Abstract
•Nitrogen cycling in reef-building corals is a function of all holobiont members.•Control of nitrogen cycling may stabilize holobiont functioning under oligotrophic and eutrophic conditions.•Anthropogenic change may sway the control of nitrogen cycling, promoting coral decline.•Elevated nitrogen fixation rates may foster coral bleaching and disease.
Corals are animals that form close mutualistic associations with endosymbiotic photosynthetic algae of the genus Symbiodinium. Together they provide the calcium carbonate framework of coral reef ecosystems. The importance of the microbiome (i.e., bacteria, archaea, fungi, and viruses) to holobiont functioning has only recently been recognized. Given that growth and density of Symbiodinium within the coral host is highly dependent on nitrogen availability, nitrogen-cycling microbes may be of fundamental importance to the stability of the coral–algae symbiosis and holobiont functioning, in particular under nutrient-enriched and -depleted scenarios. We summarize what is known about nitrogen cycling in corals and conclude that disturbance of microbial nitrogen cycling may be tightly linked to coral bleaching and disease.