Saving coral reefs
So why are we assisting to the coral bleaching phenomenon? ‘When corals are under stress, such as when the temperature of the sea surface increases and the light intensity is high, a process takes place which appears to originate from the zooxanthellae’, explains Stephane Roberty. ‘All the information that teams around the world have been able to gather indicates that this stress has an impact upon the photosynthesis mechanisms of microalgae. They thus produce excessive quantities of free radicals which will lead, through complex processes, to cell death, the digestion or expulsion of zooxanthellae by the coral. The coral loses a major part of its population of microalgae, its tissues become transparent, thus leaving its white exoskeleton visible. However, corals need to obtain a significant amount of their nutrition through symbiotic microalgae which, because they disappear, no longer play this feeding role. The coral now depends exclusively upon its consumption of zooplancton (which are not necessarily common in tropical waters) and its reserves. If the stress doesn’t last, the remaining zooxanthellae will multiply and recreate a population as before. If not, the animal dies, leaving only its exoskeleton behind, which will rapidly be colonised by other organisms. This is what we see during coral bleaching.’ Photosynthesis in questionIt is not yet possible to act on this phenomenon, because the underlying mechanisms to this reaction have not been fully identified. However, as Fabrice Franck explains, one of the stages leading to coral bleaching has recently been identified and has been the subject of a publication by the Liège team (1): ‘Under normal circumstances, in photosynthetic organisms and thus in zooxanthellae, light induces the move of electrons from water to CO2. When the light is too intense or under conditions of heat stress, the algae absorb too much light energy in relation to what they can process. The photosynthetic apparatus is thus subject to excess energy. With the help of Pierre Cardol, we have tried to understand what was the becoming of this energy.’ Defence mechanismFortunately, these microalgae possess mechanisms which reduce the impact of these free radicals. But this raises other questions: are these antioxydant capacities insufficient, and is the Mehler reaction really at the origin of the phenomenon of coral bleaching? (1) Roberty S, Bailleul B, Berne N, Franck F, Cardol P (2014) PSI Mehler reaction is the main alternative photosynthetic electron pathway in Symbiodinium sp., symbiotic dinoflagellates of cnidarians. New Phytologist 204:81-91 |
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