Calcification Strategies in Sorites orbiculus: Insights from Ultrafine Structure Observations and pH Variations

Biomineralization processes are remarkably varied across the biosphere, yet the foraminifera stands out for their intricate construction of calcium carbonate shells—a phenomenon critical for interpreting paleoceanographic applications. In our research, we delve into the latest observations on the calcification strategy employed by the porcelain-shelled foraminifera Sorites orbiculus (Forsskål in Niebuhr, 1775), offering a reevaluation of the prevailing models of chamber formation. Utilizing focused ion beam scanning electron microscopy (FIB-SEM), we meticulously documented the ultrastructural development of the shell in calcification. Our findings may challenge the traditional vesicle-mediated crystal formation hypothesis, revealing a unique and complex cotton candy-like structure at the calcification site instead.

Simultaneously, pH imaging conducted during chamber formation has disclosed a fascinating pattern of pH elevation that transcends the newly formed chambers, extending to the final stages of calcification. This suggests a more intricate and systemic approach to biomineralization than previously understood. The localized increases in pH not only coincide with the morphological intricacies of the shell but also suggest a remarkable environmental plasticity inherent to S. orbiculus. Our nuanced comprehension of calcification, grounded in precise pH measurements and detailed microstructural observations, illuminates new facets of this species' biomineralization strategy. The insights gleaned here hold profound implications for redefining elemental partitioning and isotopic fractionation in foraminiferal shells and may herald a paradigm shift in our understanding of biomineralization within porcelain foraminifera.