Coral Bleaching: When a 30,000-Year Partnership Breaks Down

A coral is a landlord, not just an animal

A reef-building coral polyp is an animal, but most of its colour and much of its energy come from a tenant: single-celled algae called zooxanthellae (genus Symbiodinium and relatives) living inside the coral's own tissue. The algae photosynthesise and pass a large fraction of the sugars they produce directly to the coral host, often supplying the majority of its daily energy budget; in return the coral provides shelter, carbon dioxide and a stable, well-lit position near the reef surface. This partnership, tens of millions of years old, is also what gives coral its colour — the white calcium carbonate skeleton underneath is normally hidden by the pigmented algae and the coral's own tissue.

Heat turns the algae toxic

Even a mild, sustained rise in sea temperature — often just 1-2°C above the normal local summer maximum — disrupts the algae's photosynthetic machinery, particularly Photosystem II. A damaged photosystem cannot safely dissipate absorbed light energy and instead leaks it as reactive oxygen species (ROS) — molecules that damage cell membranes and proteins in whatever tissue they reach, including the coral host's own cells. What was a beneficial tenant becomes, under heat stress, a source of cellular damage. The coral's defensive response is to expel the malfunctioning algae from its tissue — a reaction that protects the coral in the immediate term but removes its main food supply, and its colour, in one step.

Degree Heating Weeks: measuring accumulated stress

Because bleaching risk depends on how much heat has accumulated, not just the peak temperature on any single day, NOAA's Coral Reef Watch program tracks Degree Heating Weeks (DHW): whenever sea surface temperature exceeds the local historical summer maximum by at least 1°C, that excess is summed over a trailing 12-week window.

DHW = Σ (SST_week − MMM − 1°C)   for weeks where SST_week ≥ MMM + 1°C
      over the trailing 12-week window

MMM = Maximum Monthly Mean, the historical local summer baseline

DHW ≥ 4  →  Bleaching Warning: significant bleaching likely
DHW ≥ 8  →  Alert Level 2: widespread bleaching and mortality likely

The 12-week window matters because it reflects biology, not arbitrariness: a brief, sharp heat spike that dissipates quickly does less cumulative damage than the same total heat spread across weeks of sustained stress, because the coral has some capacity to cope with short excursions but not with prolonged ones.

Bleached is not dead — yet

A newly bleached coral is alive, pale and starving. If the marine heatwave subsides before its energy reserves are exhausted, the coral can reacquire zooxanthellae from the surrounding water or from surviving cells and gradually regain its colour and function over weeks to months. If thermal stress persists — or if a second heatwave arrives before recovery is complete — the coral's stored energy runs out, its tissue thins, and it becomes vulnerable to disease and algal overgrowth, ultimately dying and being colonised by other organisms. This is why the distinction between DHW 4 (bleaching likely) and DHW 8 (mortality likely) is operationally so important for reef managers: it marks the boundary between a reversible stress event and one that starts killing coral outright.

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