Nitrogen (N) is a limiting nutrient in highly productive tropical coral reefs despite its key role for coral metabolism. This requires efficient N (re)cycling within the coral holobionts (i.e., the coral animal with its algal symbionts and associated microbes). At the same time, high internal N concentrations can be detrimental to the coral holobiont by shifting the internal system from N limitation towards P limitation, which has been associated with increased bleaching susceptibility. The coral associated microbiome may play a key role in regulating internal N availability by either providing bioavailable N through fixation of atmospheric dinitrogen (N2) or alleviating the coral holobiont from excess N through denitrification. N2 fixation by diazotrophs is highly susceptible to shifts in environmental parameters (e.g., temperature, light), whereas the potential effects of the environment on denitrification are so far unknown. Recent research suggests that both key N-cycling pathways occur simultaneously in coral holobionts, appear to be correlated, and are likely fuelled by photosynthates provided by their algal symbionts. Based on the available evidence, I hypothesized that a stable coral holobiont is characterized by an internal balance of C and N availability, facilitated by the complex interplay between associated denitrifiers, diazotrophs and algal symbionts within the coral holobiont. A deviation from this balance can lead in an interruption of N limitation for the algal symbionts and may ultimately result in reduced coral holobiont health. Project DECODE (Determining the drivers of coral-associated denitrification), combined field studies in seasonal resolution and manipulative laboratory experiments employing a variety of methodologies from stable isotope analysis to gas chromatography, and molecular techniques.
Awards
- Deutsche Forschungsgemeinschaft