The foundation of coral reefs is an ancient symbiotic relationship between coral and microscopic algae. The single-celled, golden-brown algae, called zooxanthellae (genus Symbiodinium), are less than 0.01 millimeters in diameter, yet they are a fundamental part of most coral reef systems. The relationship between the coral and the zooxanthellae is mutualistic: Both gain something from it. This relationship works so well that, based on fossils, it’s been around for over 240 million years (since the Triassic period). While this relationship has endured the test of time, it is possibly being challenged more now than ever before, and, with it, the whole structure of coral reefs worldwide.

What Is Coral? To understand why zooxanthellae are so important to most corals, it’s important to be familiar with what corals are and what they do. Although it often looks more like a plant or a piece of scenery to us, coral is made up of many individual animals which live together in a colony, all interconnected by a thin layer of tissue.

Each individual is called a polyp, and resembles a sea anemone (which it is related to). Around their mouths, polyps have rings of tentacles which are retracted during the day, but come out at night to trap food ranging from plankton to small fish. Once caught and immobilized by the stinging tentacles, the prey is brought through the polyp’s mouth and into its stomach, or main body cavity.

The physical framework for coral reefs is limestone, which the coral is primarily responsible for making. Each polyp surrounds its body with the mineral aragonite (made from calcium and carbonate in the seawater), which can be turned into hardened limestone by other organisms. Over time, the reef grows upwards and outwards, about one centimeter a year in diameter and height. Consequently, after several hundred years of healthy growth, a coral may be several feet in diameter.

Corals and Their Zooxanthellae: So why are the zooxanthellae so important to the coral? When corals create aragonite, excess carbon dioxide is also generated, and this is consumed by the zooxanthellae. In turn, the zooxanthellae not only help the coral make aragonite more efficiently, but they also supply a source of nutrients to the coral: Through photosynthesis, the zooxanthellae convert carbon dioxide into glucose or amino acids that the coral can use.

Because of the key role of the zooxanthellae, most coral reefs must live within about 150 feet of the ocean surface, where they receive enough sunlight for the zooxanthellae to carry out photosynthesis.

The coral’s relationship with the zooxantheallae has enabled coral reefs to live in tropical, shallow waters around the world. It’s estimated the coral reefs cover about 200,000 square miles, perhaps as many as 350,000 square miles, located mostly between the Tropics of Cancer (23.4 degrees North) and Capricorn (23.5 degrees South). About 90 percent are in the Indo-Pacific. Temperatures are ideal for the reefs in these areas, with averages around 82 degrees Fahrenheit and minor fluctuations (highs of 85F and lows of 75F).

There are exceptions. Some reefs can go as far as 31 degrees North or South of the equator. Rarer still are deep-water coral reefs, which can be found nearly 1,500 feet under the ocean’s surface, and are frequently found around Norway. They’ve learned how to get by without photosynthesis.

In addition to temperature and water depth, where the coral reefs can survive is also tightly controlled by nutrient levels. Counter intuitively, coral reefs need relatively nutrient-poor waters (waters with low levels of nitrogen and phosphorous) to thrive. If nutrient levels are too high, algae can overtake the reefs. The corals’ symbiotic zooxanthellae provide the corals with nutrients, which are recycled throughout the reefs by bacteria, zooplankton, and others through countless, complex symbiotic relationships. Many other organisms in the coral reefs also have a symbiotic relationship with the zooxanthellae, including but not limited to jellyfish, sponges, and clams.

Diversity in the Reefs: Over the 240 million years that corals have had this key relationship with zooxanthellae, coral reefs have become home to an astonishing diversity of organisms, with more different species living in one area than any other marine habitat. Though estimates vary widely based on the measuring technique used, between 600,000 and 9,000,000 species are thought to live in coral reefs globally; we’ve categorized what might be only 10 percent of these.

The dominant organisms in most coral reefs are corals (both “stony,” belonging to the order Scleractinia, and “soft,” order Alcyonacea), sponges (phylum Porifera), and algae. Among corals alone there is great diversity; a given reef can have a few dozen, or hundreds, of coral species.

Because space is very limited on the reefs, competition is fierce, and a multi-million-year arms race has led to unique ways of making a living. With at least two thirds of reef fish carnivorous and willing to dine on a variety of creatures, many reef animals survive by being inconspicuous or building up massive defenses. Filter feeders, like sponges, quietly create water currents into their bodies to gather nutrients, while others, like corals, wait for prey to swim into their grasping tentacles. Mollusks and crustaceans are among those that hide in mobile, protective cases. Just as one animal develops a defense, its predator is evolving a way to bypass it. One predator of the corals, the crown-of-thorns starfish (Acanthaster planci), can bypass its aragonite defenses to devour the coral.

But the damage this starfish inflicts upon coral shies in comparison to the coral’s latest threat: people.

Reefs as Boon and Hazard:Throughout much of written history, coral reefs were noted mainly as a hazard to ships. Some observed in the 1700s that elaborate networks of life were in the reefs, but they could not be explored. In the 1800s, Charles Darwin developed several theories which still hold true today about the creation of coral reefs. Finally, about 60 years ago, the development of scuba equipment allowed much closer examination.

