UPCAT Reading Comprehension practice question 6: According to the passage, what gives a healthy reef its color?

A healthy coral is not a single organism but a partnership. The coral animal builds the limestone skeleton, while microscopic algae called zooxanthellae live inside its tissues, converting sunlight into sugars. The algae supply as much as ninety percent of the coral's energy, and they also give the reef its color.

When water temperature rises even one to two degrees Celsius above the usual summer maximum and stays there for several weeks, the partnership breaks down. The algae begin producing compounds that damage the host, and the coral expels them. What remains is translucent tissue over a white skeleton, which is why the event is called bleaching.

A bleached coral is not yet a dead coral. If temperatures fall soon enough, it can reacquire algae and recover, though it will typically grow more slowly and reproduce less for years afterward. If the heat persists, the coral starves.

The difficulty is one of interval rather than intensity. Reefs have always endured occasional hot years. What is new is the shortening gap between them, which leaves too little time for recovery before the next event begins.