Coral bleaching

Coral bleaching is the process when <a href="/facts/Coral/j7fBLrpw">corals</a> become white due to loss of <a href="/facts/Symbiosis/MrBArRGa">symbiotic</a> <a href="/facts/Algae/U1ftVKU3">algae</a> and <a href="/facts/Photosynthesis/TMV7w693">photosynthetic</a> pigments. This loss of pigment can be caused by various stressors, such as changes in water temperature, light, <a href="/facts/Salinity/X6mB1YAM">salinity</a>, or nutrients. A bleached coral is not necessarily dead, and some corals may survive. However, a bleached coral is under stress, more vulnerable to starvation and disease, and at risk of death. The leading cause of coral bleaching is <a href="/facts/Effects_of_climate_change_on_oceans/n6mDQJQq">rising ocean temperatures</a> due to <a href="/facts/Climate_change/rEN4BDE7">climate change</a>. 
Bleaching occurs when coral <a href="/facts/Polyp_(zoology)/GFMuk9Mz">polyps</a> expel the <a href="/facts/Zooxanthellae/e4qjDWyN">zooxanthellae</a> (<a href="/facts/Dinoflagellate/jcQuSgM3">dinoflagellates</a> commonly referred to as algae) that live inside their tissue, causing the coral to turn white. The zooxanthellae are <a href="/facts/Photosynthesis/TMV7w693">photosynthetic</a>, and as the water temperature rises, they begin to produce <a href="/facts/Reactive_oxygen_species/GpwSRugH">reactive oxygen species</a>. This is <a href="/facts/Toxicity/qa96rs60">toxic</a> to the coral, so the coral expels the zooxanthellae. Since the zooxanthellae produce the majority of coral colouration, the coral tissue becomes transparent, revealing the coral skeleton made of <a href="/facts/Calcium_carbonate/UQ1wLFna">calcium carbonate</a>. Most bleached corals appear bright white, but some are blue, yellow, or pink due to pigment proteins in the coral.
Bleached corals continue to live, but they are more vulnerable to disease and starvation. Zooxanthellae provide up to 90 percent of the coral's energy, so corals are deprived of nutrients when zooxanthellae are expelled. Some corals recover if conditions return to normal, and some corals can feed themselves. However, the majority of coral without zooxanthellae starve.
Normally, coral polyps live in an <a href="/facts/Endosymbiont/eWvSS8ID">endosymbiotic</a> relationship with zooxanthellae. This relationship is crucial for the health of the coral and the reef, which provide shelter for approximately 25% of all marine life. In this relationship, the coral provides the zooxanthellae with shelter. In return, the zooxanthellae provide compounds that give energy to the coral through photosynthesis. This relationship has allowed coral to survive for at least 210 million years in nutrient-poor environments. Coral bleaching is caused by the breakdown of this relationship.
The leading cause of coral bleaching is rising <a href="/facts/Ocean_temperature/D9bNcRxX">ocean temperatures</a> due to <a href="/facts/Climate_change/rEN4BDE7">climate change</a> caused by <a href="/facts/Human_impact_on_the_environment/vIAWqfPB">anthropogenic activities</a>. A temperature about 1 °C (or 2 °F) above average can cause bleaching. The ocean takes in a large portion of the <a href="/facts/Greenhouse_gas_emissions/Cu9nXPHE">carbon dioxide (CO2) emissions</a> produced by human activity. Although this uptake helps regulate global warming, it is also changing the chemistry of the ocean in ways never seen before. <a href="/facts/Ocean_acidification/GE7MrSPj">Ocean acidification</a> (OA) is the decline in seawater <a href="/facts/PH/RnSv8D7y">pH</a> caused by absorption of anthropogenic carbon dioxide from the atmosphere. This decrease in seawater pH has a significant effect on marine ecosystems.
According to the <a href="/facts/United_Nations_Environment_Programme/2P9vPoSU">United Nations Environment Programme</a>, between 2014 and 2016, the longest recorded global bleaching events killed coral on an unprecedented scale. In 2016, bleaching of coral on the <a href="/facts/Great_Barrier_Reef/3DkV4aad">Great Barrier Reef</a> killed 29 to 50 percent of the reef's coral. In 2017, the bleaching extended into the central region of the reef. The average interval between bleaching events has halved between 1980 and 2016. Coral bleaching events were recorded in 2020, 2021, and 2022 on the Great Barrier Reef and on reefs in Western Australia. Between 2023 and 2024, the fourth recorded mass bleaching event occurred, with heat stress found in each major <a href="/facts/Oceanic_basin/TKKHflL6">ocean basin</a> of both the <a href="/facts/Northern_Hemisphere/EFwyqL25">Northern Hemisphere</a> and <a href="/facts/Southern_Hemisphere/QfJ4z9L5">Southern Hemisphere</a>. The world's most bleaching-tolerant corals can be found in the southern <a href="/facts/Persian_Gulf/W78AfnC1">Persian Gulf</a>. Some of these corals bleach only when water temperatures exceed ~35 °C.

Coral bleaching open-in-new

Coral bleaching