NOAA Coral Reef Watch directly links **heat stress** to bleaching: "First observed in the early 1980s, **mass coral bleaching** ... has become one of the most visible and damaging marine ecological impacts of **persistently rising ocean temperatures**." It defines bleaching as "the process by which **corals lose the symbiotic algae** that give them their distinctive colors and main energy sources." It further notes that severe coral bleaching events are driven by the "acceleration of **heat stress events that cause mass bleaching**."

Coral bleaching describes the process whereby the coral host loses its intracellular photosynthetic microalgal symbionts (family Symbiodiniaceae) and/or their photosynthetic pigments, often in response to thermal stress. This breakdown leads to the visual whitening of corals through the loss of intracellular microalgal symbionts, which can result in coral mortality and changes in reef communities over large regions. Most recent coral‐bleaching events are caused by anomalously high seawater temperatures resulting from climate change.

Coral bleaching is a stress response of corals and involves the breakdown of the symbiosis between the coral host and its photosynthetic algal symbionts (zooxanthellae). The most common driver of mass coral bleaching events is elevated sea surface temperature associated with marine heatwaves. When temperatures exceed the usual summer maxima, corals can lose substantial proportions of their algal symbionts, leading to the characteristic white appearance and increased risk of mortality.

“Coral bleaching can be defined as the loss of symbiotic zooxanthellae and/or their photosynthetic pigments from their cnidarian host. This major disturbance of reef ecosystems is principally induced by increases in water temperature.” The authors note that “one of the most important [stressors] for mass coral bleaching is abnormal high sea surface temperatures which can act synergistically with high solar irradiance.” They describe bleaching at the cellular level as “a substantial or partial loss of the endosymbiotic algae from the coral tissues, and/or the loss or reduction of photosynthetic pigment concentrations within zooxanthellae.” They list mechanisms of symbiont loss including “expulsion by exocytosis… and host cell detachment.”

Historical field research found that severe mass coral bleaching can be predicted when the water temperature exceeds the average of the warmest historical month by more than 1°C (Goreau, 1990; Williams and Bunkley-Williams, 1990; Hayes and Goreau, 1991; Goreau, 1991; Goreau et al., 1993). Corals face a risk of bleaching when the thermal anomaly exceeds 1–2°C (Glynn and D'croz, 1990). Based on regional coral biological experiments and field observational reports, CRW defined the combination of ecologically significant coral bleaching alert thresholds as an HS threshold of 1°C and DHW threshold of 4°C-weeks; when both HS and DHW thresholds are reached, the minimum bleaching alert level is reached and ecologically significant bleaching is expected.

Here, we investigate the role of zooxanthellae in the thermal tolerance of the Indo-Pacific stony coral, Acropora millepora, in a large scale transplantation experiment and show that thermal tolerance in this species is inextricably linked to the type of algal symbionts harboured. Thus, the thermal tolerance of host–algal symbiosis appears to be dependent on the physiological characteristics of the zooxanthellae under temperature (and light) stress, with the zooxanthellae being the weakest link in the symbiotic partnership. Thermal tolerance among different zooxanthella types is in large part due to the thermal stability of the thylakoid lipid membranes of the chloroplasts, with sensitive types having greater susceptibility to attack by reactive oxygen molecules which ultimately damages host cells.

In the 1980s, scientists discovered that corals start getting stressed if the water gets only 1°C warmer than the highest temperature expected in the summertime. We call this temperature the "bleaching threshold" because the stress caused by warmer-than-normal water can cause the corals to bleach. When the water gets too warm, the zooxanthellae can't use the sun's energy as efficiently… they turn this excess energy from sunlight into chemicals (toxins called 'reactive oxygen species') that can cause damage to themselves and to Polly. So even though Polly normally needs the zooxanthellae, she has to get rid of them to survive this temperature stress. As a result, she will expel most of the zooxanthellae from her body… The entire coral soon looks pale or white, so we say that it looks 'bleached'.

The paper explains that “elevated seawater temperature is a primary trigger of coral bleaching” and that under such stress “a large number of Symbiodinium were most likely expelled due to host cell detachment, and the subsequent loss of Symbiodinium from coral tissues led to coral bleaching.” The authors found that at 30°C (moderate thermal stress) “Symbiodinium become damaged, and corals selectively digest the damaged cells or immediately expel them without digestion by exocytosis, which is most likely an adaptive mechanism in response to moderate thermal stress.” They further suggest that if stressful conditions persist “damaged Symbiodinium may accumulate within coral tissues, resulting in coral bleaching.”

