The biocalcifying process in foraminifera, especially in benthic rotaliids like the <em>Ammonia</em> genus, involves intricate interactions between organic components and inorganic crystal formation, in which cellular membranes likely play a central but still underappreciated role.</p> <p>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 <em>Ammonia </em>spp. specimens, pointing to a conserved role for annexins in calcium regulation across diverse organisms, including these protists.</p> <p>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 <em>in situ</em> carbon K-edge XANES spectroscopy on a focused ion beam (FIB) lamella from the final chamber of the <em>Ammonia confertitesta </em>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.</p> <p>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.