Foraminifera, the most ancient known calcium carbonate–producing eukaryotes, are crucial players in global biogeochemical cycles and well-used environmental indicators in biogeosciences. However, little is known about their calcification mechanisms. This impedes understanding the organismal responses to ocean acidification, which alters marine calcium carbonate production, potentially leading to biogeochemical cycle changes. We conducted comparative single-cell transcriptomics and fluorescent microscopy and identified calcium ion (Ca2+) transport/secretion genes and α-carbonic anhydrases that control calcification in a foraminifer. They actively take up Ca2+to boost mitochondrial adenosine triphosphate synthesis during calcification but need to pump excess intracellular Ca2+to the calcification site to prevent cell death. Unique α-carbonic anhydrase genes induce the generation of bicarbonate and proton from multiple CO2sources. These control mechanisms have evolved independently since the Precambrian to enable the development of large cells and calcification despite decreasing Ca2+concentrations and pH in seawater. The present findings provide previously unknown insights into the calcification mechanisms and their subsequent function in enduring ocean acidification.