Doctoral Programme on Marine Ecosystem Health and Conservation
 The MARES Researchers and their Research
Algal-coral interactions on coral reef ecosystem across multiple algal species and functional groups.
PhD Code: MARES_x45_2010:
  • Host institute 1: P11 - Université Paris Marie Curie (UPMC)
  • Host institute 2: P1 - Ghent University
Research fields:
  • T2 - Understanding biodiversity effects on the functioning of marine ecosystems
  • Claude, Elisabeth Payri
  • Olivier De Clerck
Contact Person and email: Claude, Elisabeth Payri -

Subject description
Tropical coral reefs, the rainforests of the sea, feed a large portion of the world's population, protect tropical shorelines from erosion, and harbour a vast reservoir of biodiversity providing various goods and services critical to human societies (Moberg & Folke, 1999). Unfortunately, they are also among the most fragile ecosystems and are now beset by problems ranging from local pollution and overfishing to outbreaks of coral disease and overgrowth by macrophytes (Hughes et al, 2007; Bellwood et al, 2004). Most scientists agree that those threats all result in loss of biodiversity, of seascape complexity and associated services (Dodge et al. 2008). The losses are very often associated with shifts from coral to algal dominance, and macrophytes are recurrently suggested to play a major role in the decline (Done, 1992). McCook et al. (2001) reviewing the coral-algal interactions, proposed 6 mechanisms by which algae can compete with corals: 1/overgrowing, 2/shading, 3/abrasion, 4/allelopathy, 5/recruitment barrier and 6/epithallial sloughing. All those mechanisms have been more or less documented and the recent work of Rasher and Hay (2010) demonstrate the significance of chemical ecology in the coral-algal-herbivore interactions and the impact on coral reef. Numerous native as well as non-indigeneous algae (invasive species) have been involved in algal overgrowing and reef degradation and there are a limited number of reports of the reversal of the phase shifts (Stimson & Conklin, 2008). Several groups of macroalgae including Halimeda, Caulerpa, for the greens, Dictyotales (Dictyota, Lobophora, Padina) and Fucales (Turbinaria, Sargassum) for the brown and Acanthophora, Asparagopsis, Euchema, Gracilaria and Portieria for the reds are currently associated with algal blooms and coral reef degradation. In New Caledonia, various coastal shallow reefs and outer reef slopes are facing overgrowing of certain species of Halimeda, Dictyotales (Dictyota and Padina), Sargassum and Asparagopsis deeply changing the seascape in the affected areas. The aim of the project is 1/ to better document the phase-shift and to give a good mapping of the knowledge at a local (reef) and regional (island) scale, 2/ to precise the status of the species involved paying special attention to cryptic diversity and indigenous vs introduced algal species, 3/ to assess the outcomes and mechanisms involved in seaweed-coral competition across multiple seaweed species and functional groups.
1. Mapping of knowledge. Several reefs in New Caledonia are particularly well-studied at geomorphology and habitat levels including information on coral reef community distribution, fish community structure and organisation (species diversity/trophic structure/ recruitment), and document contrasted situation regarding anthropogenic influences; part of those areas is located in Marine Protected Areas. Documentation on specific algal cover will be focused in those specific sites where most of the environmental information can be compiled into Geographical Information Systems. Distribution of algal occurrences and all associated variables will be spatially analysed to provide information about the different scenarios based on algal vs coral cover and to establish if possible explicit relationship between variables. Competition experiment developed hereafter (point3) will be carry on sites selected among these well-studied areas in contrasted sites regarding fishing pressure
2. Diversity. Given the widespread nature of cryptic diversity among species characterized by a relatively simple morphology, it is necessary to apply a barcoding approach to characterize the diversity of the algae which compete with corals for space and light. The genera of interest are reknown for their high levels of sibling species and especially the clades consisting of coral- associated species appear to remarkably speciose (e.g. Dictyota, Portieria). We hypothesize that this diversity be translated in the competitive behaviour (e.g. allelopathy) displayed by the respective species. Collected specimens will be screened for the commonly applied barcoding markers which are group specific and generated sequences will be integrated in the Barcode of Life project and serve a reference for future studies.
3. Competition experiments. The project incorporates in situ as well as laboratory experiments to assess the effect of various algae on coral health. Field experiments will document the negative effects of physical contact between coral colonies and various algae. Because visual assessment of coral bleach and mortality can be subjective we will combine the data with in situ measurements of photosynthetic efficiency (~zooxanthellae) using PAM fluorometry. To verify the allelopathic nature of the seaweed-coral competition (as opposed to abrasion or shading), the field experiments will be complemented with laboratory experiments whereby coral species are subject to lipid soluble extracts of various algae. Fish herbivory control of algal-coral interaction will be tested on fished – site vs protected areas or using caging experiments, under various scenarios regarding the balance between algal and coral cover.

Moberg, F., Folke, C. (1999). Ecol. Econ, 29:215-233.
Hughes, TP et al, (2007). Curr. Biol., 17:360-365.
Bellwood, DR et al, (2004). Nature, 429:827-833.
Dodge, RE et al. (2008). Science, 322:189b-190.
Done, T. (1992). Hydrobiologia, 247:121-132.
McCook, L. et al. (2001).Coral Reef, 19:400-417.
Rasher, D.B., Hay, M.E. (2010). PNAS, 107, 21:9683-9688
Stimson, J., Conklin, (2008).Coral Reef, 27:717-726.

Expected outcomes
-better quantification of various scenarios of algal-coral interaction and competition in coral reef ecosystem.
-enhanced algal diversity knowledge for the New Caledonian region, with a particular highlighting on species which compete with corals.
-contribution to Barcode of Life project by producing new sequences -enhanced understanding of coral-algal-herbivore interactions and allelopathy in coral decline/health
-potential new natural compounds from coral reef algae
-value-added to monitored areas within the Marine Protected Areas network in New Caledonia (UNESCO World Heritage)
-benefits to managers/end-users of MPA through dissemination of information about significance of herbivory control of algal development

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Last update on 04/12/2019 -
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