Micro- and nanometric characterization of the celestite skeleton of acantharian species (Radiolaria, Rhizaria)

Rina Fujimaki,Yuya Oaki,Hiroaki Imai,Takashi Toyofuku,Noritoshi Suzuki,Yukiko Nagai,Katsunori Kimoto,Shinji Shimode

doi:10.1038/s41598-022-06974-2

Abstract

We clarified the specific micrometric arrangement and nanometric structure of the radiolarian crystalline spines that are not a simple single crystal. A body of the celestite (SrSO4) skeleton of acantharian Acanthometra cf. multispina (Acanthometridae) composed of 20 radial spines having four blades was characterized using microfocus X-ray computed tomography. The regular arrangement of three types of spines was clarified with the connection of the blades around the root of each spine. The surface of the spines was covered with a chitin-based organic membrane to prevent from dissolution in seawater. In the nanometric scale, the mesocrystalline structure that consists of nanoscale grains having distorted single-crystal nature was revealed using scanning- and transmission electron microscopies, electron diffraction, and Raman spectroscopy. The acantharian skeletons have a crystallographically controlled architecture that is covered with a protective organic membrane. These facts are important for penetrating the nature of biogenic minerals.

Highlights

In nature, organisms produce various inorganic materials with precisely controlled morphologies from a limited selection of ubiquitous elements, such as calcium, silicon, carbon, and oxygen, under ambient conditions
Given a unit sphere (Fig. 1d) whose origin is defined at the skeletal center and whose x- and y-axes are designed to the equatorial spines, the deviation angles of tropical and polar spines from the equatorial plane are 30° and 60°, respectively
Tilted faces are assignable to the (210) plane by comparing the shape of artificially produced celestite c rystals[27]. These results suggest that the spines are not a homogeneous single crystal but a bundle of fibrous units elongated in the a-axis direction

Summary

Introduction

Organisms produce various inorganic materials with precisely controlled morphologies from a limited selection of ubiquitous elements, such as calcium, silicon, carbon, and oxygen, under ambient conditions. Specific crystallographic orientation, but uncertainty still remains regarding the spine arrangement of the celestite skeleton of the selected acantharian specimens, which was characterized using various techniques, including microfocus X-ray computed tomography (CT), TEM, scanning electron microscopy (SEM), and Raman scattering spectrometry. The arrangement of four equatorial spines (e), four diametric (= 8) tropical spines (t), and four diametric (= 8) polar spines (p) agrees with Müller’s law[5] (Fig. 1b).

Results

Conclusion