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

Xenophyophores are a group of giant deep sea protists characterized by intracellular barium sulphate (BaSO4) 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, similar to 2 mu m 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 mu m) 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 < 1.0 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 invaginations of the cell wall. Similar features have been observed in the deep sea foraminiferan Rhizammina algaeformis. Possible origins of the BaSO4 crystals and the taxonomic relationship between xenophyophores and certain foraminiferans are discussed.