Why Are Scientists Exploring
Deep Coral Fore Reefs
A key purpose of NOAA’s Ocean Exploration Initiative is to investigate the more than 95 percent of Earth’s underwater world that until now has remained virtually unknown and unseen. Such exploration may reveal clues to the origin of life on Earth, cures for human diseases, answers on how to achieve sustainable use of resources, links to our maritime history, and information to protect endangered species.
Coral reefs provide habitats for some of the most diverse biological communities on Earth. Most people have seen photographs and video images of shallow-water coral reefs, and many have visited these reefs in person. In addition to their value for recreation and tourism industries, shallow-water reefs provide other benefits that include protecting shorelines from erosion and storm damage, supplying foods that are important to many coastal communities, and providing very promising sources of powerful new antibiotic, anti-cancer and anti-inflammatory drugs (for more information about drugs from the sea, see ‘More About Biotechnology and Drugs from the Sea,’ below).
Shallow-water corals often have microscopic algae called zooxanthellae (pronounced ‘zoh-zan-THEL-ee’) living inside their soft tissues. These algae provide an important source of nutrition for many coral species, and may also be involved with the corals’ growth. Because zooxanthellae depend on sunlight for photosynthesis, scientists have historically believed that reef-building corals were confined to relatively shallow depths where there is enough sunlight to support photosynthesis. Below 150 m, light levels are not adequate to support photosynthesis. Recently, though, ocean explorers have discovered extensive mounds of living coral in depths from 400 m to 700 m-depths at which there is virtually no light at all! These deep-water corals do not contain zooxanthellae (so they are called ‘azooxanthellate’), and do not build the same types of reef that are produced by shallow-water corals. But branches of some deep-water coral species (e.g., Lophelia pertusa) grow on mounds of dead coral branches that can be several meters deep and hundreds of meters long. Recent studies indicate that the diversity of species in deep-water coral ecosystems may be comparable to that of coral reefs in shallow waters, and that there are just as many species of deep-water corals (slightly more, in fact) as there are species of shallow-water corals. For more information, activities, and lessons about coral reefs, visit the NOAA’s National Ocean Service Coral Reef Discovery Kit.
Around the world, shallow water coral reefs have been intensively studied by scientists using self-contained underwater breathing (SCUBA) equipment, while deep coral systems are being investigated with submersibles and remotely operated underwater vehicles (ROVs). Recent explorations have found a third type of coral ecosystem between depths of 50 m and 150 m: light-limited deep reefs living in what coral ecologists call the ‘twilight zone.’ These reefs have been studied much less than shallow and deep-water reefs because they are beyond the safe range of conventional SCUBA equipment, yet are too shallow and close to shore to justify the use of expensive submersibles and ROVs. The few studies of twilight zone reefs suggest that these ecosystems not only include species unique to this depth range, but may also provide important refuges and nursery habitats for corals and fishes that inhabit shallower reefs. This is particularly important in areas where shallow reefs are severely stressed (see ‘More About Threats to Coral Reefs,’ below) since twilight zone coral ecosystems may provide a natural option for recovery.
Scientific exploration of twilight zone coral reef ecosystems is urgently needed to provide information for their protection, as well as to identify potentially important sources of drugs and other biological products from organisms that are endemic to these systems. Helping to meet this need is the primary focus of the 2007 Ocean Explorer Cayman Island Twilight Zone Expedition.
The Cayman Island Twilight Zone Expedition is focused on three broad questions:
- Where are deep coral fore reef communities found?
- What is the biodiversity and distribution of living organisms in these communities?
- Which species have potential for biotechnology or as sources of new pharmaceutical products?
In addition, scientists plan to investigate a variety of secondary questions related to reproduction in deep coral reefs, photosynthetic capability, growth rates, connections between deep fore reefs and shallow water coral ecosystems, and how chemical defenses of soft corals and sponges vary in response to changing environmental conditions.
The technological keys for the Cayman Island Twilight Zone Expedition are new diving techniques that enable scientists to personally visit deep-water ecosystems without the need for expensive submersibles.
In 1943, Jacques Cousteau and Emile Gagnan invented a demand regulator which provided a way to control the release of air from a pressurized cylinder so that a diver could breathe underwater. In the following decades, self-contained underwater breathing apparatus (SCUBA) became a standard tool for marine scientists that gave personal access to underwater environments. Over the years, improvements were made to the Cousteau-Gagnan regulator, but the practical depth range of SCUBA equipment was limited by a combination of physics and human physiology.
As anyone knows who has ever tried, it is impossible to breathe underwater through a snorkel, pipe, or hose that is more than a few feet long. This is because the surrounding water exerts pressure on a diver’s body that is equal to one atmosphere (about 14 pounds per square inch) for every 33 feet of depth. To breathe through a long snorkel underwater, the muscles we use to fill our lungs have to overcome this water pressure - and they are not designed to do that.
