Composite calcite and opal test in Foraminifera (Rhizaria)

Abstract

Foraminifera are unicellular eukaryotes known to have a shell, called a test, generally made of secreted calcite (CaCO3). For the first time, we report a foraminifera with a composite calcite–opal test in the cosmopolitan and well-studied benthic species Bolivina spissa (Rotaliida), sampled from 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 (FTIR) spectroscopy, 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 itself secretes the opal layer. The layer was systematically found in all studied individuals and had no apparent substructure. Its thickness showed a growth pattern analogous to 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 subsequent 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 protective 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 palaeoenvironmental proxies using foraminiferal test composition. If silicification in Foraminifera were 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.