a General Mechanism of Regulation in Photosymbiosis
Nitrogen limitation is a key mechanism regulating the mutualism between photosymbiotic cnidarians and their algal symbionts. Algal-derived photosynthate facilitates host assimilation of inorganic nitrogen, specifically ammonium. The resulting competition between symbiotic partners for inorganic nitrogen leads to a negative feedback loop wherein symbiont density is effectively controlled through facilitation of their competitor. However, it is unclear whether this feedback loop is sufficient to stabilize the interaction absent other forms of regulation. Using a generalized modeling approach, we identify the conditions under which cnidarian-algal competition for nitrogen yields dynamically stable symbioses. We show that nitrogen limitation is sufficient to stabilize the interaction through the interplay of intraspecific (symbiont-symbiont) and interspecific (host-symbiont) competition. As host assimilation of inorganic nitrogen increases, the system shifts from being stabilized by intraspecific competition between symbionts to interspecific competition between host and symbionts, necessitating a positive coupling of symbiont density to host nitrogen assimilation via photosynthate translocation for dynamic stability.