Uncovering unseen players of coral recruitment for reef conservation – CoralMates

The first objective will combine in situ and ex situ approaches to test the species-specific effects of CCA on various coral life stages, from larvae to juveniles. In situ approaches will include: 1) surveys of natural coral recruits-CCA interactions using ultraviolet lighting (to visualise recruits as small as 1mm in diameter) coupled with detailed taxonomic identification. 2) A year-long monitoring of natural and experimentally created interactions between coral recruits of different sizes and various CCA species. Ex situ approaches will include the study of coral larval behaviour in the water column in response to species-specific CCA exudates using flow and no flow choice chamber experiments.

For the second objective, we will: 1) determine whether specific associations (or inhibitions) between CCA and corals could be related to certain microbes and/or metabolites, 2) assess the influence of CCA on the early stages of development of the coral microbiome, and 3) identify specific bacterial species and chemical extracts that impact coral larval behaviour, settlement and survival. This objective will make use of state-of-art technologies in microbiology, chemical ecology and metabolomics.

For the third objective, we will test the hypothesis that macroalgal competition (i.e., a local stressor) and elevated seawater temperature (i.e., a global stressor) alter coral settlement by affecting CCA microbiome and metabolome and the interaction between CCA and corals during settlement. We will also seek to distinguish the effects on CCA chemical signals and larval perception of these signals.

The fourth objective will combine laboratory rearing, field studies of naturally settled corals, and ecological modelling to evaluate the cost-benefits of undertaking ex situ settling on experimental substrata with beneficial CCA and/or chemicals followed by in situ outplanting versus relying on natural processes.

The projet has already produced one publication who experimentally demonstrated that not all species of crustose coralline algae are beneficial for the survival and growth of coral juveniles. We found considerable variability in both the outcome and process of competition between crustose coralline algae and corals.

Our results suggest several possible mecanisms depending on the exposure of the microhabitat. By examining the causes of death of the recruits in the experimental treatments, we conclude that, in subcryptic habitats, crustose coralline algae can reduce the survival and/or growth of coral recruits via direct competitive overgrowth, while, in exposed habitats, they can enhance coral recruitment by alleviating competition with turf algae.

Many coral reef scientists and managers equate crustose coralline algae (all species) to improved recruitment potential for corals. These results challenge this view using a robust experimental approach, with important consequences for our predictions of reef recovery potential and the way we manage reefs.

The projet is on going.

Jorissen H, Baumgartner C, Steneck RS, Nugues MM (2020) Contrasting effects of crustose coralline algae from exposed and subcryptic habitats on coral recruits. Coral Reefs DOI 10.1007/s00338-020-02002-9

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.