Composite calcite and opal test in Foraminifera (Rhizaria)
Abstract. Foraminifera are unicellular eukaryotes known to have a shell, called test, generally made of secreted calcite (CaCO3). We report for the first time a Foraminifera having a composite calcite/opal test in the cosmopolitan and well-studied benthic species Bolivina spissa (Rotaliida), sampled from the Sagami Bay in Japan at 1410 m depth. Based on comprehensive investigations including Scanning Electron Microscopy (SEM) coupled with Energy Dispersive X-ray Spectroscopy (EDS) and Fourier Transform Infrared Spectroscopy (FTIR), we inspect 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 probable Silica Deposition Vesicles (SDVs), organelles involved in opal precipitation in other silicifying organisms, confirming that the Foraminifera themself secretes the opal layer. The layer was systematically found in all studied individuals and had no apparent sub-structure. 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. We further discuss the potential function(s) of this composite test and propose that the opal layer may serve as a protection barrier against predators using either mechanical drilling or chemical etching of the calcitic test. Isotopic composition measurements performed separately on the proloculus part and the apertural side of B. spissa suggest that the presence of an opal layer may alter 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.
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