5:15 PM - 7:15 PM
[BPT03-P04] The shell formation process of the foraminifera Sorites orbiculus -With special attention to the ultra-microstructure of the site of calcification
Keywords:Foraminifera, FIB-SEM, MXCT, pH imaging
Calcareous foraminifera are single-celled organisms that live in the ocean and produce shells made mainly of calcium carbonate. The elemental and stable isotope composition of foraminiferal shells empirically reflects the environmental conditions at the time of shell formation, making them valuable proxies for reconstructing past ocean conditions, such as temperature and salinity. However, knowledge of the biological elementary processes involved in elemental uptake during calcification remains limited. Even for calcium and carbonate ions, the mechanisms of uptake are still not fully understood.
Calcareous foraminifera are broadly classified into hyaline and miliolid types. The shell formation process of miliolid foraminifera has been reported in previous studies based on observations of Calcituba polymorpha. According to these studies, needle-like calcium carbonate crystals form within intracellular vesicles and are subsequently transported outside the cell via exocytosis, where they accumulate to construct the shell wall. However, no direct observations have been made of the specific process in which these crystals are assembled into the shell structure, and no subsequent research has addressed this gap. Moreover, these findings from Calcituba polymorpha have often been extrapolated to all miliolid foraminifera without any evidence, underscoring the need for further studies that investigate potential differences among species
To understand the calcification process in miliolid foraminifera, it is necessary to clarify how elongated crystals are secreted at the site of calcification and how they contribute to the construction of the shell wall. In this study, we conducted laboratory culture experiments to investigate the shell formation process of Sorites orbiculus, a species of miliolid foraminifera. We observed and captured Timelapse images the entire process of shell formation. Additionally, we applied pH imaging to record the pH environment at the site of calcification. Ultra-microstructural observations of the calcification site were performed using electron microscopy, and three-dimensional imaging of forming and partially formed shells was conducted using the micro/nano CT system at the Super Photon ring-8 GeV (SPring-8).
Using a focused ion beam scanning electron microscope (FIB-SEM), we observed cross-sections of the shell wall during formation and discovered a porous, cotton candy-like structure in the calcifying area. This structure was fibrous, with diameters of 20-30 nm, and showed branching in multiple locations. Three-dimensional imaging revealed that this structure was not needle-like but rather elongated and fibrous. Observations of intracellular pH indicated an increase in pH near the chamber formation site, creating an environment favorable for calcification. Furthermore, transmission electron microscopy (TEM) observations of the fibrous structure revealed multiple crystalline-like structures, 7-8 nm in size, within the fibers. Diffraction pattern analysis identified these nanostructures as calcite.
These results suggest that the calcification process in Sorites orbiculus differs from the model previously proposed for Calcituba polymorpha, where intracellular vesicles form needle-like crystals to construct the shell wall. Our findings strongly indicate that the calcification modes in miliolid foraminifera are more diverse than previously thought.
Calcareous foraminifera are broadly classified into hyaline and miliolid types. The shell formation process of miliolid foraminifera has been reported in previous studies based on observations of Calcituba polymorpha. According to these studies, needle-like calcium carbonate crystals form within intracellular vesicles and are subsequently transported outside the cell via exocytosis, where they accumulate to construct the shell wall. However, no direct observations have been made of the specific process in which these crystals are assembled into the shell structure, and no subsequent research has addressed this gap. Moreover, these findings from Calcituba polymorpha have often been extrapolated to all miliolid foraminifera without any evidence, underscoring the need for further studies that investigate potential differences among species
To understand the calcification process in miliolid foraminifera, it is necessary to clarify how elongated crystals are secreted at the site of calcification and how they contribute to the construction of the shell wall. In this study, we conducted laboratory culture experiments to investigate the shell formation process of Sorites orbiculus, a species of miliolid foraminifera. We observed and captured Timelapse images the entire process of shell formation. Additionally, we applied pH imaging to record the pH environment at the site of calcification. Ultra-microstructural observations of the calcification site were performed using electron microscopy, and three-dimensional imaging of forming and partially formed shells was conducted using the micro/nano CT system at the Super Photon ring-8 GeV (SPring-8).
Using a focused ion beam scanning electron microscope (FIB-SEM), we observed cross-sections of the shell wall during formation and discovered a porous, cotton candy-like structure in the calcifying area. This structure was fibrous, with diameters of 20-30 nm, and showed branching in multiple locations. Three-dimensional imaging revealed that this structure was not needle-like but rather elongated and fibrous. Observations of intracellular pH indicated an increase in pH near the chamber formation site, creating an environment favorable for calcification. Furthermore, transmission electron microscopy (TEM) observations of the fibrous structure revealed multiple crystalline-like structures, 7-8 nm in size, within the fibers. Diffraction pattern analysis identified these nanostructures as calcite.
These results suggest that the calcification process in Sorites orbiculus differs from the model previously proposed for Calcituba polymorpha, where intracellular vesicles form needle-like crystals to construct the shell wall. Our findings strongly indicate that the calcification modes in miliolid foraminifera are more diverse than previously thought.