By Emma Simon
Supporting a quarter of all marine species, coral reefs are some of the most beautiful, diverse, and important ecosystems on the planet. Unfortunately, they are also some of the most threatened. Climate change exacerbates a number of threats to coral reefs, including infectious diseases, algal blooms, and decreased growth rates. The two biggest headline-making dangers to coral reefs right now are marine acidification and coral bleaching.
In order to understand why climate change poses such a threat to coral reefs, we must start with a basic understanding of the coral structure.
All corals belong to the phylum Cnidaria (along with anemones, jellyfish, and other nematocyst-producing animals) and the class Anthozoa. Anthozoa is divided into the subclasses of octocorals (often colloquially referred to as soft corals) and hexacorals. These subclasses are named for the number of tentacles surrounding each individual polyp. One order of hexacorals is Scleractinia, reef-building corals. These are the organisms that are responsible for the structure and habitat of coral reefs— the mental image you associate with the word “coral” is most likely a form of Scleractinia.
Corals come in a variety of shapes and sizes, but nearly all provide important habitats for reef-dependent organisms.
The subclass of octocorals includes gorgonians, or sea fans, which form elaborate and colorful branching patterns on reefs.
Corals are colonial animals; each colony is composed of a vast number of connected polyps. These polyps are soft and transparent, and range in size from one to three millimeters. They consist of a small mouth surrounded by tentacles that assist them in filter-feeding microscopic plankton from the water. The ectoderm— or outermost cellular layer— of each polyp is armed with nematocysts, cells that produce stinging nematocysts used for defense.
Each coral polyp is protected by a corallite, composed of a form of calcium carbonate (CaCO3) called calcite and produced by cells within the aboral ectoderm of the polyp— a process of biomineralization. These corallites connect between the polyps to form a calcite skeleton structure for the coral.
Each coral species has a unique Skeletal Organic Matrix (SOM) encoded in its genetic material that determines the structure and development of its corallites.
Corals obtain some of their energy through filter feeding, but a large portion comes from the symbiotic relationships they form with photosynthetic zooxanthellae, usually dinoflagellates of the genus Symbiodinium. The zooxanthellae provide nutrients in the form of carbohydrates to the coral in exchange for a place to live.
These terms can seem intimidating, but the key takeaway is that tiny, colorful, nutrient-producing plankton— zooxanthellae— live inside corals and keep them alive.
Corals’ system of biomineralization and their symbiotic relationship with zooxanthellae are threatened by increased carbon dioxide (CO2) in the oceans and rising sea surface temperatures, respectively.
When CO2 is present in seawater, it dissociates and combines with water (H2O) to form bicarbonate (HCO3-), leaving hydrogen ions (H+) as a byproduct. As the concentration of H+ increases, the pH of the water decreases. The ocean typically resides at a pH of 7.8 – 8.5, so it is considered alkaline. The term “marine acidification” could perhaps more accurately be referred to as marine “less-alkaline-ification”. The current atmospheric CO2 concentration is right around 420 ppm, and about 30% of this is absorbed by the ocean. Surface seawater has already decreased 0.1 pH units, representing a 30% change since the start of the Industrial Revolution. Even more alarming, surface seawater is predicted to decrease by an additional 0.3 pH units by the end of the century.
How does the increase in ocean acidity impact coral reefs?
Coral biomineralizes calcium carbonate through a reaction involving bicarbonate and calcium ions (Ca2+). High concentrations of H+ make it more difficult for corals to biomineralize the calcium carbonate needed for their skeletons because the H+ will react more readily with the bicarbonate than the calcium ions will to form carbonic acid (H2CO3), essentially stealing a key ingredient to coral’s calcite recipe. So coral and all other calcifying organisms are forced to work much harder when CO2, and in turn, H+, is abundantly present in the oceans.
A coral’s color comes from the photosynthetic zooxanthellae residing within its polyps.
So that sums up marine acidification. But what about our second threat to reef ecosystems: coral bleaching?
Corals are extremely temperature sensitive— they require a narrow range, around 27°C – 31°C, to maintain a healthy relationship with their zooxanthellae. When temperatures spike too far out of this range (any greater than 32°C), or spike too quickly, the corals experience oxidative stress. The zooxanthellae, recognizing the sickness within the cells of their hosts, are expelled, and the corals are left without the symbiosis responsible for their energy. The zooxanthellae are responsible for coral’s bright colors, and in their absence, all that is left is the bone-white calcium carbonate skeleton. This is referred to as coral bleaching. Corals can survive for a few weeks without their zooxanthellae. If ocean temperatures return to their preferred range within this time, the zooxanthellae will return and they can recover from bleaching. If not, the corals will die.
When a coral colony dies, algae take over the deserted skeleton.
Parrotfish, and other algae-feeding reef organisms, can assist with recovering corals by consuming algae before it can take over the diseased reef.
The good news is, corals have survived both low ocean pH and high ocean temperatures before. During a period called the Paleocene-Eocene Thermal Maximum, about 55 million years ago, massive amounts of CO2 were released into the atmosphere. Corals survived, but barely. Many species went extinct, and those that didn’t, took millions of years to reestablish their role in the marine environment. Even now, CO2 is being released at an unprecedented rate, and it is unclear whether corals will be able to recover from bleaching events as they have in the past.
Scientists are researching ways to make corals more resilient or reestablish corals in areas where they have disappeared. Organizations working towards coral reef conservation include the Coral Gardeners, NOAA, and the Coral Reef Alliance. To support the future of a quarter of all marine life, support the exploration of science. Coral reefs are far too important to let disappear.