The boring microflora in modern coral reef ecosystems: a review of its roles

Euendolithic microorganisms (boring microflora) – cyanobacteria, algae and fungi – colonize all carbonate substrates in modern coral reefs and are distributed worldwide. Recent studies showed that in dead carbonate, they are important primary producers and are fed upon by various excavating invertebrate and vertebrate grazers, contributing greatly to biodestruction processes (bioerosion) and sedimentation. Additionally, it has been shown that in some live calcifying organisms, they either inflict damages to live tissues or provide a benefit to the host, depending on the euendolithic community involved (parasitic or mutualistic relationships). Based on those recent studies, the following question is raised: Are euendoliths key organisms in the functioning and maintenance of coral reefs? Reviewed literature includes studies on (1) the mechanisms used by euendoliths to penetrate into carbonate substrates (production of acids or chelating fluids; use of the products of photosynthesis / respiration and / or calcium pumps), (2) their roles in reef bioerosion and sedimentation (major roles), (3) their metabolism (important rates of production), (4) their interactions with their live hosts (symbiosis, mutualism and / or parasitism) and (5) the effects of various environmental factors such as eutrophication, sedimentation and rising atmospheric pCO2 on euendolith activities. The review concentrates on modern coral reef ecosystems.