Open or closed: pH modulation and calcification by foraminifera

Marine calcifying organisms precipitate their shells either in equilibrium with seawater or under strict biological control. Here, we show that these two options represent two ends of a spectrum. In species with a more “closed” system, rates of H⁺ removal and Ca²⁺ uptake are high and exceed the amount of ions required for calcification. This explains the relatively low Mg/Ca of the calcite of this species by dilution of the [Mg²⁺] in the calcifying fluid. Conversely, in species with a more open system, the H⁺ and Ca²⁺ fluxes are lower, with more seawater exchanged between the environment and calcifying fluid, explaining the relatively high Mg/Ca in these foraminifera. In either of these species, mitochondria were found to be located at the site where the Ca²⁺/H⁺ exchange takes place and the mitochondrial density aligned with the rate of pumping. These findings highlight the crucial role of transmembrane transporters and mitochondria in foraminifera calcification and explain the species-specific elemental signatures.