[B-PT01] Biomineralization and Geochemistry of Proxies

[BPT01-P01] Ultrafine structure and elemental compositions of Acantharian (Radiolarian) skeleton from Kashiwajima Island, southwestern Shikoku, Japan

*Mao Fujii¹, Rie S Hori², Hiroaki Ohfuji⁵, Yasuhide Nakamura³, Mariko Nagashima⁴ (1.Graduate School of Agricultural and Life Science, The University of Tokyo, 2.Graduate School of Science and Engineering, Ehime University, 3.Estuary Research Center, Shimane University, 4.Graduate School of Science and Technology for Innovation, Yamaguchi University, 5.Department of Earth Science, Graduate School of Science, Tohoku University)

Keywords:Radiolaria, Acantharia, skeletal composition

Radiolarians are divided into two major groups based on the chemical composition of shell: the first group, Polycystine, has opal (SiO₂ + nH₂O) skeleton, preserved from Cambrian to Recent sediments, and the second one possessing celestite (SrSO₄) skeleton called Acantharia. Acantharia is one of the unicellular zooplankton taxa including ca. 50 genera and ca. 150 species. Acantharians are distributed in the oceans around the world, especially oligotrophic surface waters. However, the acantarian skeletons do not remain as fossils because they melt in seawater after died. Acantharia show high value in biomass and there are no other planktons that makes SrSO₄-skeletons. In this study, we examined of living Acantharian cells from surface seawater of Kuroshio Current off shore Kashiwa-jima Island, Kochi prefecture, Shikoku, Japan, and studied crystalline structure and chemical compositions of skeletons, in order to clarify its biomineralization process, using by TEM, FE-SEM, EDS (at Ehime University) and WDS analyses (at Yamaguchi University), combined with Focused Ion Beam (FIB) method at GRC (Ehime University).

Radiolarians were picked up from the plankton samples obtained from the surface seawater off Kashiwa-jima samples, and separated into 6 types of Acantharian Clade (A, B, C, D, E and F), under a binocular microscope. Skeletons of each Clade were put on the sample plate separately, and cut off by FIB after washed carefully by Milli-Q water. The thin films of Acantharian skeletons, in particular spines, were made and then observed with a field emission-transmission electron microscope to understand the ultrafine structure.

The crystal structure of Acantharian skeleton was confirmed as uniform Celestine minerals, possessing polarity parallel structure. However, we recognized different characters among SEM images of thin section of spines between Clade E&F and Clade D, which are probably concerned to skeletal forming process, also heterogeneous chemical compositions in the section of spines were detected. We will show these precise results of studies on Acantharia as a poster presentation.