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Caution: Do Not Bleach!

Coral Reef Bleaching / Grades 9-12 / Life Science

Focus Question

Why are coral reefs important, and what are possible explanations for the phenomenon known as “coral bleaching?”

Learning Objectives

Links to Overview Essays and Resources Useful for Student Research


Audio/Visual Materials

None, unless students require A-V equipment for their public education programs

Teaching Time

One or two 45-minute class periods

Seating Arrangement

Classroom style or groups of 4-6 students

Maximum Number of Students


Key Words

Coral reefs

Background Information

Coral reefs are one of the most biologically productive ecosystems on Earth. Most people have seen images of brightly colored fishes and other reef-dwelling organisms, yet many do not understand why these systems are personally important. Programs and articles about coral reefs typically point out benefits that include protecting shorelines from erosion and storm damage, supplying foods that are important to many coastal communities, and providing recreational and economic opportunities. These benefits are obviously important to people who live near reefs, but there is another aspect of coral reefs that can benefit everyone: the highly diverse biological communities are new sources of powerful antibiotic, anti-cancer and anti-inflammatory drugs.

The idea of coral reefs as a source of important new drugs is new to many people; but in fact, most drugs in use today come from nature. Aspirin, for example, was first isolated from the willow tree. Morphine is extracted from the opium poppy. Penicillin was discovered from common bread mold. Although almost all of the drugs derived from natural sources come from terrestrial organisms, recent systematic searches for new drugs have shown that marine invertebrates produce more antibiotic, anti-cancer, and anti-inflammatory substances than any group of terrestrial organisms. Particularly promising invertebrate groups include sponges, tunicates, ascidians, bryozoans, octocorals, and some molluscs, annelids, and echinoderms. For more information on drugs from the sea, visit

Despite their numerous benefits to humans, many coral reefs are threatened by human activities. Sewage and chemical pollution can cause overgrowth of algae, oxygen depletion, and poisoning. Fishing with heavy trawls and explosives damages the physical structure of reefs as well as the coral animals that build them. Careless tourists and boat anchors also cause mechanical damage. Thermal pollution from power plants and global warming cause physiological stress that kills coral animals and leaves the reef structure vulnerable to erosion. Many of these impacts are the result of ignorance; people simply aren’t aware of the importance of coral reefs or the consequences of their actions, but the damage and threats to reefs continues to increase on a global scale.

Some of the most severe damage appears to be caused by thermal stress. Shallow-water reef-building corals live primarily in tropical latitudes (less than 30° north or south of the equator). These corals live near the upper limit of their thermal tolerance. Abnormally high temperatures result in thermal stress, and many corals respond by expelling the symbiotic algae (zooxanthellae) that live in the corals’ tissues. Since the zooxanthellae are responsible for most of the corals’ color, corals that have expelled their algal symbionts appear to be bleached. Because zooxanthellae provide a significant portion of the corals’ food and are involved with growth processes, expelling these symbionts can have significant impacts on the corals’ health. In some cases, corals are able to survive a “bleaching” event and eventually recover. When the level of environmental stress is high and sustained, however, the corals may die.

Prior to the 1980s, coral bleaching events were isolated and appeared to be the result of short-term events such as major storms, severe tidal exposures, sedimentation, pollution, or thermal shock. Over the past twenty years, though, these events have become more widespread, and many laboratory studies have shown a direct relationship between bleaching and water temperature stress. In general, coral bleaching often occur in areas where the sea surface temperature 1°C or more above the normal maximum temperature.

In 1998, the President of the United States established the Coral Reef Task Force (CRTF) to protect and conserve coral reefs. Activities of the CRTF include mapping and monitoring coral reefs in U.S. waters, funding research on coral reef degradation, and working with governments, scientific and environmental organizations, and business to reduce coral reef destruction and restore damaged coral reefs.

As co-chair of the CRTF, and as directed by the Coral Reef Conservation Act of 2000, NOAA has the responsibility to conserve coral reef ecosystems. NOAA’s coral reef conservation efforts are carried out primarily through its Coral Reef Conservation Program (CRCP). Under this program, NOAA works with scientific, private, government, and nongovernmental organizations at the local, federal, and international levels to address conservation actions. Among other actions, the CRCP undertakes a variety of mapping and monitoring activities to understand:

The first part of this lesson is intended to:

In the second part of this lesson, students design and prepare educational programs to improve public awareness of the importance of coral reefs and what needs to be done to reduce or eliminate harmful impacts from human activities. This activity offers many opportunities for cross-curricular activities, and may be extended over several weeks or months. If time is limited, you may choose to use the first part alone.


Learning Procedure

Part 1

1: Direct students to the Corals Tutorial at:
. You may want to assign different tutorial sections to each student group. Have each student or student group complete one version of the Subject Review (downloadable), and lead a discussion to review the answers. Be sure that students understand the relationship between coral animals and their symbiotic algae (zooxanthellae), and that under thermal stress many corals will expel their zooxanthellae.

