Hermatypic corals or hard corals are the primary reef-building corals, that house single-celled microalgae, called zooxanthellae (Symbiodinium sp), the symbiotic relationship benefits both partners: The coral obtains food from the plant photosynthesis, the microalgae benefit from nutrients released as waste by the coral and both live by limiting the flow of nitrogen and other essential nutrients, zooxanthellae selectively leak amino acids, sugars, complex carbohydrates, and small peptides across the host-symbiont barrier, these compounds provide the host with a supply of energy. When conditions are stressing for coral reefs, the symbiotic algae zooxanthellae are lost from the tissues of the polyps as a response to high temperature (as a primary mechanism), low temperature, high ultraviolet light (during periods of calm clear water), and many other stimuli, this phenomenon is called “bleaching”.
Since the Industrial Revolution, the decline of coral reefs by natural factors and antropogenic activities have been considered a serious geographic problem and a new challenge to the health of coral reefs, this antropogenic activities have contributed to increase the impacts and pressures on them, and it makes changes in the composition of biogeochemical cycles. The main environmental factors affecting the development of reef building corals in a local or regional scale are high temperatures such as light, sediments, nutrients, ultraviolet radiation and salinity, all of them play significant roles and together as a synergies could intensify the bleaching event and the survivorship of reef building corals and a range of other coral reefs associated invertebrates.
In fact, elevated temperatures indicate the main cause of mass bleaching events on a regional or local scale, the increase of 1°C on the Superficial Sea Temperature for a long period could trigger mass bleaching events. As well, the intensity of various forms of solar radiation has long been suspected to play a role in bleaching events, when water is clear the penetration of UVR is high and it has preceded some heat stress on the metabolism of symbiotic algae, it produces a fail in one of the two photosystems, (PS II) and it has a complete inhibition of photosynthetic oxygen when zooxanthellae are exposed to high light under heat stress, the results begins to denature the proteins that make up the photosynthetic components in the reaction center of zooxanthellae, its failure of the ability of the dark reactions to process photosynthetic energy and increased sensitivity of these organisms to photoinhibition. High light intensities increasing light levels lead to the over-reduction of the light reactions and the production of potentially harmful products such as oxygen free radicals and if the oxygen free radicals are not detoxified by several enzyme systems will rapidly lead to cellular damage, for these reasons zooxanthellae is expulsed to avoid damage in the polyp tissues. The overriding conclusion is that bleaching is due to a lowering of the sensitivity of zooxanthellae to photoinhibition.
Nevertheless, is hard to understand the presence of bleaching events, it also varies across time and space, and it can also differ between colonies that are located side by side, at a geographic scale, even species and it depends on the dynamic of environmental variables that found in different regions of coral reefs (Brown, 1997). Scientific research keeps trying to relate the different variables with the mass bleaching events in different parts of the world to identify those gaps.
Regarding damages caused by bleaching events, it has found negative effects in the development of coral reefs, such as low rates of sexual reproduction, reduced reef productivity and growth, reduction in zooxanthellae density, loss of photosynthetic pigments, loss of partial or total color, and the denaturing of proteins, carbohydrates, and lipids, that could make changes in the production of organic compounds and causes disrupt in biogeochemical cycles and it changes the community structure.
Even with these damages, coral reefs will respond to the local stressors such as high temperatures intensities, high light, salinity, nutrients and they also show an ability to acclimatize or adapt fast enough to these changes and it will be able to survive bleach events or will die. Through bleaching events, coral reefs could survive, and their symbiotic relationship could return to normal conditions if the environmental factors lead it or to adapt the populations that are better able to tolerate higher temperatures.
As consequences of this coral bleaching event of greatest concern are acute episodes of high mortality and protracted debilitation of survivors in the form of diminished growth and reproductive rates, corals with branching growth forms, rapid growth rates, and these tissue layers appear to be most sensitive to bleaching, and usually, die if seriously bleached, but the slow-growing, thick-tissue, massive corals appear to be less sensitive and commonly recover from all but the most extreme episodes, bleaching thus selectively remove certain species from reefs and can lead to major changes in the geographic distribution of coral species and reef community structures. It is important to mention, that this phenomenon not only affects coral reefs, but reef-building corals also provide much of the primary productivity of coral reef ecosystems, and this production is consumed directly by invertebrates and fishes associated with coral reefs, and the reduction in the abundance and diversity of reef-building corals are likely to influence the majority of other coral reef organisms, furthermore bleaching events affect those organisms with the same symbiotic relationship with the microalgae Symbiodinium sp.
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