11:00 AM - 1:00 PM
[BPT03-P03] Observation of shell ultrastructure during calcification in the miliolid foraminifera Sorites sp.
Keywords:Biomineralization, foraminifera, FIB-SEM, Ultra micro structure
Observation of shell ultrastructure during calcification in the miliolid foraminifera Sorites sp.
Foraminifera is unicellular organisms with calcareous shells. They are widely distributed in the ocean. As the elemental composition of foraminiferal shells reflect the paleoenvironmental conditions, it can be used to reconstruct to paleoceanographic. However, our knowledge of the biological processes of elemental uptake associated with calcification is still limited. In particular, the full details of the role of pseudopodia and organic sheet structures of the shell during calcification remain unclear. In order to reveal the process of shell formation in hyaline foraminifera, which is one of the two types of calcareous foraminifera, we performed ultra-microstructural observations of individuals during shell formation. From these observations, we have shown that the calcification site is separated from the outside environment by an organic sheet structure and that calcium carbonate is deposited there. On the other hand, the formation process of the milky-white shells known as miliolid foraminifera, which is another type of calcareous foraminifera, has not been well studied since 1986, when Hemleben et al. showed that needle-like calcite crystals are formed in intracellular vesicles.
In this study, to clarify the process of shell formation in the porcelain-shelled foraminifera Sorites sp., we observed the calcification site using individuals during calcification.
A focused ion beam scanning electron microscope (FIB-SEM) was used to make smooth transverse sections of the shells in the process of calcification. The SEM images showed that a highly porous cotton candy-like structure was formed in the calcification sites. On the contrary to Hemleben et al. (1986), the fibrous structures ramify into several branches and thus make up cotton candy-like structure. The calcification sites were observed at high magnification (80,000x and 160,000x), and the fibrous structures that constitute the cotton candy-like structure was varied in thickness between 5 to 30 nm.
On the other hand, the shell structure of the penultimate chamber was observed to be packed with crystals and denser than the calcification chamber. In addition, the thickness of the crystals were uniformly with a thickness of about 20 nm. Unlike the chambers during calcification, no branching fibrous structure has been found so far. Elemental analyses with EDS of the cross-section showed that the Ca content was lower in the calcification chamber, while the Mg and S contents were higher than the penultimate chamber.
These results suggest that the calcification of Sorites sp does not occur in the manner proposed by Hemleben et al. (1986), in which the intracellular vesicles form needle-like crystals to construct the shell wall. This observation may reveal a novel and unknown mode of biomineralization in foraminifera. This finding may be a key to understanding elemental partitioning and isotopic fractionation in foraminiferal shells.
Foraminifera is unicellular organisms with calcareous shells. They are widely distributed in the ocean. As the elemental composition of foraminiferal shells reflect the paleoenvironmental conditions, it can be used to reconstruct to paleoceanographic. However, our knowledge of the biological processes of elemental uptake associated with calcification is still limited. In particular, the full details of the role of pseudopodia and organic sheet structures of the shell during calcification remain unclear. In order to reveal the process of shell formation in hyaline foraminifera, which is one of the two types of calcareous foraminifera, we performed ultra-microstructural observations of individuals during shell formation. From these observations, we have shown that the calcification site is separated from the outside environment by an organic sheet structure and that calcium carbonate is deposited there. On the other hand, the formation process of the milky-white shells known as miliolid foraminifera, which is another type of calcareous foraminifera, has not been well studied since 1986, when Hemleben et al. showed that needle-like calcite crystals are formed in intracellular vesicles.
In this study, to clarify the process of shell formation in the porcelain-shelled foraminifera Sorites sp., we observed the calcification site using individuals during calcification.
A focused ion beam scanning electron microscope (FIB-SEM) was used to make smooth transverse sections of the shells in the process of calcification. The SEM images showed that a highly porous cotton candy-like structure was formed in the calcification sites. On the contrary to Hemleben et al. (1986), the fibrous structures ramify into several branches and thus make up cotton candy-like structure. The calcification sites were observed at high magnification (80,000x and 160,000x), and the fibrous structures that constitute the cotton candy-like structure was varied in thickness between 5 to 30 nm.
On the other hand, the shell structure of the penultimate chamber was observed to be packed with crystals and denser than the calcification chamber. In addition, the thickness of the crystals were uniformly with a thickness of about 20 nm. Unlike the chambers during calcification, no branching fibrous structure has been found so far. Elemental analyses with EDS of the cross-section showed that the Ca content was lower in the calcification chamber, while the Mg and S contents were higher than the penultimate chamber.
These results suggest that the calcification of Sorites sp does not occur in the manner proposed by Hemleben et al. (1986), in which the intracellular vesicles form needle-like crystals to construct the shell wall. This observation may reveal a novel and unknown mode of biomineralization in foraminifera. This finding may be a key to understanding elemental partitioning and isotopic fractionation in foraminiferal shells.