A coral reef is an underwater habitat characterized by corals that construct reefs. Reefs are made up of coral polyp colonies bound together by calcium carbonate. Most coral reefs are made up of stony corals, which have polyps that cluster together. 

Coral reefs are among the world's most diversified ecosystems. Coral polyps, the invertebrates largely responsible for reef formation, come in a variety of shapes and sizes, including vast reef-building colonies, beautiful flowing fans, and even small, lonely species. Thousands of coral species have been identified; some thrive in warm, shallow tropical waters, while others survive in frigid, dark ocean depths.

Because of the richness of life found in the environments generated by corals, reefs are frequently referred to as "rainforests of the sea." Healthy coral reefs support around 25% of the ocean's fish population. In the various nooks and crannies made by corals, fish and other species find shelter, food, reproduce, and rear their young. The coral reefs of Northwest Hawaiian Island, which are part of the Papahnaumokukea National Marine Monument, demonstrate the richness of species associated with shallow-water reef habitats. This region is home to around 7,000 different kinds of fish, invertebrates, plants, sea turtles, birds, and marine mammals. Deep water reefs or mounds are less widely recognized, yet they also maintain a diverse range of sea life in a very barren world. 

Corals that reside in shallow water have a symbiotic association with photosynthetic algae called zooxanthellae that dwell in their tissues. The coral offers a safe habitat as well as the substances required for zooxanthellae for photosynthesis. In exchange, the algae create carbohydrates and oxygen, which the coral needs for sustenance. The algae also aid the coral in waste removal. Mutualism refers to a sort of symbiosis in which both partners profit from the interaction.

 Deep-sea corals lack zooxanthellae and survive in significantly deeper or colder oceanic environments. Unlike their shallow-water relatives, which rely mainly on photosynthesis to make food, deep sea corals obtain a large portion of their energy from plankton and organic materials. Below are some of the benefits derived from coral reef

Coral reefs safeguard coasts from storms and erosion, provide jobs for local populations, and give recreational possibilities. They also provide meals and fresh medications. Reefs provide food, money, and protection to about half a billion people. Local businesses benefit from fishing, diving, and snorkeling on and around reefs to the tune of hundreds of millions of dollars. The annual net economic worth of the world's coral reefs is estimated to be in the tens of billions of dollars. These habitats are culturally significant to indigenous peoples worldwide. Upon the benefit/ services coral reef provides, they face challenges and are discussed below:

Unfortunately, coral reef ecosystems are under grave danger. Diseases, predators, and storms are examples of natural risks. People are also responsible for other problems such as pollution, sedimentation, unsustainable fishing techniques, and climate change, which is boosting ocean temperatures and generating ocean acidification. Many of these hazards can induce coral bleaching and mortality, while others inflict physical harm to these fragile ecosystems. During the 2014-2017 coral bleaching disaster, exceptionally warm seas (mainly caused by a severe El Nio) impacted 70% of the world's coral reef ecosystems. Some locations were particularly heavily damaged, such as Australia's Great Barrier Reef, where hundreds of kilometers of coral bleached. 

Corals may recover from bleaching episodes if circumstances improve before they die, however the ecosystems can take many years to fully rebuild. Scientists are also experimenting with innovative methods of assisting coral reef ecosystems, such as growing coral in a nursery and then transferring it to damaged places. 

Reference:

Coral reef ecosystems | National Oceanic and Atmospheric Administration (noaa.gov)