Nitrogen Cycling in Coral Reef Ecosystems

Coral reef ecosystems thrive in nutrient-poor tropical waters through tightly regulated nitrogen transformations that support primary productivity and symbiosis. Atmospheric dinitrogen is converted into bioavailable forms by diazotrophic microbes living in coral tissues, on their surfaces and in surrounding sediments. Ammonium released by these processes fuels both host metabolism and algal symbiont growth, while subsequent nitrification and dissimilatory nitrate reduction to ammonium (DNRA) regenerate recycled nitrogen. Denitrification removes excess fixed nitrogen as gaseous dinitrogen or nitrous oxide, helping to maintain optimal nitrogen:phosphorus ratios within the holobiont. Interactions among the coral animal, Symbiodiniaceae algae and diverse bacterial assemblages create microenvironments that govern rates of fixation, regeneration and removal. Environmental stressors such as elevated temperature, acidification and eutrophication can disrupt these balances, with consequences for coral health, reef resilience and global biogeochemical cycles.

Research from Nature Portfolio

One investigation in Red Sea corals quantified the abundance of denitrifier marker genes alongside measured denitrification rates in several scleractinian species. It demonstrated that species with higher gene copy numbers exhibited elevated denitrification activity, which correlated positively with nitrogen fixation rates and Symbiodiniaceae cell densities, indicating a compensatory role in balancing new nitrogen inputs. Another study revealed active anaerobic nitrogen metabolisms within oxygenated coral tissues, documenting simultaneous ammonium oxidation, nitrate reduction and nitrous oxide production under dark conditions. Genomic analyses of isolated cobionts confirmed the presence of nitrate-respiring pathways, underscoring the significance of microoxic niches in sustaining key nitrogen conversions within the holobiont.

Nitrogen Cycling in Coral Reef Ecosystems publication trend

The graph below shows the total number of articles in nitrogen cycling in coral reef ecosystems across all publications each year (not limited to Nature Index journals).

Technical terms

Diazotroph: A microorganism capable of enzymatically converting atmospheric dinitrogen gas into ammonia or related bioavailable nitrogen.

Nitrogen fixation: The biochemical process by which dinitrogen is reduced to ammonia, providing new nitrogen to ecosystems.

Denitrification: Microbial reduction of nitrate and nitrite to gaseous nitrogen forms (N₂O, N₂), facilitating nitrogen removal.

Nitrification: A two-step microbial oxidation of ammonium to nitrite and subsequently to nitrate, regenerating oxidised nitrogen.

DNRA (Dissimilatory nitrate reduction to ammonium): Microbial conversion of nitrate directly to ammonium, retaining nitrogen within the system.

Coral holobiont: The integrated ecological unit comprising a coral animal and its associated algal and microbial symbionts.

References

1. Denitrification Aligns with N2 Fixation in Red Sea Corals. Scientific Reports (2019).
2. Discovery and quantification of anaerobic nitrogen metabolisms among oxygenated tropical Cuban stony corals. The ISME Journal: Multidisciplinary Journal of Microbial Ecology (2020).
3. Symbiodiniaceae and Ruegeria sp. Co-Cultivation to Enhance Nutrient Exchanges in Coral Holobiont. Microorganisms (2024).
4. Community structure of coral microbiomes is dependent on host morphology. Microbiome (2022).
5. Presence of algal symbionts affects denitrifying bacterial communities in the sea anemone Aiptasia coral model. ISME Communications (2022).