The Cousteau-Gagnan regulator overcame this problem by supplying air at a pressure that matched the pressure in the water (ambient pressure), so there was no additional pressure for breathing muscles to overcome. The problem is that a diver using this type of regulator breathes gas at higher pressures than at the surface. Instead of a normal pressure of one atmosphere, a diver at 33 feet is breathing air pressurized to two atmospheres; at 66 feet, the pressure is three atmospheres, and so on.
Why is this a problem? One problem is that if a diver takes a lungful of air at a depth of 66 feet and then ascends to 33 feet, the external pressure drops from three atmospheres to two atmospheres. Since the volume of a gas is inversely proportional to the pressure of the gas, as the pressure drops the volume will increase. So the air in the diver’s lungs would expand, and if the diver was holding his breath the expanding air could rupture his lungs. When this happens, air bubbles can enter the bloodstream causing a condition known as air embolism. If the bubbles block important blood vessels, the result can be paralysis or death.
Another problem is that as the pressure of a gas increases, the solubility of that gas in a liquid increases as well (Henry’s law). So if a diver breathes air from a demand regulator at 66 feet for a while, her blood will contain twice the amount of dissolved gases from the air than it did at the surface. Again, the problem comes when the pressure is reduced. Anyone who has ever opened a can of soda knows what happens when you suddenly release the pressure on a liquid containing dissolved gas: bubbles form in the liquid. If the diver rapidly ascends from 66 feet, the dissolved gases in her blood may form bubbles, creating a problem that is somewhat similar to an air embolism in that critical blood vessel may become blocked. This condition is called decompression sickness or the bends, and was first seen in miners working in pressurized coal mines (it was also a problem for workers constructing the Brooklyn Bridge, who spent hours working underwater in pressurized iron boxes called caissons, so yet another name for the condition is caissons disease). Since air is about 70% nitrogen, more nitrogen is dissolved in the blood than other gases and the bubbles of decompression sickness are bubbles of nitrogen gas. Oxygen isn’t believed to be involved, since much of the oxygen dissolved in a diver’s blood is quickly bound by hemoglobin, and normal metabolism reduces blood oxygen concentration.
Divers avoid decompression sickness by closely monitoring their dive time and depth, since they both affect the amount of gas that dissolves in the blood. Decompression tables and dive computers show how long a diver may stay at a particular depth without having a high risk of decompression sickness. If they stay longer than this time, then they have to return to the surface in stages, stopping for a specific amount of time at shallower depths (decompression stops) to allow the nitrogen diffuse out of their blood without forming bubbles.
More problems arise from the fact that as the pressure of a gas increases, the physiological effects of that gas may change; and some of these changes are bad news for divers. Oxygen, for example, becomes toxic at high pressure and can cause convulsions. Nitrogen can cause a condition called nitrogen narcosis or rapture of the deep, which is similar to alcohol intoxication.
Additional discussion of diving physiology is provided in the Now Take a Deep Breath lesson (PDF, 288 kb).
To overcome these problems, a diver must breathe something other than ordinary air, and this is where technical breathing mixtures come in. The easiest way to deal with the problem of oxygen toxicity is to reduce the proportion of oxygen in the breathing gas mixture. But this means that the proportion of some other gas would have to be increased, and increasing the proportion of nitrogen would increase problems with nitrogen narcosis. So gas mixtures for deep diving substitute helium for nitrogen. Helium is not toxic, even under the high pressures needed for deep diving. Helium also has another advantage: it is a much smaller molecule, and therefore less dense than nitrogen. As the pressure of a gas increases, so does its density; and as gas density increases so does the work needed to move that gas around. Since the density of helium is less than nitrogen, it is easier to inhale and exhale under high pressure.
Some breathing gas mixtures for deep diving consist of oxygen and helium alone, and are called Heliox. Others contain nitrogen as well, and are called Trimix. The problem with mixtures containing helium is that the small helium molecules dissolve into the blood much more quickly than nitrogen, so longer decompression times are needed. To deal with this problem, divers may turn to a Nitrox mixture that contains nitrogen and oxygen but with less nitrogen and more oxygen than ordinary air. Nitrox mixtures can be used at moderate depths without risking oxygen toxicity, and allow divers to greatly decrease the time needed for decompression.
