The Crystallography and Possible Origin of Barium Sulphate in Deep Sea Rhizopod Protists (Xenophyophorea)

Abstract

Xenophyophores are a group of giant deep sea protists characterized by intracellular barium sulphate (BaSO 4 ) crystals. X-ray diffraction, electron diffraction and electron microscopy studies have been performed on barium sulphate crystals from three xenophyophore species ( Aschemonella ramuliformis, Reticulammina labyrinthica, Galatheammina lamina ) obtained at bathyal and abyssal depths in the north-eastern Atlantic. Two populations of crystals were observed. The first were tablets, ~2μm in length and rhombic or hexagonal in outline. In both cases, the tabular face was of index (100). The second population consisted of much smaller particles (<0·5 μm) of poor crystallinity. A comparison of the larger xenophyophore crystals with synthetically grown crystals indicated that the former probably grew at low supersaturation (S<25) in solutions of low to moderate ionic strength (I<l·0 M). Some preliminary observations of the cellular organisation of A. ramuliformis are reported. The protoplasm is multinucleate and characterized by what seems to be a system of extracellular lacunae formed by imaginations of the cell wall. Similar features have been observed in the deep sea foraminiferan Rhizammina algaeformis . Possible origins of the BaSO 4 crystals and the taxonomic relationship between xenophyophores and certain foraminiferans are discussed.