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The author became fascinated by the relationship between coral and its symbiotic algae while assisting for a marine ecology course at the Marine Biological Laboratory decades ago. There, she met graduate students who would later make crucial contributions to understanding this symbiosis. This is their post- �黨�ǿ���Ƶ story.

In 1879, the German botanist Anton de Bary coined the term symbiosis. People usually think of symbiosis in terms of mutually beneficial relationships, but scientifically it’s a catch-all for any relationship—harmonious or antagonistic, parasitic or neighborly—between two different organisms. The details are left unclear.

Just two years later, while studying marine creatures under a microscope, de Bary’s compatriot Karl Andreas Heinrich Brandt realized that the small amber orbs lining their digestive tissues were not part of them, but a type of symbiotic algae. Brandt gave the cells the name “zooxanthellae,” which roughly means “little yellow cell in an animal.”

Among the creatures in whom zooxanthellae live are coral, and in the ensuing century scientists discovered that those little yellow cells donate a whopping 90 percent of the sugar they make from photosynthesis to their coral hosts. That energy is why coral colonies—which themselves are aggregations of pencil-eraser-sized polyps—can build reefs so immense they can be seen from outer space.

The break-up of the symbiosis, known as bleaching, which occurs when seawater warms, is the reason coral reefs are the first ecosystem-scale casualties of climate change. Half the world’s coral reefs largely from bleaching, and by some predictions that number will by mid-century. Yet the nature of the relationship between coral and algae remains in many ways a mystery.