[B-CG07] 微化石生物学の最前線

[BCG07-07] Composite calcite and opal test in Foraminifera (Rhizaria)

Richirt Julien^1,2、岡田 賢¹、石谷 佳之¹、植松 勝之³、多米 晃裕^3,5、小田 加也¹、磯部 紀之¹、石村 豊穂⁴、土屋 正史¹、*野牧 秀隆¹ (1.海洋研究開発機構、2.リール大学、3.マリンワークジャパン、4.京都大学、5.福井大学)

キーワード：有孔虫、珪質殻、シリカ沈着小胞、殻化学組成

We report for the first time a composite calcite and opal test in the abundant bathyal benthic foraminiferal species Bolivina spissa (Rotaliida), collected from the Sagami Bay, Japan. Based on comprehensive investigations including Scanning Electron Microscopy (SEM) coupled with Energy Dispersive X-ray Spectroscopy (EDS) and Fourier Transform Infrared Spectroscopy (FTIR), we investigated the morphology and composition of the novel opaline layer coating the inside part of the calcitic test. Using Scanning Transmission Electron Microscopy (STEM) and EDS analyses, we detected putative Silica Deposition Vesicles (SDVs), organelles involved in opal precipitation in other silicifying organisms, confirming that certain foraminifer may secrete opal. The layer was systematically found in all studied individuals and had no apparent sub-structure in the layer. Its thickness showed an analogous growth pattern with the calcitic shell of B. spissa, being the thickest in the oldest chamber (proloculus) and becoming thinner toward the younger chambers (apertural side). Its absence in the youngest chambers indicates that silicification occurs subsequently to calcification, probably discontinuously. This finding poses the question of the origin of this composite test from an evolutionary point of view. We propose that the opal layer may serve as a protection barrier against predators using mechanical drilling or chemical etching of the calcitic test. Isotopic composition measurements performed separately on the proloculus side and the apertural side of B. spissa suggest that the precipitation of an opal layer may affects on the calcitic isotopic signal and impact paleoenvironmental proxy using foraminifer’s tests composition. If silicification in Foraminifera was found to be more widespread than previously thought, it could possibly have important implications for foraminiferal evolution, palaeoceanographic reconstructions, and the silica cycle at global scale.