What is the most biodiverse ecosystem on Earth?
Many people may think of tropical rainforests, but the answer is more likely to be the ocean's coral reef ecosystem, which accounts for only about 1% of the total area of the ocean, but about 25% of marine life lives around it.
Pictured: Bleached corals
However, in the past two days, the National Oceanic and Atmospheric Administration of the United States has announced that coral reefs around the world are experiencing the fourth reef bleaching event on record.
At least 850 million people worldwide will be severely affected by the important role of coral reef ecosystems, with reduced incomes and food scarcity becoming scarce.
So, what's really going on with coral reefs, and why are they experiencing bleaching all of a sudden?
©Holobionics
What exactly is coral?
Corals look like both animals and plants, so there is some confusion in the classification, previously some people classified them as plants, but now corals are classified as cnidarians, they are animals.
However, corals are not ordinary animals, and when we say corals, we usually refer to coral "colonies", which are a group of many genetically identical polyps, which are called colony modular organisms.
The smallest unit of coral's colony module is the polyp, each of which is an individual animal, which is sac-shaped, usually only a few millimeters in diameter and a few centimeters in length at most.
Pictured: A coral polyp
Through asexual reproduction, polyps constantly produce genetically identical individuals (or modules) around themselves, and then these genetically identical polyps are linked together through living tissues to form a complete organism – what we call corals.
Although corals are modular organisms, they do not grow indiscriminately, and in the case of hard corals, for example, each polyp secretes an exoskeleton composed of calcium carbonate near its base, which grows up to have the biological characteristics of that species.
There is also a type of coral, which is soft coral, which is also a group of modular organisms, but they are not completely built on their own exoskeleton like hard corals, and these corals are more like other cnidarians.
Figure: Polyp anatomy
In addition to the fact that polyps create a coral through asexual reproduction, each coral also has the ability to reproduce sexually, and the same coral will release gametes together at night, which are fertilized freely in the water and eventually obtain polyp larvae.
Figure: Schematic diagram of © coral's sexual reproduction
Polyp larvae do not fix themselves like adult individuals, they drift freely in the ocean in the form of plankton until they catch a suitable place and then redevelop into corals.
What's going on with coral reefs and bleaching corals?
When we talk about coral reefs, they are mainly created by the exoskeleton of hard corals, and they are called reef builders.
Pictured: Coral reefs are mainly found in tropical waters
The coral lifestyle is very unique, and while some coral species have the ability to hunt and use stinging cells to attack and paralyze small prey that comes close to them and then eat them outright, they usually don't get their energy this way.
The main way corals obtain energy is symbiosis, with which zooxanthellae photosynthesize to provide corals with the energy to survive, and in exchange, corals provide them with shelter to thrive.
Coral reefs are usually found in shallow waters because their symbionts require photosynthesis, although there are also species of corals that do not rely on photosynthetic symbionts and can create coral reefs in the deep ocean.
Pictured: Zooxanthellae
In fact, many polyps are transparent, and their color is produced by symbionts, and when the coral loses its symbiont, we can see through the surface of the polyp the calcium carbonate exoskeleton inside - it appears white, which is the bleaching of corals.
Of course, coral bleaching is a general term, because not all polyps are colorless, some species have their own color - blue, pink, purple or yellow, but the presence of symbionts can also change their own color.
In this case, when the coral loses its symbiont, its color changes (it becomes its original appearance), which is also very easy to identify.
If we want to know why corals bleach, we just need to know why corals are losing their symbionts.
In fact, corals are more closely related to symbionts than we think, and those symbionts live directly in the cytoplasm of coral cells.
Coral cells are the living and breeding grounds of symbionts, they do not actively leave corals, but corals have a choice, they will use their immune system to keep the symbionts they want, and expel those who do not.
As for why corals drive away their otherwise useful symbionts, the reason is simple: these symbionts can sometimes become poisonous – really poisonous.
The most common cause of coral-expelled symbionts is that symbionts produce reactive oxygen species in the coral's cells, which are toxic to corals.
As for why symbionts are so good that they suddenly produce reactive oxygen species, the most common reason is that the water temperature changes, and as the temperature of the living environment increases, zooxanthellae will release more and more reactive oxygen species, eventually causing the corals to abandon them.
Over time, bleaching corals (which lose their symbionts) can starve to death because they can't hunt efficiently to keep them alive in the long run.
Pictured: A coral bleaching followed by a resurgence
However, when the symbionts become non-toxic, the corals will accept them again, saving themselves from starvation, and the color will return to its normal state.
If albino corals don't wait for the symbionts to return, they starve to death, then the reef will also collapse, because the coral's calcium carbonate exoskeleton will be exposed as the coral dies, and the calcium carbonate is very easy to erod.
At last
This year marks the fourth event of massive bleaching of coral reefs, beginning with satellite inspections in 1985, largely due to persistent high temperatures – last year was considered the hottest year on record.
The first three bleaching events occurred in 1998, 2010 and 2016, and together they cost 30%-50% of the world's coral reefs.
As we mentioned earlier, coral reefs are mainly built from the exoskeleton of hard corals, but hard corals grow exoskeletons very slowly, and it is very impressive to grow 1 cm per year.
Pictured: The Great Barrier Reef has experienced a severe bleaching episode
The largest coral reef system in the world is the Great Barrier Reef off the coast of Australia, which consists of more than 2,900 small reefs linking 900 islands and stretches for 2,300 kilometres.
Some believe that it would take 2 million years for corals to form a system like the Great Barrier Reef, and that the collapse of the entire Great Barrier Reef could take just a few years.
Needless to say, the reef provides abundant fishery resources, and it also protects the surrounding beaches, and you can't imagine the consequences of losing the reef.
Original report: https://www.sciencealert.com/record-breaking-ocean-heat-triggers-4th-global-coral-bleaching-event