Corals first appeared in the fossil about 530 million years ago. Corals are extremely ancient animals (cnidarians) that evovled into modern reef-building forms over the last 25 millions. Corals have experienced a wide range of ocean temperatures over this period. In the late Eocene, ~ 40 million years ago, corals experienced temperatures much higher than the worst temperatures seen today, or forecast. But today, with lower temperatures than the Eocene, corals become stressed when the temperatures are elevated above what they are used to. Yet there are corals in the inter-tidal zone in the Kimberley region of northwest Australia that have adapted to 10m tides, aerial exposure for several hours, and temperatures as high as 37 C that fluctaute daily by up to 7C. These corals not only survive but thrive and calcify at the same rate as corals in more benign conditions. These are are but one example of many corals that have adapted to harsh conditions. But corals in the Galapagos became thermally stressed and suffered a mass die off in 1984 when the temperatures were elevated above 26-27C. Corals in the Red Sea become thermally stressed when the temperatures exceed 34C. So what is happening?
Most reef-building corals contain photosynthetic cells - think plants - called zooxanthellae (the proper scientific name is Symbiodinium) that live in the coral's tissues. The zooxanthellae - the word is from the modern Latin "zoo" meaning of animals and the Greek "xanthos" meaning yellow - and the coral form a highly beneficial mutualistic relationship. The corals provide the zooxanthellae with a protected environment and biological compounds they need for photosynthesis. In return the zooxanthellae - single celled algae of the variety of yellowish-brown dinoflagellates - consume carbon dioxide, produce oxygen, and help the coral to remove its wastes. More importantly, the zooxanthellae through photosynthesis supply the corals with glucose, glycerols, and amino acids. The corals use these products to make proteins, fats and carbohydrates and produce calcium carbonate. Zooxanthellae are always present in the water column as plankton and corals can adjust its population of zooxanthellae on a daily basis to meet its needs. Approximately 90% of the nutrients for a coral comes from its zooxanthellae.
In addition to providing corals with essential nutrients, zooxanthellae are the reason for the unique and beautiful colors of many stony corals. But when the corals are physically stressed by high or low temperatures, low salinity, high light intensity, or pollutants - even most sunscreens - the corals have the ability to release the zooxanthellae back into the water, or digest them for the energy. With the 'color' gone, the corals appear to be bleached stark white, hence the term "coral bleaching." This is dramatically shown in the banner photograph for this page. Without the zooxanthellae to support their metabolic processes, corals begin to starve and if left in that condition long enough will die. Some types of branching corals can only survive for ten days in this state, others such as some massive corals, can survive for weeks or even months by feeding on zooplankton. But eventually they too will die unless the zooxanthellae return.
Scientists have discovered that coral bleachings, which were first noticed in 1979, are attributed to the increased frequency and intensity of sudden large spikes in the sea surface temperature (SST). These are temperature spikes that are well above the present conditions suitable for modern corals. These spikes are not part of a natural cycle and are attributable to global climate change. The tolerance of corals to thermal stress varies widely, depends on variety of the zooxanthellae. and is influenced by the temperature regimes of specific locations. It was initially thought there was only one species of zooxanthellae, but it has since been discovered that there is a wide variety of taxa. Some of the variety's are less heat tolerant than others. The zooxanthellae on Autralia's Great Barrier Reef are from a variety that is known to be less heat tolerant, and the GBR has experienced many coral bleachings. This all helps to explain why corals in the Galpagos bleached at 26-27C, but corals in the Red Sea can survive temperature of 32-34C without bleaching.
Over the course of its life, corals are able to obtain multiple different species of zooxanthellae. The Alternate Bleaching Hypothesis theorizes that corals expel zooxanthellae that are intolerant to present conditions, in the hopes that they may acquire more tolerant ones from the surrounding sea water. The variation in thermal tolerances of different zooxanthellae and the ability of coral hosts to adapt to changing sea temperatures are important factors determining the long-term resilience of coral reefs around the world.
If the thermal stress episode is mild or brief, corals have been known to exhibit a phenomena that has been dubbed "colourful coral bleaching." Following the loss of its zooxanthellae, some reason reason the coral produces a pigmentation that is colorful.
Once a coral reef dies, it is not likely to recover its former splendor. Coral reefs in the US Virgin Islands experienced an extreme coral bleaching event in 2005 that lead to mortality. Coral reefs in the Galapagos esperienced a catastrophic bleaching event in 1984 that they have not recovered from. Corals reefs in the Florida Keys and the Bahamas are presently (August 2023) experinecing elevated temperatures that they have never been exposed to in the last forty years, and probably much longer. A coral reef that is dead is not growing and a coral reef that is not growing will eventually disintegrate due to bioerosion. The intricate structure that provides so many species with homes and food will collapse into a heap of coral rubble. Eventually the rubble will become coral sand.
In order to quantify what constitues an excessive temperature at a particular reef that results in coral bleaching, marine biologists have derived a measure called Degree Heating Week (DHW). If the value of the DHW exceeds 1C, then coral bleaching is possible, if it exceeds 4C then coral bleaching is significant, and if it exceeds 8C coral bleaching is severe and mortality is possible. DHW begins with knowing the history of the sea surface temperature (SST), which can be readily determined from satellite sensorrecords that have been acquired since 1984. This is the single greatest advantage of DHW; the temperatures can be observed remotely without the need for in-situ observations on each and every reef around the world. The DHW recognizes that the thermal tolerance of corals is influenced by the temperature regime of specific locations, but it can not assess the heat tolerance of the zooxanthellae. Nevertheless it a powerful tool that coral reef managers use, and scuba divers can take advantage of.
Corals experience thermal stress, the main cause of bleaching, when sea surface temperatures exceed 1°C (1.8°F) above the maximum summertime mean. This stress worsens as the heat anomaly persists. Degree Heating Week (DHW) shows how much heat stress has accumulated in an area over the past 12 weeks (3 months) by adding up any temperature exceeding the bleaching threshold during that time period. When DHW reaches 4°C-weeks (7.2°F-weeks), significant coral bleaching is likely, especially in more sensitive species. When DHW exceeds 8°C-weeks (14.4°F-weeks), widespread bleaching and mortality from thermal stress may occur.
In some instances corals can recover from bleaching. If conditions return to normal, and stay that way, corals can regain their zoo, return to their bright colors and survive. To date, coral recovery from bleaching has only been observed after the heat stress subsides. Which simply means prolonged warmer temperatures and other stressors can leave the living coral in a weakened state. It can struggle to regrow, reproduce and resist disease – and is vulnerable to coral diseases and mortality.
It can take decades for coral reefs to fully recover from a bleaching event, so it is vital that these events do not occur frequently. Although corals are able to adapt and acclimate, it is unsertain if this evolutionary process will occur quickly enough to prevent major reductions of their numbers.
Sir David Attenborough On The Devastating Truth About Coral Reefs
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