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Coral Reefs: Can we live without them?

 

By:  MerrieBeth Neely and Lisa Young

We are in serious trouble. As if our everyday challenges aren't enough - getting chomped by a parrotfish, raked by a roving sea star, or dizzied by a hurricane. These days we get slashed by store-bought fins, sliced with boat anchors, and suffocated by sewage. Ours is not an easy life, and it's not getting any easier.

If corals could talk, that's what they might say, and this article is dedicated to explaining why.

First, let's define coral. There are two types of corals: those that have skeletons and build reefs (stony corals), and those that do not (soft corals). When you think of a coral reef, you might picture the exotic creatures that live there like lobsters, eels, and angelfish. But what exactly is a coral? Is it an animal, a plant, or a rock?

Well, for reef-building corals, the answer is all three! Corals are colonies of tiny, delicate animals called polyps. Each polyp has a simple body structure: a mouth, a body sac, and tentacles to capture food. Now here's the plant part. Thousands of microscopic algae called zooxanthellae (pronounced zoo-zan-thell-ay) live within the polyp's inner tissues. The coral and the algae have a mutually beneficial (symbiotic) relationship. The coral host layer provides carbon dioxide and nutrients that the zooxanthellae use in photosynthesis, the light-driven energy process in which plants use carbon dioxide and water to produce carbohydrates and oxygen. The coral uses the oxygen for respiration, and the carbohydrates for much of its daily energy supply. Now here's where the rock part comes in: The energy helps the coral host build its protective carbonate (or limestone) skeleton from calcium and bicarbonate in the seawater. Rarely growing more than a half inch per year, these individual skeletons fuse over time to form a reef.

So how do humans alter the lives of these unique rock-like, plant-harboring animals?

Just as we need clean air to breathe and function properly, corals need clear, clean water to make a living. Clear, because corals need sunlight to enable their zooxanthellae to photosynthesize and their polyps to grow. Clean, because too many particles in the water can smother the polyp tissues. Unfortunately, humans do a lot to muck up the water. Activities such as chopping down trees and dredging waterways dump coral-clogging silt into the water. Nutrients from our fertilizers and sewage encourage the growth of other kinds of algae in the water above the coral that prevent light from reaching the zooxanthellae inside.

Furthermore, one person's recreation can mean a coral community's demise. In just seconds, careless boaters and divers can destroy layers of coral skeletons that took the tiny reef-builders years to create. Damage might come in the form of a boat anchor breaking apart a coral head, or even a careless kick from a snorkeling fin.

 

 

 

 

 

A huge ship splintering the reef to pieces as it runs aground can destroy thousands of years of growth (Photos courtesy of Walt Jaap, Florida Fish and Wildlife Conservation Commission, Photo taken 1984, Key Largo, foreign cargo vessel carrying animal feed grounded on Molasses Reef).  It is also a common practice in many parts of the world to squirt poisons over the corals or explode reefs to harvest desirable fish and reef organisms for the profitable aquarium trade. Over-harvesting of fish and shellfish for the seafood industry alters the complex food web of the coral reef community.

Other threats by humans occur over longer time scales. For example, high temperatures and pollution are lethal to corals - they grow even more sluggishly than normal and often become susceptible to disease (Disease and sunscreen sidebars). And this might sound strange, but your car is one factor in a whole suite of human-induced factors that make the life of a coral that much more difficult. Here's what we mean.

Corals and other reef organisms such as clams, snails and algae build hard limestone "houses" on top of the skeletons built by their parents, grandparents, and so on down the family line. They build these "houses" through a complex chemical reaction involving calcium, carbon, and oxygen. Since the industrial revolution, we've been pumping carbon dioxide and other greenhouse gases into the atmosphere at an unprecedented rate by running our cars, burning fossil fuels, and clear-cutting the forests. Many scientists believe that this is slowly warming the earth's atmosphere and oceans. The thin veneer of water at the ocean surface exchanges gases with the atmosphere in a two-way process; therefore, increasing the concentration of greenhouse gases in the atmosphere ultimately changes the chemical composition in the water. Because carbon dioxide is a weak acid when dissolved in water, when we increase the concentration of carbon dioxide in the atmosphere we actually make the oceans more acidic. The corals have a tough time in this type of environment and must use up more energy just to maintain their skeletons, let alone build them.

Furthermore, as greenhouse gases heat up the planet like an electric blanket that's always on, more polar ice caps melt, which adds water to the oceans and raises sea level. This makes it tough for reefs to grow fast enough to keep up.

