Coral-associated denitrification is seasonally variable and species-specific.

Nitrogen (N) plays a critical role in coral growth, but maintaining an N-limited state is essential for coral-algal symbiosis stability. Coral-associated denitrifiers may help regulate excess N, though denitrification in corals remains poorly quantified. To better understand species-specific and environmental effects on the denitrification pathway, we investigated year-long denitrification dynamics in four Red Sea corals, using acetylene inhibition assays alongside physiological and environmental measurements. All species exhibited measurable denitrification activity, ranging from 0 - 0.8 nmol N cm-2 h-1 for Stylophora pistillata and Acropora sp., 0 - 0.4 nmol N cm-2 h-1 for Millepora dichotoma, and 0 - 2.0 nmol N cm-2 h- 1 for Dendrophylliidae indet. Denitrification displayed seasonal patterns, with higher rates in the spring/summer compared to autumn/winter, and we identified temperature, dissolved organic carbon (DOC) and nitrate availability as key environmental drivers. Lastly, we observed up to fivefold higher denitrification rates in the fully heterotrophic azooxanthellate coral Dendrophylliidae indet, than among mixotrophic species harbouring Symbiodiniaceae. Our findings indicate that denitrifiers use both photosynthetically derived and environmental carbon (C), with DOC central in maintaining tight coupling of C and N cycling in coral holobionts. Additionally, denitrification is modulated by environmental conditions, highlighting its vulnerability to environmental change.