Corals produce dispersive, planktonic larvae, which must recruit onto appropriate benthic surfaces to complete their life cycle. The recovery of coral populations thus critically depends on the settlement (when larvae first attach to the benthos) and subsequent survival (recruitment) of new individuals. Together with scleractinian corals, crustose coralline algae (CCA) are major framework builders and carbonate producers on tropical reefs. They also host a diversity of microorganisms that are thought to play a role in coral settlement and metamorphosis. A recent study suggests that CCA species facilitating coral settlement have higher abundances of bacteria that inhibit the growth and/or biofilm formation of coral pathogens. Vice versa, species inhibiting coral settlement have higher abundances of coral pathogens and cyanobacteria. Certain macroalgae that compete with adult corals are also known to induce coral bleaching and disease and several coral pathogens have been found on macroalgal surfaces. Thus coral larvae could use differences in bacterial community composition on CCA species to assess the suitability of settlement substrates and selectively settle on CCA species that contain beneficial bacteria. Bacterial recognition by coral larvae could operate via chemicals produced by these bacteria. However, presently suggested compounds (e.g. tetrabromopyrrole) have been shown to induce coral settlement and metamorphosis without attachment. Two classes of CCA cell-wall inducing compounds have been proposed as more effective inducers, but none of these compounds have been fully characterised. Thus, the different microbial and chemical cues that mediate the positive interaction between corals and certain CCA species are still unclear.
This project proposes to combine field observations, laboratory experiments and state-of-art technology in microbiology, genetic, chemical ecology and metabolomics to uncover the intimate links between corals and CCA. Our first two aims are: 1) to identify the species of CCA playing a key role in coral recruitment and 2) to identify the microbes and biomolecules mediating this interaction. Climate stressors could affect the fragile nature of the interaction between CCA and corals and interact with local scale stressors to disrupt coral recruitment processes. Thus, our third aim is: 3) to determine how local and global stressors affect the chemical and microbial interactions between CCA and corals. Finally, our fourth aim is: 4) to assess whether these « unseen players » (algae, microbes and biomolecules) could be used to improve coral reef restoration. This aim includes a process-based approach that evaluates the cost-benefits of active restoration of corals versus allowing natural processes of colonisation to occur unaided.
Given the vulnerability of coral reefs to local and global disturbances, this project responds to the pressing need to understand the mechanisms of coral recruitment to aid reef recovery and sustain ecosystem resilience. It will contribute to a better knowledge of the players (from organisms to molecules) and mechanisms driving the ecology, functioning and evolution of the environment and its associated biodiversity in order to better anticipate the impacts of human use and climate change. Coral-algal-microbial interactions have been extensively studied over the past two decades, often showing negative effects of algae and microbes on corals and precipitating shifts from coral to algal dominance. Here, our research focuses on the positive interactions between these organisms. It will significantly improve our knowledge of the role of microorganisms and chemicals in the fine-scale dynamics of coral recruitment and allow the discovery of new molecules that have fundamental and applied interests. The final aim will enable the development of restoration approaches that could reduce the use of wild corals and benefit local economies.
Madame Maggy Nugues (Centre de recherche insulaire et observatoire de l'environnement)
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
LECOB Laboratoire d'Ecogéochimie des Environnements Benthiques
CRIOBE Centre de recherche insulaire et observatoire de l'environnement
ENTROPIE Ecologie marine tropicale dans les Océans Pacifique et Indien
Help of the ANR 600,494 euros
Beginning and duration of the scientific project: March 2019 - 48 Months