Fig 6. A predicted model of metabolic interactions between host and symbiont at different stages.

(A) Aposymbiotic stage. Host cells synthesize amino acids from food-derived choline. The ammonium produced from the catabolism of amino acids is converted to urea and evacuated from host cells. (B) Early symbiotic stage. The animal host is partially colonized by symbiont cells which produce glucose via photosynthesis. A large proportion of the glucose remains in the symbiont for its own proliferation, while a minimum amount of glucose is transferred to host and enables host-dependent ammonium assimilation. The production of amino acids frees host from its dependence on food-derived choline. (C) Fully symbiotic stage. The proliferation of symbiont cells requires more nitrogen to produce more glucose. This further enhances the capability of the host to assimilate ammonium and control the nitrogen availability to symbiont. The process reaches an equilibrium eventually where symbiont cell density is stable. (D) Stressed stage. The balance of this system depends on the photosynthesis of symbiont and the translocation of photosynthates to the host. Stresses, such as temperature stress imposed by climate change, reduce photosynthetic efficiency of the symbionts and photosynthate translocation, resulting in reduced ammonium assimilation by the host and increased ammonium availability to the symbionts. The increased availability of nitrogen to the symbionts can induce cell proliferation and further reduction of photosynthate translocation which exacerbates the metabolic imbalance of the system and initiates a vicious cycle.