Calcifying organisms, which build their shells and skeletons from calcium carbonate, are negatively affected by an increase in atmospheric carbon dioxide primarily through ocean acidification. When CO2 levels rise in the atmosphere, more CO2 dissolves into seawater, lowering the pH and making the ocean more acidic. This acidification reduces the availability of carbonate ions (CO3^2-), which are essential for calcifiers to produce their calcium carbonate structures
. As a result, calcification rates in many marine organisms decrease, making it harder for them to build and maintain their shells or skeletons. Some shells may even begin to dissolve under these conditions due to increased dissolution rates
. This effect is particularly pronounced in early life stages of organisms like bivalves, which have high energetic costs for initial shell formation
. However, responses vary among species. Some calcifiers show reduced calcification or net dissolution, while others may increase calcification or show no change, depending on species-specific physiology, ability to regulate pH at calcification sites, shell mineral composition, and environmental factors
. For example, coccolithophores exhibit mixed responses to elevated CO2, with some strains showing decreased calcification and others showing increased or unchanged calcification
. Additionally, organisms with skeletons containing high magnesium levels tend to be more vulnerable to acidification, and warming oceans may exacerbate this vulnerability
. Polar regions, where calcium carbonate is more soluble, are especially at risk, affecting key species like bryozoans and pteropods
. In summary, increased atmospheric CO2 leads to ocean acidification, which generally impairs the ability of calcifying organisms to form and maintain calcium carbonate shells and skeletons, though the degree of impact varies widely among species and environmental conditions
