INTRODUCTION  

     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 -CCA) and destructive forces (mostly bioerosion). Unfortunately this equilibrium is increasingly threatened by anthropogenic activities and global change factors such as ocean acidification and rising sea surface temperature. 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. About 1/15 of the world population depends on coral reef resources (food and economic resources). Additonally, reefs form natural barriers protecting coasts and tropical islands against storms, cyclones and tsunami. These marine "oasis" are hot spots of biodiversity and represent about 170 billions of US dollars with a high potential for 'pharmaceutical' molecules.

     The rise of coral mortality induces an increase in surface areas available for colonization by the agents of bioerosion. Bioerosion is much more efficient on dead substrates than on live ones. Compare to constructive forces, bioerosion process has received relatively less attention. Recent works 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 Coral Reefs (in Articles) and Pandolfi et al. 2011 Science vol. 333 for instance).

              Healthy reef dominated by corals      Healthy reef dominated by CCA               Bleached corals

     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 taxonomy and ecological characteristics of macroborers and grazers. In contrast, a few studies focussed on boring microflora ( microborers or eu-endolithic organisms). In 2000s several studies highlighted the important role played by boring microflora in the carbonate biogeochemical cycle in coral reef ecosystems. These agents are one of the main actors of reef bioerosion (Tribollet & Golubic 2005, Coral Reefs), especially the boring siphonous chlorophyta of the genus Ostreobium (Tribollet 2008, Microbial Ecology). They are important primary producers (Tribollet et al. 2006) and are one of the main food resources to grazers, especially grazing fishes (parrotfishes) (Bruggemann et al. 1994, MEPS; Tribollet & Golubic 2005; Grange et al. 2015). Recent experiments showed that they positively respond to eutrophication (Carreiro-Silva et al. 2005, 2009, Coral Reefs & MEPS, respectively) and ocean acidification (Tribollet et al. 2009, Global Biogeoch. Cycles) suggesting that reefs may shift from net calcification towards net dissolution before the end of the century. More studies are needed to better predict the response of boring microflora to anthropogenic and climatic factors, and to better understand their roles in the reef carbonate budget. Such studies will allow better predicting the fate of coral reefs in the context of climate change.