Microbially facilitated nitrogen cycling in tropical corals

Area of Science:

• Marine Biology
• Microbial Ecology
• Biogeochemistry

Background:

• Tropical scleractinian corals host diverse microbial symbionts forming a 'microbiome'.
• This coral microbiome has the potential for various nitrogen (N) transformations, including denitrification, nitrification, nitrogen fixation, and dissimilatory nitrate reduction to ammonium (DNRA).
• Direct evidence for active N-cycling processes within tropical corals remains limited.

Purpose of the Study:

• To quantify rates of nitrogen cycling processes in tropical scleractinian corals.
• To verify the activity of denitrification, nitrification, nitrogen fixation, and DNRA within coral holobionts.
• To elucidate the role of coral-associated microbes in reef nutrient cycling.

Main Methods:

• Stable isotope techniques.
• Nutrient uptake calculations.
• Metagenomic analysis (nrfA gene abundance).

Main Results:

• Denitrification was detected but at very low rates, suggesting limited N removal by this process.
• Nitrogen fixation activity was relatively high, confirming corals as net N importers.
• Low net nitrification was observed, but substantial gross nitrification may be masked by nitrate consumption.
• Significant DNRA activity was calculated based on nrfA gene abundance and N fluxes.

Conclusions:

• The coral microbiome actively participates in multiple N-cycling pathways.
• Corals are significant importers of nitrogen to reef systems due to nitrogen fixation.
• DNRA is a substantial process in tropical corals, with critical implications for reef N budgets.
• This study provides a comprehensive quantification of N-cycling processes in tropical corals.