17:15 〜 18:30
[BPT01-P02] Ultra-microstructure observation of miliolid foraminifera (Sorites sp.) during shell formation process
キーワード：バイオミネラリゼーション、有孔虫、石灰化、集束イオンビーム、微細構造
Foraminifera are unicellular organisms with calcareous shells that live mainly in the marine environment. The elemental composition of foraminiferal shells contributes significantly to paleoceanographic reconstruction analysis. Nowadays, there are many unclear points remained about foraminiferal shell formation process. There are two types of foraminifera with different shell structures: hyaline foraminifera and porcelaneous (miliolid) foraminifera. Regarding the process of forming a milky white porcelaneous shell called miliolid foraminifera, Hemleben et al. (1986) showed that needle-like crystals of calcium carbonate were formed in intracellular vesicles. Since then, research of this field was not progressing very much (with some exception.). Therefore, in this study, in order to clarify the shell formation process of the large benthic foraminifera Sorites sp., which is a member of miliolid foraminifera, the ultra-fine structure of the shell formation site was observed using a specimen during the calcification.
According to time-lapse observations by optical microscopy, the shell formation of Sorites sp. involves the formation of cytoplasm in the form of a new chamber, framed by two organic sheets. Crystals are deposited between these two sheets.
In SEM and FIB-SEM observations, we observed the specimen in the process of deposition of the crystals in the above shell formation process. The cross-sectional observation revealed a structure with five major layers. From the outermost to inner surfaces, a relatively thick layer of organic matter (layer a), sheet mixture of needle-like crystals and organic matter on the surface of the shell (layer b), a crystal-filled layer occupying most of the shell (layer c), a sheet of organic matter on the inner surface (layer d), and spherical cytoplasm distributed on the inner surface (layer e). The primary organic sheet is not present in layer c as seen in hyaline foraminifera.
In the layer a, there is a network structure of pseudopods woven together and a membrane with holes in places between them. Spherical organic structures are present on the sheet. The spherical structure has a variability of size about 0.1µm-1µm. The thickness of the layers also varies from 0.1-3µm. Layer b was found that the outer side of the shell formation site. This was a thin organic film structure. This structure likely corresponds to the Outer Organic Lining (OOL) of hyaline foraminifera. However, its thickness seems thinner than hyaline species. The structure looks like a sheet of needle-like crystals with organic material as a binder. The crystals are arranged in a way that is sometimes random and sometimes flowing. The needle-like crystals was about 700 nm. The layer c is filled with needle-like material, but in some places it is spongy with low filling density and scattered voids. The electron density of this needle-like material seems to be somewhat lower than that of a normal crystal. Whether they are pseudopodia or crystals will require further analysis. Layer d is a few nanometers thick and appears to be lining the inside of the layer c, but its structure still needs to be examined. The structure and other details still need to be investigated. The layer e seems to have pseudopodia and cytoplasm attached to the layer d.
These findings will deepen our understanding of biomineralization of foraminifera and support to consider the conditions under which elemental partitioning and isotopic fractionation occur.
According to time-lapse observations by optical microscopy, the shell formation of Sorites sp. involves the formation of cytoplasm in the form of a new chamber, framed by two organic sheets. Crystals are deposited between these two sheets.
In SEM and FIB-SEM observations, we observed the specimen in the process of deposition of the crystals in the above shell formation process. The cross-sectional observation revealed a structure with five major layers. From the outermost to inner surfaces, a relatively thick layer of organic matter (layer a), sheet mixture of needle-like crystals and organic matter on the surface of the shell (layer b), a crystal-filled layer occupying most of the shell (layer c), a sheet of organic matter on the inner surface (layer d), and spherical cytoplasm distributed on the inner surface (layer e). The primary organic sheet is not present in layer c as seen in hyaline foraminifera.
In the layer a, there is a network structure of pseudopods woven together and a membrane with holes in places between them. Spherical organic structures are present on the sheet. The spherical structure has a variability of size about 0.1µm-1µm. The thickness of the layers also varies from 0.1-3µm. Layer b was found that the outer side of the shell formation site. This was a thin organic film structure. This structure likely corresponds to the Outer Organic Lining (OOL) of hyaline foraminifera. However, its thickness seems thinner than hyaline species. The structure looks like a sheet of needle-like crystals with organic material as a binder. The crystals are arranged in a way that is sometimes random and sometimes flowing. The needle-like crystals was about 700 nm. The layer c is filled with needle-like material, but in some places it is spongy with low filling density and scattered voids. The electron density of this needle-like material seems to be somewhat lower than that of a normal crystal. Whether they are pseudopodia or crystals will require further analysis. Layer d is a few nanometers thick and appears to be lining the inside of the layer c, but its structure still needs to be examined. The structure and other details still need to be investigated. The layer e seems to have pseudopodia and cytoplasm attached to the layer d.
These findings will deepen our understanding of biomineralization of foraminifera and support to consider the conditions under which elemental partitioning and isotopic fractionation occur.