Vibrant coral reef ecosystems, like this one off a tiny atoll in the Pacific Ocean called Helen Reef, are threatened by changing ocean conditions, which include warming temperatures and more acidic seawater. (Hannah Barkley, Woods Hole Oceanographic Institution)

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Vibrant coral reef ecosystems, like this one off a tiny atoll in the Pacific Ocean called Helen Reef, are threatened by changing ocean conditions, which include warming temperatures and more acidic seawater. (Hannah Barkley, Woods Hole Oceanographic Institution)