Coral Bleaching
Why heat-stressed corals expel their symbiotic algae and turn white, what triggers mass bleaching events, and what recovery does and does not look like.
A healthy reef is built by animals that are mostly transparent: the color of a coral reef comes overwhelmingly from single-celled algae (Symbiodiniaceae) living inside the coral's own tissues, photosynthesizing and handing over up to 90% of their sugar production to their host. Bleaching is what happens when that symbiosis breaks: under stress — most commonly heat — the coral expels its algae, and the white limestone skeleton shows through the transparent tissue. A bleached coral is starving, not dead.
The mechanism
Coral algae photosynthesize and supply the host with sugars; the host supplies nutrients and a sunlit home. When water temperature exceeds the coral's local summer maximum by about 1 °C for a few weeks, the photosynthetic machinery of the algae starts producing reactive oxygen species that poison the symbiosis, and the coral physically ejects them. The threshold varies by species, region, and thermal history — corals acclimatized to variable temperatures bleach later — but the relationship is monotonic: warmer and longer is worse. Thermal stress accumulates across a season, measured in degree-heating-weeks.
Events and recovery
Mass bleaching is a global-scale phenomenon: the 1998, 2016, 2017, and 2020–2022 events each hit every tropical ocean basin. The 2016 event alone damaged the Great Barrier Reef severely enough to shift its composition — fast-growing tabular and staghorn corals were hit hardest, and recovery, where it happens, takes a decade or more. Recovery depends on larvae arriving from surviving reefs, water quality, and above all the absence of a repeat heat wave before the community re-establishes.
What follows from the science
Recovery is possible locally (protect herbivores that graze competing algae, reduce nutrient runoff, restore herbivore populations), but the driver is global: ocean warming under the greenhouse effect. Repeated bleaching without cooling intervals converts reefs from coral-dominated to algae-dominated states, which changes fisheries, coastline protection, and biodiversity across the tropics. Related reading: ecological succession for how reef communities re-assemble after disturbance.
Tags
climate change coral ecology symbiosis