17:15 〜 18:45
[BPT02-P07] Ultrastructure observation of site of calcification and inter/extra-cellular pH imaging in Sorites orbiculus
キーワード：石灰化、有孔虫、バイオミネラリゼーション、pH、微細構造
Foraminifera are unicellular organisms with calcareous teststests. As the elemental composition of foraminiferal tests reflects the paleoenvironmental conditions, it can be used as a paleoceanographic proxy. Despite the increasing number of studies biological experiments and electron microscopy observations, our comprehension of the biological processes governing calcium and elemental uptake during calcification is far from complete. The chamber formation process of miliolids foraminifera have not been well studied since Hemleben et al. (1986) revealed the formation of needle-like calcite crystals in the intracellular vesicles. The chamber formation model is based on the results for a type of miliolid, Calcituba polymorpha, and it is necessary to increase knowledge to extend it to other porcelaneous foraminifera. In fact, Parker (2017) focused on the variable shell structure of miliolid species, and it is suggested that the chamber formation mechanisms of miliolids may also be diverse. But there is still limited knowledge on how their shells are organised with long, elongated calcium carbonate crystals. If the long, thin crystals are secreted from the vesicles and intertwined each other, it is necessary to observe how they are arranged into the test wall. To clarify the detailed process of chamber formation in the porcelaneous-shelled miliolid foraminifera, we observed the ultrastructure of the calcification site and pH imaging during calcification of Sorites orbiculus.
The series of intracellular pH observation showed an increase in pH at the place of a newly forming chamber (i.e. 0) in S. orbiculus. However, in S. orbiculus, the increase in pH is also observed in the penultimate chamber (i.e. -1) and the chamber before the penultimate chamber (i.e. -2). Extracellular pH observations showed a decrease in pH around foraminifera.
A focused ion beam scanning electron microscope (FIB-SEM) was used to make smooth transverse sections of the chamber wall during the process of calcification. The FIB-SEM images showed that a highly porous cotton candy-like structure was formed in the calcification site. The structure was highly branched and fibrous. On the other hand, the test structure of the penultimate chamber was packed with crystals and denser than the newly calcifying chamber. In addition, no crystal-like morphology has so far been observed in the intracellular vesicles. TEM observation of the branched fibrous structures at the site of shell formation revealed 7-8 nm crystal-like structures inside the fibrous structures. It was identified as calcite from the diffraction pattern. These results strongly suggest that calcification of Sorites orbiculus does not occur in the manner suggested by the mechanism associated with C. polymorpha, 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 and also be a key to understanding elemental partitioning and isotopic fractionation in foraminiferal shells.
The series of intracellular pH observation showed an increase in pH at the place of a newly forming chamber (i.e. 0) in S. orbiculus. However, in S. orbiculus, the increase in pH is also observed in the penultimate chamber (i.e. -1) and the chamber before the penultimate chamber (i.e. -2). Extracellular pH observations showed a decrease in pH around foraminifera.
A focused ion beam scanning electron microscope (FIB-SEM) was used to make smooth transverse sections of the chamber wall during the process of calcification. The FIB-SEM images showed that a highly porous cotton candy-like structure was formed in the calcification site. The structure was highly branched and fibrous. On the other hand, the test structure of the penultimate chamber was packed with crystals and denser than the newly calcifying chamber. In addition, no crystal-like morphology has so far been observed in the intracellular vesicles. TEM observation of the branched fibrous structures at the site of shell formation revealed 7-8 nm crystal-like structures inside the fibrous structures. It was identified as calcite from the diffraction pattern. These results strongly suggest that calcification of Sorites orbiculus does not occur in the manner suggested by the mechanism associated with C. polymorpha, 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 and also be a key to understanding elemental partitioning and isotopic fractionation in foraminiferal shells.