How do organisms like coral and shellfish use carbon to make their shell?

As carbon dioxide from the burning of fossil fuels has accumulated in the atmosphere, some of it has been absorbed by the ocean. As the gas dissolves in the water, it forms a weak acid (the same kind that's in bubbly soft drinks), causing the ocean itself to become gradually more acidic.
As ocean water becomes more acidic, it also lowers the amount of calcium carbonate available to aquatic animals that use the mineral to build shells or skeletons, such as corals. These organisms can be important links in the marine food chain.

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