What is Coral Bleaching?
The coral host (animal) reacts to higher-than-normal temperatures by expelling the symbiotic algae (plant) that live in their tissues and then turn completely white.
The symbiotic algae, called zooxanthellae, are photosynthetic and provide their host coral with food in return for protection. Their presence gives the corals a distinctive brown/ dark colour, and fluorescent pigments or secondary metabolites, produced by the host add the vibrant colours that characterise corals.
Prolonged stressful environmental conditions cause a breakdown in this symbiotic relationship, first revealing the fluorescent pigments and then leaving the white calcium carbonate (aragonite) skeleton visible through the coral tissue. Any symbiotic (containing zooxanthellae) marine animal can bleach, such as giant clams.
Image from https://oceanservice.noaa.gov/facts/coral_bleach.html
When does it occur?
Coral bleaching occurs when corals are stressed by changes in environmental conditions.
The major causes are climate change-induced warming of sea surface waters, pollution, freshwater flooding, and extremely low tides.
Image from https://www.marineconservation.org.au/coral-bleaching/
Bleached corals can still obtain energy?
Bleached corals can no longer obtain energy from photosynthesis, and if bleaching persists for an extended period, corals will starve and die (of course there are exceptions, as corals have a range of feeding strategies from entirely autotrophic to entirely heterotrophic and anything in between).
For those corals that survive, bleaching can deplete the corals’ energy resource to the extent that corals do not reproduce for one or two years. The threat to corals increases as the bleaching events become more frequent because they have no time to recover.
As said, stress can be caused by unusually high or low sea temperatures; high or low light levels; and the presence of freshwater or pollutants. But the extent of coral bleaching depends on above-average temperatures, the duration of high-water temperatures, the specific coral and the history of the reef where the corals are growing.
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