Coral reefs have been a boon for the millions of people living in poorer, tropical countries, where many individuals make only about a dollar a day. Not only are the reefs highly productive, providing shelter and sustenance to edible fish, and not only do they provide shoreline protection from ocean storms, but tourism can account for half to two-thirds of all the foreign income to some of these countries, such as the Republic of Seychelles and the Republic of Maldives.

Warming and Acidification Destroy Reefs: While overfishing and water pollution certainly damage the coral reefs, the biggest threat is global warming, which is destroying the reefs mainly through increasing the water temperature and acidifying the waters. Due to the many challenges the coral reefs are now facing, it’s thought that between one third and one fifth may already be destroyed or unable to recover, and another quarter may soon join them. The coral reefs may be the first major ecosystem to go extinct in modern times, unless major changes are made.

Over the last 20 to 30 years, there has been a significant increase in coral bleaching. Bleaching occurs when the water temperatures increase only two to three degrees Fahrenheit above the normal temperatures. For reasons not fully understood, this causes the zooxanthellae, which are normally bound in the coral’s endodermis cells, to be expelled from the coral.

If the coral does not get the zooxanthellae back within a month, the coral dies. The remaining coral skeleton is white, or “bleached.” In 1998, an El Nino year, around 16 percent of the coral reefs worldwide were destroyed by bleaching. In the August 27, 2010, issue of the magazine Science, Clive Wilkinson, a reef expert at Australia’s Reef and Rainforest Research Center in Townsville, predicted that this year may be worse than 1998, which would be especially disastrous for the many reefs that have not yet recovered from 1998. If temperatures continue to increase, as some experts predict they will, in about 25 to 60 years most reefs will be damaged too greatly to ever recover.

However, there is some evidence to suggest that there may be zooxanthellae that are temperature tolerant, and more resistant to bleaching; this has been especially seen in the fast-growing coral Acropora.

Over the last 200 years, the amount of carbon dioxide in the atmosphere has increased by 30 percent. Much of this has been taken up by the oceans and is greatly acidifying them. When carbon dioxide dissolves in seawater, it forms carbonic acid, which makes the water more acidic. This is the opposite of what corals need to create aragonite. Consequently, it’s predicted that in 30 to 50 years, coral’s ability to create aragonite will be decreased by about 30 percent.

Run-off Kills and So Do Melting Ice Caps:In addition, nutrient enrichment, overfishing, pollution, and disruption of shorelines all contribute to damaging the coral reefs. As mentioned above, waters with high nutrient levels (such as from farm fertilizer run-off) promote algal growth. Additionally, the crown-of-thorns starfish, that key coral predator, thrives in high nutrient waters.

Overfishing and the use of cyanide to “anaesthetize” reef fish for catching and selling to aquarium hobbyists are also problems. At least 25 million saltwater fish are traded annually, over 95 percent of them wild-caught. Unsurprisingly, chronic oil spills correlate with high mortality rates in coral reefs. Lastly, indigestible plastics, which make up about 90 percent of the floating material in the oceans, strangle and starve marine animals.

And then there are the rising sea levels due to melting of the polar ice caps. While sea levels have risen and fallen much in the last thousand years, the global change occurring now is much quicker. Sea levels are now increasing by about 3 millimeters a year, compared to an average of 1.7 millimeters over the 20th Century. While the reefs can grow up to 10 millimeters vertically every year, this is only when they are healthy. Given the current complications discussed above, it remains to be seen whether the corals can keep up with the increasing sea levels, or will fall behind and perish.

What Can We Do? While the future for the coral reefs certainly looks grim, action can still be taken. In 2004, Australia extended protections across its Great Barrier Reef. Among other measures taken, the protected “no-take” area was increased from 5 percent of the reef to 33 percent. The United States created the Coral Reef Conservation Act of 2000, which works through the National Oceanic and Atmospheric Administration to promote reef management, map reef locations, create outreach and education programs, and more. Many researchers are also working to better understand how the corals are affected by global warming and pollutants. Locally, in the Ecology, Evolution, and Marine Biology department at the University of California, Santa Barbara, Dr. Gretchen Hofmann studies the effects of acidification and global warming on ocean waters, and Dr. Sally MacIntyre investigates how such waters are affected by nutrient transport and pollutants.

On an individual level, some of the best actions to take to combat global warming in general are to be aware of energy consumption: reduce, reuse, and recycle. To be more active, volunteer for a local cleanup program. This Saturday, September 25, is 2010 California Coastal Cleanup Day, in association with United Way’s Day of Caring.

For more on coral reefs, see Charles R. C. Sheppard, Simon K. Davy, and Graham M. Pilling’s book, The Biology of Coral Reefs, the book Endangered Oceans by the Opposing Viewpoints Series, Martin Hovland’s book Deep-Water Coral Reefs: Unique Biodiversity Hot-Spots, the August 27, 2010 Science magazine article “Hard Summer for Corals Kindles Fears for Survival of Reefs,” Wikipedia’s article “Coral reefs,” or BBC’s The Blue Planet program “Coral Seas.”

Biology Bytes author Teisha Rowland is a science writer, blogger at All Things Stem Cell, and graduate student in molecular, cellular, and developmental biology at UCSB, where she studies stem cells. Send any ideas for future columns to her at


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