Explain the mechanism of ocean acidification and its potential impacts on marine calcifiers.

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When carbon dioxide from the atmosphere dissolves in seawater, it forms carbonic acid, which partially dissociates to release hydrogen ions. The added hydrogen ions lower the pH of the ocean, making it more acidic. These extra hydrogen ions also react with carbonate ions to form bicarbonate, reducing the availability of carbonate needed by calcifying organisms to build their shells and skeletons. Because calcium carbonate minerals (such as aragonite and calcite) rely on carbonate ions, the saturation state of these minerals declines as the water becomes more acidic. In undersaturated conditions, calcification becomes harder and existing shells can even start to dissolve. The consequence for marine calcifiers is slower growth, weaker shells or skeletons, and greater vulnerability to stressors like warming and hypoxia. This can lead to shifts in marine communities and threaten habitats such as coral reefs. The other statements misstate the chemistry: CO2 does not make oceans more basic, carbonate ion availability does not increase with more CO2, and pH does not stay constant as CO2 rises.