Proton pumping accompanies calcification in foraminifera

Abstract

Ongoing ocean acidification is widely reported to reduce the ability of calcifying marine organisms to produce their shells and skeletons. Whereas increased dissolution due to acidification is a largely inorganic process, strong organismal control over biomineralization influences calcification and hence complicates predicting the response of marine calcifyers. Here we show that calcification is driven by rapid transformation of bicarbonate into carbonate inside the cytoplasm, achieved by active outward proton pumping. Moreover, this proton flux is maintained over a wide range of pCO₂ levels. We furthermore show that a V-type H⁺ ATPase is responsible for the proton flux and thereby calcification. External transformation of bicarbonate into CO₂ due to the proton pumping implies that biomineralization does not rely on availability of carbonate ions, but total dissolved CO₂ may not reduce calcification, thereby potentially maintaining the current global marine carbonate production.

Figure 1. Reduction in pH during foraminiferal calcification.

Representative images showing the time-resolved decrease in pH of seawater surrounding a calcifying specimen of Ammonia sp. over a period of 320 min. The pH values are imaged using dissolved HPTS and reported on the seawater scale. The incubated specimen shows (a) the two-dimensional variability in pH around the shell when building a new chamber and (b) the translated, spatially integrated change in pH versus distance from the foraminifer along the white dotted line shown in a. At the start of calcification, surrounding pH is still ∼7.8 outside the foraminifer, decreasing to 6.9 after 4 h and subsequently gradually increasing again 6 h after the onset of calcification. It is noteworthy that minimum pH values are found closest to the newly precipitated chamber (N). In addition, a zone of reduced pH encloses the complete shell, also where no new chamber is being produced. The gradient in pH, increasing with distance from the specimen, is mainly caused by protons diffusing away from the site where the new calcite is precipitated. Scale bars, 100 μm. The false-colour scale bar represents pH. The b/w foraminifer is superimposed on false-colour pH images.

Figure 2. Calculated proton flux from the reduction in environmental pH.

(a) Time series of proton flux during chamber formation and (b) the corresponding cumulative proton flux (Specimen no. 3 in Table 1). These estimates are based on analysis of the pH image series by theoretical fitting of the decreased pH as a function of distance from the foraminifer. Error bars indicate s.d.

Figure 3. Proton pumping-based model of foraminiferal calcification.

During calcification of a new calcitic layer (CL) on a primary organic sheet (POS), the protective envelope (PE) separates the growing calcite surface from the surrounding seawater. The chemical composition at the SOC, created by the PE, is characterized by active, outward proton pumping (I). The reduced pH in the foraminiferal microenvironment shifts the inorganic carbon speciation (II), thereby increasing pCO₂ directly outside the PE. The large gradient in pCO₂ across the PE results in diffusion of CO₂ into the SOC (III). Once inside, the CO₂ reacts to form CO₃²⁻ due to the high pH (IV) sustaining CaCO₃ precipitation by reacting with the Ca²⁺. The reduction in pH is seen over the entire foraminifer (inset), suggesting that this model applies to the complete surface of the shell of a rotalid foraminifer producing a new chamber.