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Coral Reefs and Ocean Acidification

Coral reefs cover less than one percent of the ocean floor, yet they support an estimated quarter of all marine species, from small fish to sharks and sea turtles. This extraordinary variety of life depends entirely on structures built by coral polyps, tiny soft-bodied animals that live in large colonies and secrete calcium carbonate to form hard external skeletons. Over thousands of years, the skeletons left behind by countless generations of polyps accumulate into the vast limestone structures we recognize as reefs.

One of the more subtle threats facing reefs today is ocean acidification, a process distinct from, though related to, the warming of ocean waters. When carbon dioxide from the atmosphere dissolves into seawater, it forms carbonic acid, which lowers the water's pH and reduces the concentration of carbonate ions available in the ocean. Because coral polyps rely on these carbonate ions to build their calcium carbonate skeletons, a less alkaline ocean makes skeleton-building a slower and more energy-intensive process. In laboratory experiments that simulate the acidity levels predicted for the end of this century, coral growth rates have been shown to drop substantially, and in some cases, existing coral skeletons even begin to dissolve.

It is important to distinguish acidification from coral bleaching, a phenomenon more commonly discussed in the media. Bleaching occurs when unusually warm water causes corals to expel the colorful algae living within their tissues, leaving behind a pale, "bleached" skeleton; a bleached coral is not necessarily dead, but it is under severe stress and more vulnerable to disease. Acidification, by contrast, does not immediately change a coral's color, but it steadily weakens the physical structure of the reef over years or decades, making reefs more fragile and less able to recover from storms or bleaching events. The two processes, driven by the same underlying cause, the accumulation of carbon dioxide, therefore attack reefs through different mechanisms at the same time.

Despite this bleak picture, marine biologists have found some reason for cautious optimism. Certain coral species and even individual colonies within the same species show measurably greater tolerance to both warming and acidification than their neighbors. Researchers are now studying these naturally resilient corals in the hope of understanding the genetic and physiological traits that allow them to cope, with some scientists proposing that resilient colonies could eventually be used to help restore damaged reefs elsewhere. Whether such approaches can be scaled up quickly enough to offset the pace of ocean change, however, remains uncertain.