Assessing the effects of iron enrichment across holobiont compartments reveals reduced microbial nitrogen fixation in the Red Sea coral Pocillopora verrucosa

• Published in: Ecology and evolution Vol. 7; no. 16; pp. 6614 - 6621
• Main Authors: Rädecker, Nils, Pogoreutz, Claudia, Ziegler, Maren, Ashok, Ananya, Barreto, Marcelle M., Chaidez, Veronica, Grupstra, Carsten G. B., Ng, Yi Mei, Perna, Gabriela, Aranda, Manuel, Voolstra, Christian R.
• Format: Journal Article
• Language: English
• Published: England John Wiley & Sons, Inc 01.08.2017; Wiley Open Access; John Wiley and Sons Inc
• Subjects: Coral reefs; Corals; diazotroph; Dinoflagellates; Enrichment; Environmental conditions; Environmental Sciences; Feedback loops; holobiont; Iron; Life Sciences; Microorganisms; Negative feedback; Nitrogen; Nitrogen fixation; Nutrient availability; nutrient limitation; Nutrients; Original Research; Photosynthesis; Pocillopora verrucosa; Population density; Productivity; Recycling; Respiration; Symbiodinium; symbiosis; Zooxanthellae; Red Sea; nutrient limitation; holobiont; coral reefs; diazotroph; symbiosis; Symbiodinium
• ISSN: 2045-7758; 2045-7758
• DOI: 10.1002/ece3.3293

The productivity of coral reefs in oligotrophic tropical waters is sustained by an efficient uptake and recycling of nutrients. In reef‐building corals, the engineers of these ecosystems, this nutrient recycling is facilitated by a constant exchange of nutrients between the animal host and endosymbiotic photosynthetic dinoflagellates (zooxanthellae), bacteria, and other microbes. Due to the complex interactions in this so‐called coral holobiont, it has proven difficult to understand the environmental limitations of productivity in corals. Among others, the micronutrient iron has been proposed to limit primary productivity due to its essential role in photosynthesis and bacterial processes. Here, we tested the effect of iron enrichment on the physiology of the coral Pocillopora verrucosa from the central Red Sea during a 12‐day experiment. Contrary to previous reports, we did not see an increase in zooxanthellae population density or gross photosynthesis. Conversely, respiration rates were significantly increased, and microbial nitrogen fixation was significantly decreased. Taken together, our data suggest that iron is not a limiting factor of primary productivity in Red Sea corals. Rather, increased metabolic demands in response to iron enrichment, as evidenced by increased respiration rates, may reduce carbon (i.e., energy) availability in the coral holobiont, resulting in reduced microbial nitrogen fixation. This decrease in nitrogen supply in turn may exacerbate the limitation of other nutrients, creating a negative feedback loop. Thereby, our results highlight that the effects of iron enrichment appear to be strongly dependent on local environmental conditions and ultimately may depend on the availability of other nutrients. We here set out to identify the effects of iron enrichment in the coral Pocillopora verrucosa, a foundation species on the nutrient‐poor reefs of the Red Sea. In contrast to previous reports, we did not observe effects on gross photosynthesis and the algal symbionts. Conversely, overall respiration rates increased and gross nitrogen fixation by associated microbes dropped by >80 %. Based on these data, we propose that the Red Sea coral P. verrucosa is not iron‐limited in its natural environment, which may be attributed to high iron concentrations relative to the limiting macronutrients.