Uncovering the Functional Roles of Plasma Membrane Proteins in Foraminiferal biocalcification

Abstract. The biocalcifying process in foraminifera, especially in benthic rotaliids like the Ammonia genus, involves intricate interactions between organic components and inorganic crystal formation, in which cellular membranes likely play a central but still underappreciated role.

We established a new extraction protocol enabling the isolation of membrane-associated and cytoplasmic proteins, as confirmed by proteomic analyses, revealing a clear spatial separation of protein functions. In particular, biochemical studies have identified an Annexin A13-like protein in membrane extracts of Ammonia spp. specimens, pointing to a conserved role for annexins in calcium regulation across diverse organisms, including these protists.

This study identifies membrane-associated proteins whose functions are likely linked to foraminiferal shell formation, potentially involving (i) annexin-mediated calcium transport at the site of biocalcification and (ii) regulation of ion flux through vesicle-based transport. Finally, we examined the Primary Organic Sheet (POS) using in situ carbon K-edge XANES spectroscopy on a focused ion beam (FIB) lamella from the final chamber of the Ammonia confertitesta test. The detection of lipid components suggests that this organic shell layer may partly derive from the plasma membrane, indicating its contribution to molecules forming the POS structure and composition.

Taken together, biochemical, proteomic, and ultrastructural evidence indicate that the plasma membrane, in addition to the well-established role of the shell organic layers, may play an active and sustained role in regulating foraminiferal biomineralization. These findings complete published models of foraminiferal biocalcification and support a framework in which the membrane and its associated proteins may represent central players in shell formation.