Nitrogen fixation in the mucus of Red Sea corals

Scleractinian corals are essential constituents of tropical reef ecological diversity. They live in close association with diazotrophs [dinitrogen (N₂)-fixing microbes], which can fix high rates of N₂. Whether corals benefit from this extrinsic nitrogen source is still under debate. Until now, N₂ fixation rates have been indirectly estimated using the acetylene reduction assay, which does not permit assessment of the amount of nitrogen incorporated into the different compartments of the coral holobiont. In the present study, the ¹⁵N₂ technique was applied for the first time on three Red Sea coral species. Significant 15N enrichment was measured in particles released by corals to the surrounding seawater. N₂ fixation rates were species specific and as high as 1.6-2 ng N day^-1 l^-1. However, no significant enrichment was measured in the symbiotic dinoflagellates or the coral host tissues, suggesting that corals do not benefit from diazotrophic N₂ fixation.