Coral is a marine organism that builds complex and diverse underwater structures known as coral reefs. These formations are created by living animals, not merely geological features.
The Building Block of Coral Skeletons
Coral skeletons are primarily composed of calcium carbonate (CaCO₃), specifically in a crystalline form called aragonite. This material provides the rigid framework that supports the coral colony. The process by which corals form these hard external skeletons is known as calcification.
To create calcium carbonate, coral polyps extract calcium ions (Ca²⁺) and carbonate ions (CO₃²⁻) directly from seawater. These ions are then combined within a specialized space between the coral’s cells and its existing skeleton. The coral actively regulates the chemistry within this space to facilitate the formation of aragonite crystals. As new layers of calcium carbonate are secreted beneath the polyp, the skeleton grows larger, gradually building the extensive structures seen in reefs.
The Living Components of Coral
While the calcium carbonate skeleton provides the physical structure, the coral itself is a soft-bodied animal called a polyp. These tiny invertebrates live in colonies and secrete the calcium carbonate that forms the reef. Each polyp has a sac-like body with a central mouth surrounded by tentacles, which it uses to capture food particles.
Many reef-building corals engage in a symbiotic relationship with microscopic algae called zooxanthellae, which live within their tissues. These algae perform photosynthesis, converting sunlight into energy-rich compounds. Up to 90% of the organic material produced by the zooxanthellae is transferred to the coral host, providing a significant portion of the coral’s energy needs and aiding in the calcification process. Zooxanthellae also contribute to the vibrant colors observed in many corals.
The Vital Role of Coral Structures
The hard, intricate structures of coral reefs are among the most diverse ecosystems on Earth. These formations provide habitats for a vast array of marine life, including thousands of fish species that depend on them for shelter, feeding, and breeding.
Coral reefs also serve as natural breakwaters, absorbing wave energy. This barrier protects coastlines from erosion, storms, and floods. Studies indicate that reefs can reduce wave energy by up to 97%, limiting damage to coastal communities and infrastructure. The structural complexity of reefs also helps stabilize shoreline sediments, maintaining beaches and protecting other coastal ecosystems.
How Ocean Changes Impact Coral Skeletons
Coral skeletons are vulnerable to changes in ocean chemistry, particularly ocean acidification. As the ocean absorbs more carbon dioxide (CO₂) from the atmosphere, its pH decreases, making it more acidic. This reduces the availability of carbonate ions in the seawater, which are building blocks for coral skeletons.
With fewer carbonate ions available, corals must expend more energy to build and maintain their skeletons, leading to slower growth and thinner structures. These weaker skeletons are more susceptible to damage from physical forces and bioerosion. While coral bleaching primarily involves the expulsion of zooxanthellae due to stress, it indirectly impacts the skeleton by depriving the coral of the energy needed for calcification.