TOEFL Reading Academic Passage Practice 002Coral Reef Ecosystems

Coral reefs are among the most biodiverse ecosystems on Earth, often called the “rainforests of the sea.” These underwater structures are built by colonies of tiny animals called coral polyps, which secrete calcium carbonate to form hard skeletons. Over thousands of years, these skeletons accumulate to create massive reef structures that provide habitat for countless marine species.

The relationship between corals and their environment is delicate and complex. Most reef-building corals live in symbiosis with microscopic algae called zooxanthellae that reside within coral tissues. These algae perform photosynthesis, providing the coral with up to ninety percent of its energy needs. In return, the coral provides the algae with a protected environment and access to sunlight. This mutually beneficial relationship is so crucial that corals cannot survive in deep waters where light cannot penetrate.

Coral reefs face numerous threats in the modern world. Rising ocean temperatures cause coral bleaching, a stress response in which corals expel their zooxanthellae, turning white and losing their primary energy source. While corals can recover from brief bleaching events if temperatures return to normal, prolonged or repeated bleaching often leads to coral death. Ocean acidification, caused by increased carbon dioxide absorption, makes it harder for corals to build their calcium carbonate skeletons. Physical damage from destructive fishing practices, pollution from coastal development, and predation by crown-of-thorns starfish further threaten reef health. The loss of coral reefs would be catastrophic not only for marine biodiversity but also for the hundreds of millions of people who depend on reefs for food, coastal protection, and economic benefits from tourism.