How does ocean warming lead to coral bleaching?

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Ocean Warming Triggers Coral Bleaching by Disrupting Essential Symbiosis

Ocean warming, a direct consequence of increased greenhouse gas emissions, leads to coral bleaching by inducing thermal stress on coral polyps. This stress causes the coral to expel the symbiotic algae, known as zooxanthellae, which reside within their tissues. These algae are crucial, providing the coral with up to 90% of its energy through photosynthesis and imparting its vibrant colours. Without them, the coral turns white, becoming vulnerable to disease and starvation, and its long-term survival is jeopardized.
The Symbiotic Relationship and Heat Stress

Coral polyps and zooxanthellae exist in a mutually beneficial relationship. The coral provides a protected environment and compounds needed for photosynthesis, while the zooxanthellae supply the coral with essential nutrients and energy. When ocean temperatures rise above a certain threshold, even by a small amount for an extended period, this delicate symbiosis breaks down. The excess heat damages the photosynthetic apparatus within the zooxanthellae, leading to the production of harmful reactive oxygen species. In response to this cellular damage and oxidative stress, the coral expels the algae to protect itself.
Physiological Mechanisms of Expulsion

The expulsion of zooxanthellae is a complex physiological response to heat stress. Elevated temperatures disrupt the normal functioning of the chloroplasts within the algae, leading to photoinhibition and the generation of toxic byproducts. The coral host then triggers a process that leads to the detachment and expulsion of these stressed or damaged algal cells. While this is a survival mechanism to mitigate cellular damage from reactive oxygen species, it leaves the coral without its primary food source and characteristic pigmentation. The severity of the bleaching is directly related to the intensity and duration of the heat stress event.
Long-Term Consequences of Bleaching

Even if ocean temperatures return to normal and corals manage to reabsorb algae, the recovery process can be slow and is not guaranteed. Persistent or repeated bleaching events significantly reduce the coral's ability to reproduce and increase its susceptibility to diseases. In extreme cases, such as the current unprecedented event affecting 84% of the world's reefs, mass mortality can occur, leading to the collapse of entire reef ecosystems. This loss has profound implications, as coral reefs support marine biodiversity and provide critical resources and coastal protection for hundreds of millions of people globally.

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How does ocean warming lead to coral bleaching?

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