Most of the ocean is dark and cold. Some organisms have adapted to the extreme conditions of the deep ocean, but the vast majority of marine life exists in the sunlit and relatively thin upper layer of the ocean. At the bottom edge of that thin layer are deep-sea corals. A lesser-known group of coral in comparison to their tropical relatives, deep-sea corals provide habitat that supports rich ecosystems along rocky bottom continental shelves, slopes, and seamounts. Deep-sea corals are diverse, slow-growing, fragile, and have adapted to living at the threshold of the deep sea. However, as the chemistry at this threshold changes, it is uncertain whether deep-sea corals and the ecosystems they support will be able to adapt.
The extreme conditions of the deep sea include a distinctive carbon chemistry that seawater accumulates over an approximately 1000-year journey. This journey begins at the poles where surface ocean waters become cold and dense, and then sink where they merge with deep ocean currents. While traveling at depth throughout the oceans, deep-ocean waters accumulate organic matter (mostly dead plankton) raining down from the surface waters above and the products of respiring that organic matter. Carbon dioxide is one of the products of respiration and reacts with seawater to reduce the pH and the concentration of carbonate ions.
Calcium carbonate minerals are the chemical building blocks of many marine organisms. Shellfish, coral, and many types of plankton all extract calcium carbonate from seawater to form their shells and skeletons. The lack of these building blocks is another important chemical signature of the deep ocean, and we call the depth where this change in chemistry occurs in the ocean the “saturation horizon”. Above this saturation horizon, the water is supersaturated with these minerals, meaning it has abundant building blocks for marine organisms to build their skeletons and shells. This is where most deep-sea corals live. Below that horizon, the deep ocean water is undersaturated with these minerals making it more difficult for organisms to build their skeletons and shells, and in some areas of the deep ocean, skeletons and shells dissolve.
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Adrienne Sutton, NOAA, Full article.
