In water, carbon connects with water molecules (H2O), forming bicarbonate (HCO3) and hydrogen (H) and eventually through a chemical reaction links with calcium to form calcium carbonate (CaCO3). The latter one is part of the marine life, especially the ones with shells.
CO2 also interacts with corals, which are calcifying organisms. As the corals build their structures, CO2 gets released in the calcification process. But as oceans absorb more and more CO2 from the atmosphere, it links with calcium, distracting natural calcification of corals.