When water temperatures get too high, even by as little as 1–2 degrees Celsius above the normal maximum for a prolonged period, corals become stressed. In response, they expel the symbiotic algae, called zooxanthellae, that live in their tissues and provide them with food through photosynthesis. Losing their algae causes corals to turn white, a phenomenon known as coral bleaching. Bleached corals are more vulnerable to disease and can die if stressful conditions persist.

This paper describes bleaching as follows: “Coral bleaching involves the loss of symbiotic dinoflagellates (genus Symbiodinium) and/or their photosynthetic pigments from the coral host tissue, resulting in the whitening of coral colonies.” It notes that “elevated sea temperatures are the primary cause of mass coral bleaching events worldwide,” with bleaching occurring when sea temperatures exceed the usual maximum by as little as 1–2 °C for prolonged periods, which leads to stress and expulsion or loss of the symbiotic algae.

Coral bleaching occurs when the thermal tolerance of corals and their symbionts is exceeded (Baker et al. 2008). When the temperatures of the water rise above a certain threshold, the coral ability to retain the zooxanthellae decreases. The environment becomes unstable for the zooxanthellae and they are forced to leave the corals. The higher temperatures cause weakening in the endoderm which in turn causes cell death. This leads to the expulsion of the algal symbionts into the immediate surroundings.

Summarizing experimental work on corals under heat stress, the article explains that researchers “demonstrated that corals more actively digest and expel damaged symbiotic zooxanthellae under conditions of thermal stress, and that this is likely to be a mechanism that helps corals to cope with environmental change.” It reports that at 27°C (non-thermal stress) expelled zooxanthellae are part of a normal regulatory process, but at 30°C “Symbiodinium were damaged and corals selectively digested the damaged cells or immediately expelled them without digestion by exocytosis.” The authors propose that when this response cannot keep up with prolonged thermal stress, the accumulation and loss of damaged zooxanthellae “might be a possible mechanism underlying coral bleaching during prolonged moderate thermal stress.”

Corals can survive in water temperatures up to 35 °C, however the optimal growth temperature for corals is around 25 °C. Researchers have determined that, for any given area, water temperatures of 1 °C above the expected summertime maximum temperature is stressful to corals. This thermal stress can cause coral bleaching, or the loss of zooxanthellae from the coral tissues. Since the zooxanthellae are what give the coral tissue their color, the loss of them make the coral appear white.

This university educational module summarizes the mechanism: "Under normal conditions, the zooxanthellae algae living in coral tissue absorb energy from the sun and use it for photosynthesis. However, **when the water gets too warm, zooxanthellae can produce toxins, which are harmful to both the algae and their coral hosts. For self-preservation, the coral polyps must expel the zooxanthellae**, even though they rely on these algae for key life processes." It explains that without zooxanthellae, "coral reefs appear white ... This is why we call this process coral 'bleaching.'"

Discussing the 2023 global bleaching-level heat stress, NOAA Climate.gov notes that "The intensity and duration of this **heat stress caused entire coral reefs to bleach**, with some experiencing widespread death, in Florida and around the world." It explains that coral bleaching occurs when **ocean temperatures become high enough, for long enough, that corals lose the symbiotic algae** in their tissues. These algae normally provide most of the corals’ food and color, so their loss under thermal stress leads to a pale or white appearance.

This process is known as coral bleaching and occurs when the coral must expel its zooxanthellae from its tissues because of a combination of thermal stress and high solar irradiance. Specifically, corals bleach when water temperatures exceed the longterm mean maximum summer sea surface temperatures by 1–2 or 2–3 degrees Celsius for a specific period of time (the bleaching threshold). During these periods of high temperatures, coral zooxanthellae produce high levels of oxygen reactive species (ROS) that damage coral cells and tissues. These high concentrations become toxic to the coral and the coral must expel its zooxanthellae in order to avoid further cellular damage and death.

When ocean temperatures exceed normal summer maximums by just 1–2°C for sustained periods, this ancient partnership breaks down catastrophically. The elevated temperatures damage the photosynthetic machinery within zooxanthellae, causing them to produce excess reactive oxygen species (ROS)—essentially toxic molecules that harm both the algal symbiont and the coral host (Buerger et al., 2020). In response, corals expel their zooxanthellae partners as a survival mechanism. Without these algal symbionts, corals lose both their color and their primary food source, leaving behind ghostly white skeletons.

Across marine biology textbooks and review articles, coral bleaching is consistently defined as the loss of symbiotic dinoflagellate algae (zooxanthellae) or their pigments from coral tissue under environmental stress, particularly elevated sea temperature. The mechanism commonly described involves thermal stress impairing photosynthesis in zooxanthellae, generating reactive oxygen species that damage both algal and coral cells, which in turn leads the coral host to expel the algae, leaving the white calcium carbonate skeleton visible.