Coral reef ecosystems depend on symbiosis between coral animals and intracellular photosynthetic algae (zooxanthellae). The algae provide up to 90% of the coral's energy; the coral provides the algae with shelter and access to nutrients. This partnership is highly sensitive to thermal stress; temperature increases above seasonal maxima cause bleaching (symbiont loss) and can kill reefs if sustained.
From your study of marine food webs, you know that primary production — the conversion of sunlight and nutrients into organic matter — is the foundation of ocean ecosystems. Coral reefs thrive in tropical waters that are paradoxically nutrient-poor, often called "marine deserts." The mystery of how reefs support such extraordinary biodiversity in these barren waters is resolved by one of nature's most elegant partnerships: the symbiosis between coral animals and photosynthetic algae called zooxanthellae (genus *Symbiodinium* and related genera).
Corals are animals — specifically, colonies of tiny polyps related to jellyfish and anemones. Each polyp is a simple cylinder with a mouth surrounded by tentacles that can capture zooplankton. But this animal diet provides only a fraction of the coral's energy needs. The real engine is inside the coral's own cells: millions of single-celled zooxanthellae algae live within the polyp's tissue in a relationship called endosymbiosis. These algae photosynthesize, using sunlight and CO₂ to produce sugars and oxygen, and they transfer up to 90% of their photosynthetic products directly to the coral host. In return, the coral provides the algae with a protected environment, access to sunlight (corals grow toward light), and a steady supply of CO₂ and nitrogen-rich waste products that the algae use as fertilizer. This tight nutrient recycling between animal and plant is why reefs can be so productive in nutrient-poor water — very little is lost to the surrounding environment.
The zooxanthellae also give corals their color. Healthy corals display browns, greens, and sometimes vivid hues because of the pigments in their algal symbionts. Coral bleaching occurs when this partnership breaks down, almost always triggered by thermal stress. When water temperatures rise just 1–2°C above the normal seasonal maximum for several weeks, the zooxanthellae become physiologically stressed and begin producing toxic reactive oxygen species. The coral responds by expelling its symbionts, turning white — "bleached" — as the transparent animal tissue reveals the white calcium carbonate skeleton beneath. A bleached coral is not dead, but it is starving: without its algae, it has lost its primary energy source. If temperatures return to normal within a few weeks, the coral can reacquire zooxanthellae and recover. If the stress persists, the coral dies.
Coral reefs are sometimes called the "rainforests of the sea" because they support roughly 25% of all marine species despite covering less than 1% of the ocean floor. The three-dimensional calcium carbonate framework that corals build over centuries provides habitat — crevices, overhangs, caves — for thousands of fish species, invertebrates, and algae. This framework depends entirely on the coral-zooxanthellae symbiosis, because the energy from photosynthesis is what allows corals to calcify fast enough to build reef structures that keep pace with erosion and sea-level change. When bleaching kills corals, the framework degrades, and the entire ecosystem — from the smallest shrimp to the sharks — loses its structural foundation. This is why rising ocean temperatures represent an existential threat to reef ecosystems worldwide.