Foraminifera are calcifying organisms that play a role in the marine inorganic-carbon cycle and are widely used to reconstruct paleoclimates. However, the fundamental process by which they calcify remains essentially unknown. Here we use inhibitors to show that an enzyme is speeding up the conversion between bicarbonate and CO₂. This helps the foraminifera acquire sufficient carbon for calcification and might aid their tolerance to elevated CO₂ level.