Coral Reef Bleaching
By guest writer, N. M.P
Coral reef bleaching is the whitening of corals, which is a common stress response of corals to various disturbances.
In a coral reef, there is a mutually beneficial partnership between the polyps (coral animal) and zooxanthellae (millions of tiny, single-celled algae). The polyps grow a hard carbonate exoskeleton that provides support and protection for the body of each polyp. In turn, the zooxanthellae live within the tissues of the polyps and like any other land plants, use light and dissolved carbon dioxide in the process of photosynthesis to generate energy-rich compounds and oxygen. Part of these nutrients is transferred to the host coral animal.
In this way, the coral protects the zooxanthellae, while the zooxanthellae nourish the coral. Reefs grow best in shallow warm waters where enough light can reach the coral and its zooxanthellae.
Coral reef bleaching occurs when the zooxanthellae die on a large scale or when the photosynthetic pigments within the zooxanthellae are damaged on a large scale.
Coral reef bleaching events are on the rise in both frequency and extent worldwide in the past 20 years. Recent accelerated coral reef decline are mainly associated with man-made causes like overexploitation of the seas, overfishing, nutrient overloading, etc. In turn, the natural disturbances to the coral reefs include violent storms, flooding, high and low temperature extremes, El Nino Southern Oscillation (ENSO) events, etc.
Global climate change, global warming and increased ultra-violet exposure from ozone depletion have been largely linked to the increase in coral reef bleaching events in recent years. Besides rising ocean temperature often associated with global warming, increases in sea levels can also suppress coral growth or destroy coral reefs by drowning the reefs or reducing the levels of light reaching the reefs.
When the corals are subjected to stressors, for example thermal stress brought about by increase in the temperatures surrounding the corals, the normal photosynthetic processes in the zooxanthellae are disrupted. The photosynthesis process breaks down and the zooxanthellae may start producing oxygen radicals instead. These oxygen radicals damage both the photosynthetic systems in the zooxanthellae and the zooxanthellae itself, leading to bleaching. “Superoxides” such as hydrogen peroxide are also created as toxic by-products of this process. These toxins results in coral stress reactions which lead to further coral reef bleaching.
When coral reefs bleach, the tissues of the coral become transparent and they appear white. When coral polyps are injured due to heat stress, they could expel large numbers of zooxanthellae from their tissues.
If the high temperatures are short-lived, the remaining zooxanthellae that is left behind within the coral tissue divide rapidly once the heat stress is removed and the coral slowly regains its color and survives. The corals that survive bleaching may continue to suffer long-term effects like reduced growth and reproduction and they can be more vulnerable to diseases.
If the stressful conditions are prolonged or particularly severe, and the density of zooxanthellae remains low, many corals will die. There is great variation in the resilience of different coral species and colonies toward environmental stressors.
Many coral reef provinces have been permanently damaged by warm sea temperatures. One of the most severely affected areas is in the Indian Ocean. Up to 90% of coral cover has been lost in the Maldives, Sri Lanka, Kenya and Tanzania and in the Seychelles. Major coral reef bleaching occurred in Hawaii from 1996 and in 2002. Significant bleaching occurred in the Mediterranean Sea in 1996.
Studies show that if global temperatures increase by 2 °C, many corals may not be able to adapt quickly enough physiologically or genetically. That could mean the death of many of our coral reefs on this planet.
I think that we should take action now to save our coral reefs because if we do not, one of the most attractive and life-teeming environments on this planet will be lost forever.
Read related articles on climate change effect and the other consequences of global warming.
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