The Great Barrier Reef, a UNESCO World Heritage Site with an area larger than that of the UK (348,700 km squared), is usually a widely studied area. A study was conducted by the University of Sydney to assess the coral reefs situated on One Tree Island, which lies off the coast of Queensland in Australia, and was published in 2025. It was found that in 2024, over 40% out of 462 coral reef colonies in One Tree Island had died. This was the worst amount of mass coral reef death in this coral reef system ever recorded, and was mainly due to the effects of coral bleaching.

Coral bleaching is a stress-induced reaction by the coral in response to high temperatures. Corals exist symbiotically with a type of algae called zooxanthellae; this process works by the algae utilising photosynthesis in order to produce both sugars and the building blocks for proteins, amino acids, which the coral then uses in order to survive and build up its skeleton. The algae benefits from the coral via the coral providing a safe habitat within its tissues as well as producing waste products from the nutrients it takes from algae photosynthesis, which provides the algae with nutrients like phosphates that it needs.

The coral bleaching process occurs when a thermal tolerance level has been surpassed by the temperature, the photosynthetic process that the algae use produces toxic oxygen-based products, which causes oxidative stress. This build-up of toxicity causes the coral to expel the algae from itself, which gives rise to the change in colour most commonly expressed in coral bleaching. Corals have their distinctive colours due to the important pigments that the algae contain, with the colour expressed being the colour of solar energy not absorbed (but reflected) in its photosynthesis process. This is also seen in plants and leaves. When the algae is expelled, the corals turn white, and the coral has to survive without the algae, of which the coral has a high mortality rate. This leaves the coral skeleton exposed to potential harm like diseases, of which the black band disease degraded the tissue of Goniopora coral on One Tree Island. Recovery, though, is still possible, and different species of zooxanthellae have different, and potentially higher, resistances to temperature increases.

Corals generally have a period of about 20 years in order to recover from coral bleaching and other issues such as diseases, but climate change is increasing the amount of stressful events on the corals, which prevent them from recovering. With a high sensitivity to an increase in temperature of 0.1°C for bleaching, an increase in 1.5°C in global temperatures caused by human activities may prevent future generations of algae from adapting to these new temperatures, and thus cause far more global coral mortality, of which it has been feared will lead to the decrease of 70-90% of coral reefs at this 1.5°C temperature increase.

It is not only the increase in temperature through climate change that has caused a worldwide increase in coral bleaching, but other wider environmental factors such as ocean acidification through an increase in ocean carbon dioxide levels, which can cause inhibition to coral structural growth. However, despite all of this, studies have provided evidence to suggest that, in spite of an increase in coral bleaching, coral reefs are being able to adapt, using a high genetic variety in their species to become more resistant by selectively choosing which zooxanthellae types to associate with, which was found to have happened in Panama.

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