 |
What are the effects of coral bleaching?
| Coral bleaching is terminal to reefs because of the reliance of coral on zooxanthellae. Coral provides zooxanthellae with a protected living environment in direct sunlight, and also contributes carbon dioxide and ammonia to the zooxanthellae, which are necessary for photosynthesis. Zooxanthellae reciprocate by providing the host coral with nutrients and compounds that are essential for its growth; zooxanthellae supply the coral with a source of energy through photosynthetically derived carbon compounds as well as the appropriate amino acids needed for ultra-violet light protection and improved calcification (Zielke et al. 2005). |  |
 |
Corals can also receive nutrients through predation by using stinging cells on their tentacles, called nematocysts, to catch planktonic organisms (Stachowicz & Lindquist 2000), however, only a small fraction of the sugars and amino acids that corals receive are through this acquisition strategy, and therefore it cannot survive without its photosynthetic partner (West & Salm 2003). It was once thought that only one dinoflagellate species, Symbiodinium microadriaticum, could exist symbiotically with coral, however, now many species of dinoflagellate algae have been found living within corals, and possibly more than one species within the same individual coral (Rowan & Powers 1991). |
|
When host coral tissue is depleted of its symbiotic zooxanthellae, or a reduction in the photosynthetic pigment within the zooxanthellae occurs, the coral starts to “bleach” (Kleppel et al. 1989). Most coral species begin to appear bleached when they have lost over sixty percent of their zooxanthellae, and each zooxanthellae typically loses fifty to eight percent of its pigments (Glynn 1996). A dissociation of the coral-algal mutualism and cell adhesion dysfunction occurs, resulting in a detachment of the coral’s endodermal cells with the symbiotic zooxanthellae (Gates et al. 2005). Following the detachment of the zooxanthellae, the coral appears white and bleached due to the calcium carbonate skeleton that shows through the transparent cnidarian tissue, which lacks pigment. If the bleaching is not too severe, the coral can proceed in “starvation mode” for up to four months and conserve energy until conditions improve and it can regain algae (Economist 2000). However, if the conditions do not ameliorate and zooxanthellae depletion is prolonged, the host coral will die. |
