A novel nitrogen concentrating mechanism in the coral-algae symbiosome

Coral algal symbionts are hosted inside the symbiosome of gastrodermal cells, an intracellular compartment that isolates algae from the external environment and allows host cells to control the delivery of metabolites to their symbionts. However, the underlying molecular mechanisms are largely unknown. Here, we report the diel trafficking of NH3-transporting Rhesus (Rh) channels between the cytoplasm and the symbiosome membrane in the coral Acropora yongei, which matches established patterns of nitrogen delivery to endosymbionts. Heterologous expression in Xenopus oocytes established that A. yongei Rh (ayRhp1) is a channel that facilitates NH3 diffusion across membranes following its partial pressure gradient. Immunostaining revealed ayRhp1 is widely distributed throughout coral tissues and most abundantly present in oral ectodermal cells, desmocytes, and gastrodermal cells. In the latter, ayRhp1 was observed in the symbiosome membrane of alga-containing cells. Together with V-type H+-ATPases that make the symbiosome highly acidic (pH~4), ayRhp1 constitutes an NH4+-trapping mechanism analogous to that in mammalian renal tubule. Remarkably, ayRhp1 presence in the symbiosome membrane was higher during the day than the night. This indicates a regulatory mechanism that facilitates NH4+ delivery to alga during the day, likely to sustain high turnover rates of photosynthetic proteins, while restricting NH4+ delivery at night to maintain the endosymbiotic algae in a nitrogen-limited stage that stagnates their growth. The dynamic trafficking of proteins to and away from the symbiosome membrane is a previously unknown mechanism that contributes to metabolic regulation between symbiotic partners. Significance StatementThe endosymbiotic relationship between corals and algae relies on the coordinated exchange of metabolites. Disruption of these metabolic exchanges can result in interruption of the symbiosis; however, the underlying molecular mechanisms are poorly understood. Here we report that Acropora yongei coral host cells express ammonia-transporting channel proteins (ayRhp1), which traffic to and away from the symbiosome membrane surrounding the endosymbiotic algae. In conjunction with the acidic symbiosome microenvironment, this mechanism allows host cells to regulate nitrogen delivery to endosymbionts sustaining essential functions while restricting growth. This work provides novel mechanistic information about metabolic regulation of animal-algae symbioses, and advances our understanding of physiological mechanisms that might determine coral local adaptation, resilience, and vulnerability to environmental stress including climate change.