Scleractinian corals are the key builders of coral reef ecosystems. They live in association with microbes that are beneficial for coral health and disease resistance. Diazotrophs may be particularly important as these microbes are able to fix dinitrogen (N2) and may provide this essential nutrient to the coral host. To better understand the role of diazotrophs in nitrogen provision to the coral animal, we assessed which diazotrophs are associated with the common reef-building coral Stylophora pistillata along a depth gradient - from shallow (5m) to mesophotic (50m) depth. This was combined with an analysis of the overall prokaryotic community to assess the relative abundance of diazotrophs in the microbial communities and physiological measurements.