[B-PT03] Biomineralization and Geochemistry of Proxies

[BPT03-03] pH imaging of the miliolid foraminifera Sorites sp. during calcification

*Yukiko Nagai¹, Remi Tsubaki², Kazuhiko Fujita⁴, Takashi Toyofuku^1,3 (1.JAMSTEC, 2.Eusapia Co., Ltd., 3.TUMSAT, 4.University of the Ryukyus)

Keywords:Biomineralization, Foraminifera, Calcification, pH, Imaging

Calcareous foraminifera are unicellular organisms with testes (shells) composed primarily of calcium carbonate (CaCO3) found in the ocean. The elemental composition of foraminifera test has contributed significantly to paleoceanography as a paleoenvironmental proxy of temperatures and other physico^chemical parameters. However, our knowledge of the biological processes of element uptake during calcification remains limited. We have studied the calcification process in hyaline foraminifera (Ammonia sp.) using intracellular and extracellular pH observations on living individuals in the process of chamberl formation. These observations revealed that a high pH environment was created at the site of calcification, creating a microenvironment in which calcium carbonate is easily deposited. Conversely, the outside of the test is a low pH environment, which facilitates the uptake of carbonate as a carbon dioxide (Toyofuku et al. 2017). On the other hand, pH observations have been made on miliolid foraminifera with the exception of Yuji et al. (2012MS). No new calcification model for miliolid foraminifera has been proposed for the shell formation process since Hemleben et al. (1986) showed that needle-like crystals of high-Mg calcite are formed in intracellular vesicles. In this study, we first observed pH using fluorescent indicator to document the calcification process in the miliolid larger benthic foraminifera, Sorites sp.
The series of pH observation showed an increase in pH at the place of newly forming chamber in Sorites sp. This is similar phenomena to the increase in pH during calcification in Ammonia sp. However, in Sorites sp., the increase in pH is also in the penultimate chamber and the chamber before the penultimate chamber. In the penultimate chamber and the one before it, the pH increased loci are similar to the test morphology, which is different from Ammonia sp. where the pH up regulations are observed only in the cytoplasm.
The present results indicate that the pH is also increased during calcification in the miliolid Sorites sp as well as the hyaline species. The FIB-SEM microfabrication of the calcification site and observation of the cross section showed that the crystals were not mature in the penultimate chamber and the chamber before it, and that there were small gaps between the crystals. These results suggesting that calcification in Sorites sp. may continue even after (between two) chamber formation event (Nagai et al., 2022; JpGU). This observation may have uncovered a new, previously unknown aspect of biomineralization in foraminifera. This finding may provide one insight into the question of when the elemental and isotopic composition of foraminifera shells is determined.