The Cousteau-Gagnan regulator and its modern descendants are called open circuit regulators, because when a diver exhales the gas is released into the surrounding water. In a closed-circuit system, on the other hand, carbon dioxide in the exhaled gas is removed by a device called a scrubber, and the remaining gas is returned to the diver for another breath. Since only about 25% of the oxygen in a single breath is actually consumed, most of the oxygen supplied by open-circuit systems is lost in exhaled gas. Closed-circuit systems recapture this oxygen, and so a diver can carry much less breathing gas when using these systems. Oxygen consumed by the diver is replenished either from a supply of pure oxygen, or by chemicals in the scrubber that release oxygen as part of the reactions that absorb carbon dioxide. Modern closed circuit rebreathers constantly monitor oxygen levels in the breathing mixture and are able to adjust the oxygen concentration to a level that is optimum for the divers’ depth. The result is much shorter decompression times and much less risk of gas toxicity.
Scientists on the Cayman Island Twilight Zone Expedition will use both open- and closed-circuit diving systems, as well as special breathing mixtures such as Trimix.
More About Threats to Coral Reefs
Even though they provide numerous benefits to humans, coral reefs are threatened by human activities. Sewage and chemical pollution can cause overgrowth of algae, oxygen depletion, and poisoning of shallow-water reefs. These reefs are also damaged by careless tourists and boat anchors. 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 degrees north or south of the equator) where water temperatures are close to the maximum temperature that corals can tolerate. Abnormally high temperatures result in thermal stress, and many corals respond by expelling their zooxanthellae. Since the zooxanthellae are responsible for most of the corals’ color, corals that have expelled their algal symbionts appear to be bleached. Because zooxanthellae are important to corals’ nutrition and growth, 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. But if other types of stress are present and the stress is sustained, 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 20 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 events often occur in areas where the sea surface temperature rises 1degree C or more above the normal maximum temperature.
Deep-water coral ecosystems may also be in serious danger. Commercial fisheries, particularly fisheries that use trawling gear, cause severe damage to both shallow and deep-water habitats. Ironically, some scientists believe that destruction of deep-sea corals by bottom trawlers is responsible for the decline of major fisheries such as cod. Deep-sea coral communities can also be damaged by oil and mineral exploration, ocean dumping, and unregulated collecting. Other impacts may result from efforts to mitigate increasing levels of atmospheric carbon dioxide. One proposed mitigation is to sequester large quantities of the gas in the deep ocean, either by injecting liquid carbon dioxide into deep ocean areas where it would form a stable layer on the sea floor or by dropping torpedo-shaped blocks of solid carbon dioxide through the water column to eventually penetrate deep into benthic sediments. While the actual impacts are not known, some scientists speculate that since coral skeletons are made of calcium carbonate, their growth would probably decrease if more carbon dioxide were dissolved in the ocean.
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. Using high-resolution satellite imagery and Global Positioning Satellite (GPS) technology, the National Oceanic and Atmospheric Administration (NOAA) has made comprehensive maps of reefs in Puerto Rico, the U.S. Virgin Islands, the eight main Hawaiian Islands and the Northwestern Hawaiian Islands. Maps of all shallow U.S. coral reefs are expected to be completed by 2009. NOAA monitors reefs using a system of specially designed buoys that measure air temperature, wind speed and direction, barometric pressure, sea temperature, salinity and tidal level, and transmit these data every hour to scientists. Satellites are also used to monitor changes in sea surface temperatures and algal blooms that can damage reefs. Research and restoration projects on selected coral reefs are conducted by NOAA’s National Undersea Research Program.
More About Biotechnology and Drugs from the Sea
Biotechnology is defined as the industrial use of living organisms or biological techniques developed through basic research. Marine biotechnology involves products or processes that are derived from living ocean organisms, such as artificial bone (from corals), cosmetic ingredients (from crustaceans and soft corals), glues (from mussels), and food supplements (from fish and algae). A particularly exciting area of marine biotechnology is the search for new drugs that can be used to treat cancer, inflammatory diseases, and other human health problems. Most drugs in use today come from nature. While almost all of these drugs are derived from terrestrial plants and microbes, 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.
Many of these organisms are sessile, which means that they do not move much, if at all. This may give a clue about why they produce powerful substances: An animal that is stuck in one place needs some way to repel predators; and powerful chemicals could be one solution. Another possibility is that since many of these species are filter feeders, they are exposed to all sorts of parasites and disease-causing bacteria in the water; so the powerful chemicals may be a defense against parasites or antibiotics against disease-causing organisms. Competition for space may explain why some of these animals produce anti-cancer substances: If two species are competing for the same piece of bottom space, it would be helpful to produce a substance that would attack rapidly dividing cells of the competing organism. Since cancer cells often divide more rapidly than normal cells, the same substance might have anti-cancer properties. For more information about drugs from the sea, visit the Ocean Explorer Web site for the 2003 Deep Sea Medicines Expedition.
For More Information
Contact Paula Keener-Chavis, national education coordinator for the NOAA Office of Ocean Exploration, for more information.
Other lesson plans developed for this Web site are available in the Education Section.