Proteomic characterization of a foraminiferal test's organic matrix

Foraminifera are unicellular protists capable of precipitating calcite tests, which fossilize and preserve geochemical signatures of past environmental conditions dating back to the Cambrian period. The biomineralization mechanisms responsible for the mineral structures, which are key to interpreting palaeoceanographic signals, are poorly understood. Here, we present an extensive analysis of the test-bound proteins. Using liquid chromatography-tandem mass spectrometry, we identify 373 test-bound proteins in the large benthic foraminifer Amphistegina lobifera, the majority of which are highly acidic and rich in negatively charged residues. We detect proteins involved in vesicle formation and active Ca²⁺ trafficking, but in contrast, do not find similar proteins involved in Mg²⁺ transport. Considering findings from this study and previous ones, we propose a dual ion transport model involving seawater vacuolization, followed by the active release of Ca²⁺ from the initial vacuoles and subsequent uptake into newly formed Ca-rich vesicles that consequently enrich the calcification fluid. We further speculate that Mg²⁺ passively leaks through the membrane from the remaining Mg-rich vesicles, into the calcifying fluid, at much lower concentrations than in seawater. This hypothesis could not only explain the low Mg/Ca ratio in foraminiferal tests compared to inorganic calcite, but could possibly also account for its elevated sensitivity to temperature compared with inorganically precipitated CaCO₃.