What is the most biodiverse ecosystem on Earth?
Many might think of tropical rainforests, but the more likely answer is actually the coral reef ecosystems of the oceans.
Coral reefs, covering only about 1% of the ocean's total area, support approximately 25% of marine life.
Image: Bleached coral
The National Oceanic and Atmospheric Administration (NOAA) has announced that coral reefs globally are experiencing the fourth recorded mass coral bleaching event.
Due to the vital role coral reef ecosystems play, at least 850 million people globally are facing severe impacts, including reduced income and increased food scarcity.
So, what exactly are coral reefs, and why do they suddenly undergo bleaching?
Holobionics
What exactly are corals?
Corals, with their appearance straddling the line between animal and plant, have led to some confusion in classification. While they were once considered plants, corals are now classified as cnidarians, which are animals.
However, corals are not your typical animals. When we refer to corals, we're usually talking about a "colony" of corals, which is a collective formed by numerous genetically identical coral polyps. This is known as a modular organism.
The smallest unit of a coral colony is the coral polyp. Each polyp is an individual animal, typically presenting a sac-like structure, with diameters often just a few millimeters and lengths reaching only a few centimeters.
Image: A coral polyp
Corals reproduce asexually by continuously producing genetically identical individuals (or modules) around themselves. These genetically identical coral polyps are then connected by living tissue to form a complete organism - what we call coral.
Although corals are modular organisms, they don't grow haphazardly.
Take hard corals, for example. Each coral polyp secretes a skeleton made of calcium carbonate near its base, and these skeletons grow to exhibit the biological characteristics of the species.
There's another type of coral known as soft corals. Like hard corals, they are modular organisms, but they don't fully build themselves upon their skeletons like hard corals do. Instead, these corals resemble other cnidarians more closely.
Image: Anatomy of a Coral Polyp
In addition to creating coral through asexual reproduction, each coral polyp also has the ability for sexual reproduction. Members of the same coral species release gametes together at night, which freely fertilize in the water, ultimately resulting in coral larvae.
Image: Illustration of Coral Sexual Reproduction
Coral larvae, unlike adult individuals, do not anchor themselves. They drift freely in the ocean in the form of plankton until they settle in a suitable location and develop into coral.
What about Coral Reefs and Coral Bleaching?
When we talk about coral reefs, we're mainly referring to structures built by the external skeletons of hard corals, which are known as reef builders.
Image: Coral reefs are mainly distributed in tropical waters.
The coral lifestyle is quite unique. While some coral species possess hunting abilities and use their cnidocytes to attack and paralyze small prey nearby, then consume them directly, they typically don't rely on this method for energy acquisition.
The primary means through which corals obtain energy is through symbiosis. They form symbiotic relationships with zooxanthellae, which engage in photosynthesis to provide energy for the corals to survive. In exchange, the corals offer shelter for their reproduction.
Because coral symbionts rely on photosynthesis, coral reefs are usually found in shallow waters. However, there are some coral species that don't depend on photosynthetic symbionts and can create coral reefs in deep-sea environments.
Image: Coral
In fact, many coral polyps themselves are transparent, and their colors are produced by symbiotic organisms. When corals lose their symbiotic organisms, we can see through the surface of the coral polyps to the calcium carbonate skeleton inside, which appears white. This is what we call coral bleaching.
Of course, coral bleaching is a broad term because not all coral polyps are colorless. Some species of coral polyps have their own colors - blue, pink, purple, or yellow. However, the presence of symbiotic organisms also changes their colors.
In this case, when corals lose their symbiotic organisms, their colors also change (returning to their original state), making it very easy to identify.
If we want to understand why corals bleach, we just need to know why they lose their symbiotic algae.
In fact, the relationship between corals and their symbionts is even closer than we imagine. These symbionts live directly within the coral cells' cytoplasm.
Coral cells serve as the living and breeding grounds for these symbionts. They don't voluntarily leave the corals, but corals have the power of choice. They use their immune systems to retain desired symbionts and expel unwanted ones.
As for why corals would expel symbionts that were once beneficial, the reason is quite simple—they can sometimes become toxic. Yes, really toxic.
The most common reason for corals to expel symbionts is when the symbionts produce reactive oxygen species within the coral cells, which is toxic to the corals.
So why would symbionts suddenly start producing reactive oxygen species? The most common cause is changes in water temperature. As environmental temperatures rise, the zooxanthellae release increasing amounts of reactive oxygen species, eventually causing corals to eject them.
Over time, bleached corals (those that have lost their symbionts) may starve to death because their hunting efficiency cannot sustain their long-term survival.
However, once the symbiotic organisms become non-toxic again, the corals will readmit them, avoiding starvation and returning to their normal coloration.
If bleached corals do not wait for the return of symbiotic organisms and instead starve en masse, coral reefs will collapse. This is because the calcium carbonate exoskeleton of corals will be exposed as the corals die, and calcium carbonate is highly susceptible to corrosion.
Finally,
Since the satellite monitoring of coral reefs began in 1985, this year marks the fourth mass coral bleaching event, primarily attributed to prolonged high temperatures—last year being considered the hottest on record.
The previous three bleaching events occurred in 1998, 2010, and 2016, collectively resulting in the loss of 30% to 50% of global coral reefs.
As mentioned earlier, coral reefs are primarily built by the exoskeletons of hard corals, and the growth of these hard corals' exoskeletons is extremely slow, with an impressive growth rate of only 1 centimeter per year.
Image: The Great Barrier Reef has experienced a severe bleaching event
The largest coral reef system in the world is the Great Barrier Reef, located off the coast of Australia. It consists of over 2,900 individual reefs and encompasses 900 islands, stretching over 2,300 kilometers.
Some believe that it took around 2 million years for the coral to form a system like the Great Barrier Reef, yet its collapse could occur in just a few short years.
The importance of coral reefs extends beyond providing abundant fishery resources; they also play a crucial role in protecting surrounding beaches. The consequences of losing coral reefs are unimaginable.
