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 are microbes that live in association with the coral host and may help regulate excess N, though denitrification in corals remains poorly understood. 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 Tubastrea coccinea . We observed seasonal trends in denitrification activity, with generally higher rates in the spring/summer compared to autumn/winter, and identified temperature, dissolved organic carbon (DOC) and nitrate availability as key environmental drivers. Lastly, we observed up to 5 times higher denitrification rates in the fully heterotrophic azooxanthellate species T. coccinea than in the three mixotrophic zooxanthellate species. Our findings show that denitrifiers use both photosynthetically derived and environmental 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.