Reef-building corals host symbiotic algae (zooxanthellae)
that give them colour and much of their energy. Sustained heat
stress above ~1–2°C over normal summer maximums causes corals to
expel these algae — "bleaching" — leaving a pale white skeleton
that starves without intervention or recovery.
Zooxanthellae are photosynthetic: they need enough light to
produce sugars for their coral host, but the same algae become
a liability under heat stress, over-producing reactive oxygen
that damages coral tissue — so bleaching risk rises fastest when
high light and high temperature combine. Too little light,
conversely, starves the colony even in cool water, since the
algae can't photosynthesise enough to sustain it.
- Water temperature — heat stress above the coral's
normal range triggers zooxanthellae expulsion and bleaching.
- Light intensity — drives the algae's photosynthesis;
too little light starves the colony, too much amplifies heat
stress damage.
- Growth rate — controls how densely the colony
branches, standing in for how fast a healthy colony can extend
its skeleton under good conditions.
Real-world bleaching events on the Great Barrier Reef (2016,
2017, 2020, 2022, 2024) have killed large fractions of shallow
coral cover during marine heatwaves. Restoration projects now
grow heat-resistant coral fragments on nurseries and outplant
them to damaged reefs to help colonies re-establish.