Coral reefs nitrogen cycling traced by stable isotopes-labeled compounds

Grant Data
Project Title
Coral reefs nitrogen cycling traced by stable isotopes-labeled compounds
Principal Investigator
Dr Thibodeau, Benoit   (Principal investigator)
Dr Duprey Nicolas Noel   (Co-Investigator)
Dr Baker David Michael   (Co-Investigator)
Start Date
Completion Date
Conference Title
Presentation Title
Environmental Science, Oceanography, Coral reef, Biogeochemistry, Stable Isotope
Environmental Studies and Science,Earth Sciences
Physical Sciences
Block Grant Earmarked for Research (104)
HKU Project Code
Grant Type
Seed Fund for Basic Research
Funding Year
Studies about coral reefs have generally focused on the cycling of dissolved inorganic nitrogen (DIN = nitrate (NO3-) + nitrite (NO2-) + ammonium (NH4+)) with a typical water column concentration range of 0.1 to 1 μmol L-1 (2 to 9 μmol L-1 in Hong Kong). While, the concentrations of dissolved organic nitrogen ([DON]) can be high (~5 μmol L-1), and often exceed those of DIN in coral reefs, data on the sources, sinks and turnover of DON are scarce due to the technical challenges linked to their analysis. This paucity of data has prevented the identification of DON sources and sinks and thus limited the evaluation of the role of DON in sustaining the high productivity of coral reefs (Atkinson 2011; Suzuki and Casareto 2011). Recently, the first DON stable isotope measurements over a coral reef suggested that a significant amount of DON might be released from N-fixer organisms over relatively pristine reefs, thus reefs can act as a source of DON (Thibodeau et al., 2013). To our knowledge, no DON investigation has been carried in Hong Kong waters. The main goal of this study is to identify how nitrogen cycles between the different forms of dissolved nitrogen and how it is incorporated in coral reefs around Hong Kong. This project will provide basic knowledge about the N-cycle under Hong Kong human-impacted conditions and thus will lay down the premises for our future larger N-cycle-oriented projects. We aim to respectively quantify the importance of DIN and DON in the coral nutrient consumption. Moreover, our experimental set-up is also suitable to investigate the origins of the oxygen atom of nitrified nitrate, which has been a subject of debate for the last decades. This highly valuable information will help us build an accurate dynamic isotopic model including both 15N and 18O of dissolved nitrate. Bullet point Objectives: ‧ Quantify the rate of release & uptake of nitrate, ammonia and DON ‧ Quantify the rate of N-assimilation by the coral and identify which N-pool contributed the most ‧ Identify the origin of the oxygen atom in nitrified nitrate (water vs atmosphere) ‧ Build a model of the basic N-cycle in HK coral reefs (exploratory)