Coral reefs are one of the most biologically diverse ecosystems present on earth. Within the hard reef framework of limestone, built by coral animals and their symbiotic zooxanthellae plants, are habitats for at least 25 percent of all marine species. In addition to their intrinsic natural value, coral reefs provide important benefits to society from tourism, fisheries and coastline protection during storms. Recent estimates suggest that the net economic benefit of the world's coral reefs is approximately $30 billion annually

Over the last few decades, at least 10% of tropical and subtropical coral reefs have been lost, and those that remain show signs of moderate to severe deterioration. The degradation of the world's coral reefs has been caused by a host of human-related (anthropogenic) factors, including urban coastline development and habitat modification, pollution, nutrient and sediment overloading, direct destruction, and over-fishing. These environmental pressures, along with natural phenomena, such as El Niño events, increase the susceptibility of corals to disease and bleaching, when corals lose their zooxanthellae partners.

The long-term sustainability of these valuable coral reefs is also threatened by global climate change induced by the release of carbon dioxide into the atmosphere from human activities. Rising ocean temperatures have been implicated in coral disease and mass coral bleaching, and worldwide research efforts have been targeted at understanding the interactions and impacts on coral reef ecosystems from global warming, rising sea levels and anthropogenic factors. But a hidden danger to the long-term sustainability of the world's coral reefs has recently emerged: the increasing acidification of the ocean.

Over the last century or so, the release of carbon dioxide from fossil fuel use and deforestation has led to an increase in atmospheric carbon dioxide levels. Because carbon dioxide is a soluble gas, it is rapidly absorbed by seawater, and anthropogenic carbon dioxide has accumulated in the world's oceans. For example, changes in oceanic carbon dioxide levels have been observed in the surface waters off Bermuda by scientists in BIOS's Bermuda Atlantic Time-series Study . Over the next century, atmospheric carbon dioxide is likely to double from about 370 to about 700 parts per million, with similar increases in the levels of carbon dioxide in the ocean. The problem for marine ecosystems lies in the finding that the acidity of the ocean has and will continue to change, with acidity increasing by the end of this century to levels not observed for millions of years.

The limestone, or calcium carbonate, structures of coral reefs are constructed by coral animals from the calcium and carbonate dissolved in seawater. In the future, the increasing acidity of the ocean will reduce the amount of carbonate available in seawater, and there is growing concern that these changes in the chemistry of the ocean will reduce the ability of coral animals to construct limestone reefs and the habitats they provide. Some scientists have warned, for example, that the Great Barrier Reef may be lost sometime between 2050 and 2100, or at least the ecosystem changed in profound ways.

But because of the complex interactions between coral reef biology and this global-scale stress, it is not certain how individual species or the whole reef will be impacted by the future acidification of the ocean. At present, limited resources globally have been focused on studying the impact of carbon dioxide on coral reefs and other marine ecosystems.

Recent experimental studies conducted both here in Bermuda, by Dr. Alexandra Amat and BIOS Senior Research Scientist Dr. Nick Bates, and elsewhere show a decrease in the ability of some coral species to produce calcium carbonate under lower dissolved carbonate conditions and higher ocean acidity.

The studies so far reveal a potentially dire outlook for coral reefs due to ocean acidification. But all is perhaps not bleak for the future of our coral reefs. BIOS's preliminary studies suggest that some coral species are adaptable to changing ocean chemistry and potentially capable of calcifying at healthy rates in the future. Other researchers have recently demonstrated that very low voltage electricity passing through wire cages can help corals to produce calcium carbonate. Nonetheless, the potential impact of increased ocean acidification on coral reefs and other species that secrete hard calcium carbonate skeletons is an important, emerging question in marine science.