Recall that reef-building corals are part plant. The tiny zooxanthellae need sunlight to make energy via photosynthesis. Also recall that a coral reef is essentially composed of "life over death" - the living coral polyps live on top of the older rocky fossils left from prior generations. The top layer of a coral reef must stay within the uppermost layer of water through which sunlight penetrates (euphotic zone). Therefore, if sea level rises the slow-growing corals will begin to slip below the euphotic zone and essentially "drown."

The phenomena of sea level rise and reef "drownings" are not new. Historically they have occurred as natural events unrelated to human activity. Paleoceanographers (marine scientists who study fossil reefs), can point to countless examples of sea level rise and reef drownings in the fossil reef record (See sidebar on paleoceanography at bottom of article). However, tomorrow's paleoceanographers may classify the current global warming as anthropogenic (man-made) because of the tremendous amount of greenhouse gases released by human activities today.

It's not all gloom and doom though! With more education and better management practices, we can curb the dramatic toll we are taking on coral reef communities. It's in our best interest to do so. Coral reefs are diverse, beautiful communities that not only decorate the marine environment, but are integral in sustaining its health. For example, reefs serve as nurseries for populations of finfish and shellfish; they act as a breakwater, protecting sensitive shorelines; and they produce large amounts of sediment (sand) for beaches. Many mysteries in the intricate life of a coral reef have yet to be uncovered. They may even help save our lives! Just like rainforests, coral reefs may contain the cures to many of today's and tomorrow's illnesses. For example, coral is now used for bone grafts, and extracts from a host of reef organisms are being tested for their potential to fight the growth of tumors.

So consider passing along the coral's message the next time you discuss coral reefs in science class. Yes, they can live without us, but we wouldn't want to live without them.


Sunscreen sidebar: SPF - It's not just for humans anymore
 

 

 

 

 

 

Corals can get sunburned in the water just like people. We apply lotion to prevent getting burned, but corals can't exactly run down to the corner store to buy some sunscreen. What's a coral to do? They make their own!
Although not well understood, the plant part of the coral is believed to manufacture mycosporine-like amino acids (MAA's) that are transparent to visible light, but absorb ultraviolet (UV) light. This enables the zooxanthellae to photosynthesize all day long without having their photosystems (where photosynthesis takes place) damaged by the nasty UV rays. Corals that live at shallow depths where there is more UV light penetrating the surface produce more MAA's than those that dwell deeper. It's interesting to note that high water temperatures can cause MAA's to degrade. This leaves the coral with less protection against UV rays and a greater chance of bleaching, which occurs when the zooxanthellae that give the coral its color are expelled from the coral so they look whitish. (Photos of bleached coral courtesy of Melanie Dotherow, University of South Florida)

 
 

Disease sidebar: Stress and disease in corals

The health of coral reefs is declining on a global scale.

  Many coral diseases such as black-band disease, characterized by black bands containing bacteria, cyanobacteria (blue-green algae) and fungi that spread over a coral and leave only the limestone skeleton behind, can kill living coral polyps on a reef.

  Coral bleaching events have been linked to periods of high water temperature, calm winds and high solar radiation (particularly UV light) which cause the coral to expel its zooxanthellae.

 

 

 

 

 

  Crown of Thorns Starfish, which dine on the live coral polyps, can decimate the living coral veneer as they migrate over the reef. Invasions of this coral predator are an increasingly common occurrence in the Pacific Ocean. In the photographs, observe how the starfish are advancing on the greenish coral from Palau; the bright white area is where the starfish have eaten off the living coral tissue. (Photos courtesy of Charles Birkeland, University of Guam)
 

Paleoceanography sidebar: History of the Deep

Did you know that major coral reefs once flourished where Texas, Michigan and Pennsylvania are now located? And at one time when sea level was much lower, Native Americans in western Florida lived along shorelines that are now underwater in the Gulf of Mexico! Paleoceanographers figured out why old reefs are found on top of mountains or deserts today, and why ancient shorelines are now submerged. They study the development - or history - of the world's oceans using sediment layers and fossils as clues to what the earth used to be like. For instance, geologic records (sequences or cores of sediments studied by paleoceanographers) indicate that no pre-Cambrian organisms were made of carbonate whereas today there are lots of carbonate-bearing organisms in the oceans. This tells us that the pre-Cambrian ocean environment was very different chemically than it is today.

Breaks in the geologic record (hiatuses) point to sudden and dramatic changes in the earth's climate or ocean circulation. But remember that geologically speaking, "sudden" changes might occur over thousands - or even millions - of years! These changes wipe out certain types of animals and allow others to flourish. Hiatuses are also clues to catastrophic events in the history of the earth. Please praise paleoceanographers profusely for pinpointing our planet's prolific pebble profiles! (Photo courtesy of Pam Hallock-Muller, University of South Florida; Photo shows the site of a former coral reef slope, Canning Basin, Western Australia, Sindjana National Park)

 

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