biostudies-literatureUnknown9(7)Dubicka ZAlexander von Humboldt-StiftungAlfred-Wegener-Institut für Polar und MeeresforschungCalcium carbonate minerals produced by marine organisms play a central role in the global carbon cycle and carbonate sedimentation, which influence the climate by regulating atmospheric CO₂ levels. Foraminifera are important marine single-celled organisms that have produced calcite shells for over 300 million years. Here, we present new observations promoting our understanding for foraminiferal biocalcification by studying <i>Amphistegina lessonii</i>. We integrated in vivo confocal autofluorescence and dye fluorescence imaging with elemental analysis of the cell supporting the concept that the calcite shells of foraminifera are produced via deposition of intracellularly formed Mg-rich amorphous calcium carbonate (Mg-ACC) particles that transform into a stable mineral phase. ThiHeliyone18331https://www.ebi.ac.uk/biostudies/studies/S-EPMC10375801biostudies-literatureMg-rich amorphous to Mg-low crystalline CaCO₃ pathway in foraminifera.PMC10375801Dubicka ZBijma JBickmeyer UBojanowski MJfalseMg-rich amorphous to Mg-low crystalline CaCO₃ pathway in foraminifera.Calcium carbonate minerals produced by marine organisms play a central role in the global carbon cycle and carbonate sedimentation, which influence the climate by regulating atmospheric CO₂ levels. Foraminifera are important marine single-celled organisms that have produced calcite shells for over 300 million years. Here, we present new observations promoting our understanding for foraminiferal biocalcification by studying <i>Amphistegina lessonii</i>. We integrated in vivo confocal autofluorescence and dye fluorescence imaging with elemental analysis of the cell supporting the concept that the calcite shells of foraminifera are produced via deposition of intracellularly formed Mg-rich amorphous calcium carbonate (Mg-ACC) particles that transform into a stable mineral phase. Thi2023-01-01T00:00:00Z2023 Jul2025-04-22T13:30:33.306Z2025-04-06T00:43:03.04ZS-EPMC103758013751976010.1016/j.heliyon.2023.e18331