Coral Reef Bleaching: Symbiosis Under Thermal Stress

A partnership the coral cannot live without

Reef-building coral is an animal, but most of its energy comes from single-celled algae called zooxanthellae living inside its tissue in a mutualistic symbiosis. The algae photosynthesise using sunlight and pass a large share of the sugars they produce directly to the coral host, which in turn supplies the algae with a sheltered, nutrient-rich home and carbon dioxide. The algae's pigments are also what give healthy coral most of its colour — coral tissue itself is largely translucent.

What bleaching actually is

Coral bleaching is the coral expelling its zooxanthellae, or the algae's photosynthetic pigments breaking down, in response to stress — most commonly sustained water temperatures roughly 1 to 2°C above the local summer maximum for several weeks. Elevated heat damages the algae's photosynthetic machinery, causing it to produce excess reactive oxygen species that become toxic to both the algae and the coral host, and the coral responds by ejecting the compromised algae. Without them, the translucent coral tissue reveals the white calcium carbonate skeleton underneath — hence "bleaching", even though the coral itself is not dead at this point, only stripped of its main energy source and its colour.

A race between stress duration and recovery

Bleached coral is not doomed automatically. If water temperatures return to normal reasonably quickly, most coral can reacquire zooxanthellae, either from algae surviving elsewhere in its tissue or by taking up new algae from the surrounding water, and recover full colour and function over weeks to months. But a bleached coral has lost most of its energy supply, so the longer the stress persists, the more it must rely on stored energy reserves; prolonged or repeated bleaching before those reserves and the algal population can recover leads to starvation, increased vulnerability to disease, and eventually coral death.

mild/short heat stress   → bleaching, algae recolonise, coral recovers
moderate/repeated stress → recovery incomplete between events, growth stalls
severe/prolonged stress  → energy reserves exhausted, coral starves and dies

The tipping point: a phase shift, not just coral loss

A healthy reef is a race between calcification — the rate at which coral deposits new calcium carbonate skeleton and grows the reef structure — and the combined rate of physical erosion, dissolution and biological bioerosion (grazing, boring organisms) that wears the structure down. Widespread coral death from repeated severe bleaching removes the organisms doing the calcifying while erosion continues regardless, and the reef's net growth can flip from positive to negative. Dead coral skeleton also provides open substrate that fast-growing macroalgae readily colonise, and once algae dominate, they can physically prevent coral larvae from settling and re-establishing, locking the reef into an algae-dominated state that persists even if water temperatures later improve.

Why some reefs recover and others do not

Recovery capacity depends on how much time elapses between severe bleaching events relative to how long full recovery takes — typically a decade or more for badly affected reefs — plus local water quality, since excess nutrients and sediment from coastal runoff favour algae over coral recruits and slow recolonisation. Reef sites with naturally more thermally tolerant coral or algae symbiont strains, or with cooler local currents, tend to bleach less severely under the same regional heat event, which is part of why bleaching severity varies considerably even across reefs experiencing similar average sea temperatures.

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