Corals are marine invertebrates that belong to the phylum Cnidaria. They are sessile animals that live in colonies and can be found in tropical and subtropical waters around the world. Corals build hard, calcium carbonate skeletons that form the basis of coral reefs.

Corals are important in aquatic ecosystems for several reasons:

1. Biodiversity: Coral reefs are among the most diverse ecosystems on the planet, with a wide variety of species of fish, invertebrates, and plants living in and around them. They provide habitat and food for many species, and are therefore critical to the health of marine ecosystems.

2. Protection: Coral reefs act as natural barriers that protect coastlines from storms and erosion. They can also help to buffer against the effects of climate change, such as sea level rise and ocean acidification.

3. Economic value: Coral reefs are important for fisheries, tourism, and recreation, and are therefore a valuable economic resource.

4. Carbon cycling: Coral reefs are also important for the cycling of carbon in the ocean. They absorb and store large amounts of carbon, which helps to reduce the levels of carbon dioxide in the atmosphere and mitigate the impacts of climate change.

However, coral reefs are currently facing many threats, including overfishing, pollution, climate change, and ocean acidification. 

Coral bleaching is a process by which coral polyps expel the symbiotic algae that live inside them, causing the coral to turn white or pale. The symbiotic algae, also known as zooxanthellae, provide the coral with nutrients through photosynthesis, and also give the coral its color. When the coral polyps expel the algae, the coral loses its main source of nutrition, which can lead to death if the process is severe or prolonged.

Coral bleaching is primarily caused by environmental stress, such as increased sea surface temperatures, pollution, ocean acidification, and overexposure to sunlight. When water temperatures rise or become too warm, the coral becomes stressed and may expel its symbiotic algae. In addition, pollution, ocean acidification, and overexposure to sunlight can also cause stress on the coral, leading to bleaching.

Coral bleaching has significant impacts on marine ecosystems, as coral reefs provide important habitats and nurseries for a variety of marine life. When coral reefs are damaged or destroyed, the biodiversity of the surrounding ecosystem can also be negatively impacted. Coral bleaching can also have economic impacts, as coral reefs provide significant benefits to human populations, including fisheries, tourism, and coastal protection.

To address coral bleaching, it is important to reduce the underlying causes of environmental stress that can lead to bleaching. This includes reducing greenhouse gas emissions that contribute to climate change, reducing pollution in the oceans, and protecting coral reefs from overexposure to sunlight and physical damage. In addition, efforts to restore damaged coral reefs can help to rebuild the ecosystem and protect marine biodiversity.