Partitioning of nitrogen sources to algal endosymbionts of corals with long-term 15N-labelling and a mixing model

•We model nitrogen cycling in coral endosymbionts from 15N-labelling experiments.•Algal endosymbionts derive more than half of their nitrogen from the animal host.•The contribution of host-derived nitrogen differs depending on the coral species.•Phosphate addition to seawater enhances endosymbiotic nitrogen uptake from the seawater.•A large nitrogen pool in corals supports the endosymbiotic photosynthesis.

Corals are critical in supporting high productivity and biodiversity in oligotrophic seawaters by recycling nutrients. Here, we constructed a model of nitrogen cycling in coral endosymbionts, which provides a mechanism for the efficient nutrient metabolism of corals. First, we conducted a long-term 15N-labelling experiment on corals using flow-through aquaria, where 15N-labelled nitrate was continuously supplied to the corals Porites cylindrica and Montipora digitata for two months. After the labelling experiment, 15N isotope ratios were measured in the algal endosymbionts and their animal host and the nitrogen (N) dynamics through the endosymbiont cells were modelled. The model calculations showed that the algal endosymbionts in P. cylindrica and M. digitata derived an average of 80% and 50% of their N from the animal host, respectively. The finding indicates that the large N pool in the coral animal tissue plays an important role in supporting endosymbiotic function, photosynthesis, and consequent coral reef growth. The species-specific difference in available internal N was attributed to N biomass of the animal host per unit coral surface area. Algal N uptake from seawater was enhanced by 20% with the addition of phosphate in seawater. Thus, active endosymbiotic algal photosynthesis in oligotrophic seawater may be related to their dependence on the host-derived phosphorus as well as N.