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key concepts

1.1

Living Organisms Share Similarities and a Common Origin

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Biologists Investigate Life through Experiments That Test Hypotheses

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Understanding Biology Is Important for Health, Well-Being, and Public-Policy Decisions

Corals in Hot Water

Coral reefs support the largest diversity of life in the oceans. They provide fisheries and storm protection for about a billion people, and are a magnificent source of natural beauty. But coral reefs are endangered. Over the past 20 years about half of the world’s reef-building corals have been destroyed by rising water temperature and other factors. High temperatures disrupt a fascinating aspect of coral biology. Corals are animals, yet most corals have algae (dinoflagellates) growing within their cells. Dinoflagellates use the energy of sunlight to produce carbohydrates. Corals provide a home for the dinoflagellates, which in turn provide nutrients for the corals. When high temperature impairs the dinoflagellates, the corals eject them—a process called bleaching. Then, without nutrients from the dinoflagellates, the corals die, unless they can take in new dinoflagellates that are more resistant to the higher temperatures.

Understanding the effects of heat on corals was the motivation of graduate student Rachael Bay and her colleagues working with Professor Steve Palumbi at Stanford University’s Hopkins Marine Station. While studying corals in small back-reef pools in American Samoa, they observed that during low tides some pools reached higher temperatures than others. The researchers predicted that corals in the warmer pools had mechanisms enabling them to resist bleaching. To test their prediction, the researchers brought corals into the laboratory, subjected them to temperature fluctuations, and showed that the corals from the warm pools were more resistant to bleaching. They also transplanted corals between the different temperature environments in nature. As a result of these experiments, Rachael and her colleagues proposed that two different processes contribute to the different abilities of the corals to survive heat stress. First, corals growing in the warmer pools could have genetic traits contributing to heat resistance, and second, individual corals may have the same genetic make-up but differ in their ability to adjust to their environment by changing the expression levels of certain genes. More knowledge of the mechanisms of heat stress and heat resistance in corals could lead to new strategies to decrease their losses as their environments change.

How might experiments on heat stress in corals be used to predict the response of corals to global warming?