[B-PT03] Biomineralization, Geochemistry, and Environmental Studies

[BPT03-04] Native-state ultrastructure characterization of foraminiferal biomineralization system

*Daniel Francois Do Nascimento Silva^1,2, Gert-Jan Reichart^1,2, Helen Elisabeth King², Rona Roverts³, Ben Joosten³, Shaina Vivienne To³, Nico Sommerdijk³, Lennart Jan de Nooijer¹ (1.NIOZ, 2.Utrecht Univ., 3.Radboud Univ.)

Keywords:Biomineralization, Foraminifera, Cell biology, Ocean acidification

Foraminifera are major contributors to marine calcium carbonate (CaCO3) production, playing a key role in the ocean carbon cycle. However, anthropogenic CO2 emissions and the resulting acidification of surface waters are expected to reduce their capacity to calcify by the end of the 21^st century, with potentially far-reaching implications for the carbon cycle. Physiological adaptation is crucial in this context, and understanding their calcium and carbon-uptake mechanisms is essential for assessing their role in future marine carbon cycling. Here, we examine the three-dimensional (3D) structure of the calcification site in a benthic foraminifer (Ammonia tepida) using cryogenic focused ion beam/scanning electron microscopy (cryo-FIB/SEM). High-pressure freezing preserves cellular ultrastructure near its native state, then serial FIB milling and SEM imaging at nanometer resolution reveal membrane structure and morphology, providing key insights into the isolation of the calcification site. Notably, our findings show that while the organic layers enclosing the SOC are largely continuous, occasional ~30 nm gaps expose the calcifying fluid to seawater, limiting cellular control over its chemistry. These results have important implications for both the biomineralization process of this species and our understanding of their response to increased dissolved CO₂ levels.