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.

Figure 1. Coral bleaching and sea surface temperature, June 2019-December 2019. Source: http://reefgis.reefbase.org/