Briefly explain the purpose and activities of the U. S. Coral Reef Task Force (CRTF) and the NOAA Coral Reef Conservation Program (CRCP), and highlight the monitoring functions that are intended to identify reef areas threatened by thermal stress or algal blooms (visit for more information on the CRTF and for more information on the CRCP).

2: Tell students that their assignment is to investigate possible explanations for zooxanthellae expulsion by corals under stress, and prepare a written report outlining at least one hypothesis that explains this behavior. The report should explain:

If you want to provide a starting point for this research, the following resources will be useful: (NOAA Coral Reef Watch Program and Satellite Monitoring of Coral Bleaching)

(Roadmap to Resources: Corals) (article: Coral Bleaching: the Whys, the Hows and What Next?) (article: Climate Change, Human Impacts, and the Resilience of Coral Reefs) (article: Coral Bleaching: Causes, Consequences and Response)

3: Lead a discussion of students’ research results. Written reports should include some of the following points:

Ask students to discuss why coral reefs are at risk, and what they think can or should be done to reduce or eliminate the negative impacts of human activity on coral reefs. There is a strong possibility that a significant part of the current risk to coral reef systems is the result of human activity, particularly as it relates to climate change. Meaningful actions to address this type of issue depend upon widespread understanding of the problem and commitment to workable solutions. Public education is an important step toward building this sort of understanding and commitment. Have students brainstorm what “key messages” might form part of a public education program about coral reefs, what audiences should be targeted to receive these messages, and how these messages might be most effectively delivered to these audiences.

Part 2

Have students or student groups prepare one or more public education programs about coral reefs, based on the results of their brainstorming sessions in Step 3. Encourage students to consider various media, including publications, visual presentations, drama, music, etc. You may want to have an entire class work on a single program, or have smaller groups work on multiple programs using the medium (or media) or their choice. There are many possibilities, depending upon the target audiences. These presentations also offer cross-curricular opportunities, particularly with social studies, English language arts, and fine arts. Whatever media students choose to work with, their final presentation should be accompanied by a list of sources for the information they present. A good starting point for this activity is the Roadmap to Resources: Corals (), which provides links to many other sources of coral reef data and information.


The Bridge Connection

The Bridge is a growing collection online marine education resources. It provides educators with a convenient source of useful information on global, national, and regional marine science topics. Educators and scientists review sites selected for the Bridge to insure that they are accurate and current.

The “Me” Connection

Have students write a short essay on why coral reefs are personally important, what personal actions may contribute to human-caused threats to coral reefs, and what they could do to reduce these threats.


1. The symbiotic relationship between zooxanthellae and coral polyps can be a springboard for discussing other types of cellular symbionts. Many biology students tend to overlook microbial associations in natural communities, but there is mounting evidence that eukaryotic organisms were (and are) the result of symbiotic associations between prokaryotic organisms. At some point in these associations, one (or more) species (called endosymbionts) entered the cells of another species, and performed useful functions. Each species had its own DNA, and when these organisms reproduced, both were replicated. Eventually, the individual identities of the species disappeared, resulting in a new type of organism. This sort of transformation has actually been seen in the laboratory, and is described in Margulis and Sagan (1986). For more information and a lesson plan devoted to this topic, visit

2.  Have students or student groups prepare a report on a specific aspect of coral biology, ecology, or management. Some possible topics include:

See Roadmap to Resources: Corals () for links to information on these and other relevant topics.

3. For more lesson plans and activities related to coral reefs, visit the education web pages for NOAA’s Ocean Explorer Cayman Islands Twilight Zone 2007 Expedition at



The National Ocean Service education Web site’s Roadmap to Resources about corals, with links to many other sources of coral reef data, background information, and reports
Diamante-Fabunan, D. 2000. Coral Bleaching: the Whys, the Hows and What Next? OverSeas, The Online Magazine for Sustainable Seas. – “Coral Bleaching: Causes, consequences and response;” a collection of papers from the ninth international coral reef symposium.
“The State of Deep Coral Ecosystems of the United States,” 2007 report from NOAA providing new insight into the complex and biologically rich habitats found in deeper waters off the U.S. and elsewhere around the world.,0,7842752.special – “Altered Oceans,” five-part series from the Los Angeles Times on the condition of Earth’s ocean; published July 30 – August 3, 2006

National Science Education Standards

Content Standard A: Science as Inquiry

Content Standard C: Life Science

Content Standard E: Science and Technology

Content Standard F: Science in Personal and Social Perspectives

Ocean Literacy Essential Principles and Fundamental Concepts

Essential Principle 1. The Earth has one big ocean with many features.

Essential Principle 5. The ocean supports a great diversity of life and ecosystems.

Essential Principle 6. The ocean and humans are inextricably interconnected.

Essential Principle 7. The ocean is largely unexplored.