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Coral reefs are among the most diverse and complexe marine ecosystems. Their maintenance results from the balance between constructive forces (principally growth and calcification of corals and red coralline algae) and destructive forces (mostly bioerosion). More and more anthropogenic activities and global change threat the equilibrium between those forces. Factors such as overfishing, eutrophication, sedimentation, rising sea surface temperature and pCO2 contribute to the mortality of corals and an accelarated degradation of reef framework putting in jeopardy the survival of coastal and insular human populations. 1/6 of the world population depends on coral reef resources. Coral reefs provide food and economic resources and are natural barriers against storms, cyclones and tsunami. The rise of coral mortality induces an increase in variety and surface area available for colonization by the agents of bioerosion; bioerosion being intensified on dead substrates than on live ones. In comparison to constructive forces, bioerosion has received relatively less attention. Recently studies have emphasized the importance of studying simultaneously constructive and destructive forces in order to assess the state of health of coral reefs as well as their carbonate balance (see Tribollet and Golubic 2005 RESUME). healthy reef dominated by corals healthy reef dominated by algae bleached corals (maybe dead)    Bioerosion is due to a variety of organisms, which comprises grazers (gastropods, sea urchins and fish), macroborers (sponges, polychaetes, bivalves) and microborers (cyanobacteria, algae, fungi). Bioerosion studies assessed in particular the taxonomy and ecological characteristics of macroborers and grazers. In contrast, a few studies focussed on the boring microflora (= microborers = endolithic organisms). parrotfish (grazer) sea urchin (grazer) boring sponges (macro-) boring bivalves (macro-) boring microflora (micro-borers)   
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The purpose of my researches is to highlight the role of the boring microflora in coral reef functioning and persistence. Therefore, I investigate: 1- its biodiversity 2- its role as primary producers and agents of bioerosion in different substrates 3- its interaction with the other borers, macroalgae and live coral tissues 4- the effects of environmental factors such as light intensity, eutrophication, sedimentation and rising atmospheric pCO2 on its activities (photosynthesis, dissolution of carbonates). |
I. Boring microflora are one of the main agents of bioerosion in dead carbonate substrates (dead corals, coral rubbles), under different environmental conditions (gradient of eutrophication-sedimentation on the northern GBR, Australia) and over time (3 years period). See Tribollet et al. (2002) and Tribollet and Golubic (2005) - RESUME. II. The boring microflora and grazers are interdependant. Patterns of grazing and microbioerosion as well as total bioerosion are similar; microborers in synergy with grazers being the main agents of bioerosion. Rates of grazing, microbioerosion and total bioerosion increase with distance from shore on the northern Great Barrier Reef, i.e. from turbid-eutrophic inshore reefs to oligotrophic ("pristine") offshore reefs. In contrast, macrobioerosion rates decrease with distance from shore. Macroborers are mainly filter feeders and depositivores which depend on the water column. Turbid reefs, where a lot of organic particles are in suspension, allow the development of important communities of macroborers while such conditions limit the development of endolithic communities. See Tribollet et al. (2002) and Tribollet and Golubic (2005) - RESUME. III. The principal microborer in the long-term process of microbioerosion in dead corals of the Great Barrier Reef is the chlorophyte Ostreobium quekettii. Its contribution to microbioerosion rates varies between 70% and 90% and rates can reach 1 kg CaCO3 dissolved per square meter after one and three years of exposure. IV. Boring microflora are major primary producers in dead corals and coral rubbles of Hawaiian coral reefs (Kaneohe Bay). Their contribution in dead coral production varies between 32% and 46% depending on the epilithic communities (turf algal communities or coralline algae, depending on the environmental conditions of the reefs studied). See Tribollet et al. (2006) J. Phycol. - RESUME. V. High atmospheric pCO2 (750 ppm) appears to have no effect on endoliths net photosynthetic rates under experimental conditions and over a 3 months period. Endolithic phototrophs may be at saturation under normal pCO2 or were limited by tank nutrient concentrations. In contrast, high pCO2 decreased net photosynthetic rates of crustose coralline algae. See Tribollet et al. (2006) GCB - RESUME. |
