Context

Global warming poses a more significant threat to coral growth and reef accretion than ocean acidification (OA), according to a new study. 

How heat stress affects corals more than ocean acidification?

• Ocean acidification slows the rate at which coral reefs generate calcium carbonate, thus slowing the growth of coral skeletons.
• Heat stress directly affects coral performance in hospite exacerbation of light stress in the symbionts, whereas ocean acidification induces moderate effects on coral metabolism, some of them even positive.
• As temperatures rise, mass coral bleaching events and infectious disease outbreaks are becoming more frequent.

Role of corals: 

• Corals are animals. Coral reefs boost biodiversity, buffer storms and support the livelihoods of over one billion people.
• Coral reefs do not absorb carbon and do not play a direct role in climate change mitigation. However, they are important for climate adaptation.
• In the last decade, the world lost about 14 per cent of its coral reefs.

What is Coral Bleaching?

• It occurs when abnormal environmental conditions, such as warmer sea temperatures, cause coral polyps to expel algae (zooxanthellae) living in their tissues, causing the coral to turn completely white.
• Normally, coral polyps live in an endosymbiotic relationship with this algae crucial for the health of the coral and the reef as the algae provides up to 90% of the coral's energy.
• When a coral bleaches, it is not dead. Corals can survive a bleaching event, but they are under more stress and are subject to mortality.
• Corals can recover if the water temperature drops and the algae are able to recolonise the coral reefs.

Factors responsible for Coral Bleaching:

• Increased water temperature (most commonly due to global warming), or reduced water temperatures
• Oxygen starvation caused by an increase in zooplankton levels
• Increased solar irradiance (photosynthetic active radiation and ultraviolet light)
• Increased sedimentation (due to silt runoff)
• Bacterial infections
• Changes in salinity
• Herbicides
• Extreme low tide and exposure
• Cyanide fishing
• Pollutants such as oxybenzone, butylparaben, octyl methoxycinnamate, or enzacamene: four common sunscreen ingredients that are nonbiodegradable and can wash off of skin
• Ocean acidification due to elevated levels of CO2 caused by air pollution
• Being exposed to Oil or other chemical spills

Required measures:

• A better understanding of the underlying mechanisms that enable corals to tolerate heat can significantly improve coral reef conservation and restoration efforts.