Why coral bleaching occurs

Corals are marine invertebrates belonging to the anemone tribe that live in genetically identical colonies. It creates an amazing living ecosystem on the seabed, which benefits up to a quarter of all marine species. In recent years, however, we hear more and more in the news about the so-called bleaching of corals. What is it and why does coral bleaching occur at all?

Coral exists only in the polyp stage. It is typically only a few millimeters wide and a few centimeters long. The oral cavity, which serves both for food intake and waste excretion, is surrounded by several tentacles. Inside the coral is the stomach. Some corals are attached from below to a solid calcium carbonate substrate, which they form themselves. After thousands to millions of years, so-called coral reefs are formed.^[1]

Shallow-water corals consume up to 90% of their food through symbiosis with algae called zooxanthellae. These algae most often live directly in the outer cells of the coral, where they get first through the oral cavity and then ingested by cellular phagocytosis. Zooxantelae work on the principle of photosynthesis, ie with the help of carbon dioxide formed by coral and sunlight, they create nutrients that the coral cell uses in cellular respiration.^[2][3]

Due to the stressful situations to which the coral is exposed, the symbiosis is disrupted. Zooxanthellae produce higher levels of free oxygen radicals that damage the coral. The result is expulsion or disposal of zooxanthellae. But zooxanthellae are the cause of the color diversity of corals. This process, which we call coral bleaching, reveals the true white color of the coral. The affected coral begins to starve, and if the adverse conditions that led to the bleaching last too long, the coral eventually dies. Instead, the coral reef is covered with algae, which prevents the corals from returning until the entire reef eventually begins to disintegrate.^[2][4]

Coral bleaching is caused by a number of events, the most significant of which is an increase in ocean temperature due to global warming. Events can be divided into local or global. The local cause of coral bleaching is being regulated by legislation of the states of which coral reefs are a part. Against global change, however, this legislation is short-lived and coral populations are expected to hit a dramatic decline in this century. There are estimates that the total economic benefit of corals for humans is up to $ 172 billion. Coral reefs are a natural barrier against sea storms, serve as a nesting ground for a huge number of economically beneficial animals and, among other things, generate profits in the form of tourism.^[4]

Coral bleaching can be cause by:^[4]

• Water temperature change
• Oxygen reduction by zooplankton overgrowth due to overfishing
• Increased solar irradiance
• Increased sedimentation
• Bacterial infections
• Change in water salinity
• Occurrence of herbicides
• Extreme low tide
• Sodium cyanide fishing
• Acidification of the oceans due to increased CO₂ concentrations
• Exposure to chemicals such as washed sunscreen