Ultrastructural examination of chamber morphogenesis and biomineralization in the planktonic foraminiferOrbulina universa

Pre-sphere, trochospiralOrbulina universa (d'Orbigny) were hand-collected between June and September, 1982 to 1984, from surface waters of the Pacific Ocean ca. 2 km off Santa Catalina Island, California, USA. The formation of a spherical chamber by this planktonic foraminifer was studied with light and electron microscopy. Chamber morphogenesis is preceded by the secretion of a cytoplasmic layer rich in mitochondria. The layer expands away from the pre-existing foraminiferal shell surface in a spherical pattern. Full cytoplasm expansion is followed by the secretion of an organic matrix (OM) approximately 200 m above the original shell surface. Cytoplasm, mitochondria and/or vesicles may play a role in OM secretion. Calcification and chamber thickening, new spine growth and pore development are documented and continue for a period of 1–7 d. Approximately 24 h prior to gamete release, the foraminiferal spines are resorbed and a veneer of calcite is deposited on the shell surface. An extensive intracellular organelle unique to foraminifera, the fibrillar system, is examined and is hypothesized to play a role in the biomineralization process. The taxonomic and functional significance of theO. universa spherical chamber is discussed with respect to other foraminiferal species.