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Coral reefs face myriad threats, and researchers in Israel have shed light on yet another: light pollution.

Artificial lights illuminate almost a quarter of the world’s coastlines and their prevalence is expected to grow. These lights are a problem for a wide variety of animals, from nesting turtles to seabirds. To find out their effect on corals, researchers led by Oren Levy, a marine biologist at Bar-Ilan University in Israel, dove into reefs in the Gulf of Aqaba, in the northern Red Sea, and plucked out mature colonies of two species: Acropora eurystoma and Pocillopora damicornis.

On a clear night in the Gulf of Aqaba, light pollution from coastal cities such as Eilat, Israel, and Aqaba, Jordan, extends at least a dozen kilometers offshore. This heavy dose of nighttime light may be low compared to daylight, but corals are particularly sensitive to natural light such as that from the moon, which many reef-building species use to synchronize their reproduction, says Levy.

In experiments, Levy and his colleagues tested the precise physiological consequences of light pollution. In one, they exposed coral colonies to white LED strips that were turned on from sunset to sunrise. In another, they shone different colored LEDs—white, blue, and yellow.

Compared to corals exposed only to natural light, those exposed to artificial lights at night experienced oxidative stress and lower photosynthetic performance, with P. damicornis being slightly more sensitive than A. eurystoma. Blue and white lights had a more severe effect on coral physiology than yellow lights.

Oxidative stress is the result of an imbalance in the body between oxygen free radicals (reactive molecules that contain oxygen) and antioxidant defenses. When oxygen free radicals are overproduced in corals as a result of exposure to artificial lights at night, they can cause damage to lipids as shown in this study, but can also potentially damage DNA and ultimately lead to cell death, Levy explains. Light pollution hinders corals’ ability to spawn. But with appropriate policies, light pollution can be managed to reduce stress on corals, he says.

Chris Langdon, a marine biologist at the University of Miami in Florida who was not involved in the study, found the production of oxygen free radicals “completely unexpected” given the use of low levels of light and the lack of thermal stress in the experiments.

The result highlights how sensitive corals are to small changes in light, says Tim McClanahan, a zoologist at the Wildlife Conservation Society based in New York, New York. “Corals do adapt, but at a cost,” he says. The presence of artificial light, he adds, “increases their sensitivity to climate warming stresses.”