a, b, When atmospheric CO2 enters the sea surface (a), it undergoes a series of reversible chemical reactions known as the seawater carbonate buffer system (b), which releases protons (H+) that acidify the sea water. c, Coccolithophores and other algae assimilate CO2 to produce organic carbon through photosynthesis. d, Coccolithophores also perform calcification reactions, in which two bicarbonate ions (HCO3−) are converted into one calcium carbonate (CaCO3) and one CO2 molecule. The CaCO3 is incorporated into coccoliths in the algal shell. The CO2 from calcification is released, and can either contribute to ocean acidification or degas back to the atmosphere (e), contributing to global warming. f, Biogenic particles from coccolithophores and other phytoplankton sink from the ocean surface. The ratio of CaCO3 to organic carbon in this 'rain' of biogenic particles is a critical parameter in the marine carbon cycle. Beaufort et al.1 show that coccolithophores produce less calcium carbonate at higher seawater concentrations of CO2.