Marine animals can use carbon dioxide to make their shell.
Carbon dioxide (CO2 ) is considered a trace gas in the atmosphere because it is much less abundant than oxygen or nitrogen. The oceans contain about 50 times more CO2 than the atmosphere and 19 times more than the land biosphere. Tiny marine plants called phytoplankton use this carbon dioxide for photosynthesis. Phytoplankton is the base of the marine food web. Microbial organisms such as bacteria and zooplankton consume the sinking, carbon-rich particles and convert the organic carbon back to its original inorganic form.