In August, the surface of the Arabian Gulf can pass 35 degrees Celsius. On most of the planet, water that warm destroys coral reefs within weeks. In the Gulf, reefs are still there. Understanding why has become one of the most useful questions in marine biology.
A partnership inside a stone
A coral is an animal — a colony of soft polyps, each one a tiny relative of the jellyfish — that builds a skeleton of calcium carbonate. Inside the polyps live single-celled algae called zooxanthellae. The algae photosynthesise and hand over sugars; the coral supplies shelter, carbon dioxide and nitrogen compounds. This mutualism is what makes reefs productive enough to feed a quarter of all marine species from clear, nutrient-poor water.
The partnership is also the weak point. When water is too warm for too long, photosynthesis inside the algae produces damaging reactive molecules. The coral expels its algae, loses its colour and turns bone white. That is bleaching. A bleached coral is starving, not dead — if temperatures fall soon enough, algae can return.
Gulf corals do it differently
Gulf reefs are dominated by a small number of tough genera such as Porites, Cyphastrea and Platygyra. Many host a heat-tolerant algal partner, Durusdinium thermophilum, first described from these waters. Some colonies also carry proteins that mop up reactive oxygen, and populations appear to have been selected over thousands of years for genes linked to heat tolerance.
The trade-off is diversity. The Gulf holds far fewer coral species than the Red Sea or the Indian Ocean, and its corals grow slowly. Toughness costs something.
Pressure from more than heat
Reefs off Qatar face dredging, coastal construction, sedimentation that blocks light, warm brine from desalination plants and occasional harmful algal blooms. Restoration projects now grow coral fragments in nurseries and transplant them onto artificial structures, while marine protected areas limit anchoring and fishing over sensitive reefs.
Scientists elsewhere watch closely. If the rest of the world's oceans warm to Gulf-like temperatures later this century, the organisms that already survive here may hold information about what survival requires — and about the limits of adaptation.