過去の微小藻類が作る石灰質微化石の結晶方位の解析 : 石灰質ナンノ化石Florisphaera profundaとDiscoaster属の結晶構造とその意味

抄録

Selected area electron diffraction (SAED), high-resolution transmission electron microscopic and scanning electron microscopic analyses were performed to determine crystal faces of two nannoliths from Florisphaera profunda and discoasters, the exoskeltons of a certain unicellelar alga, calcareous nannofossils. A specimen of F profunda is a pentagonal-shaped plate and its [0001] direction (c-axis) is oriented parallel to its surface and in the elongation axis of the plate-like element. Its crystallographic nature mostly resembles a proximal shield element of Emiliania huxleyi, one of the most common coccolithophorids observed in the present ocean. On the other hand, a nannolith of Discoaster specimens is a star-shaped calcite crystal and is composed of several numbers of bent, bar-shaped calcite elements, called 'rays.' Based on results of the SAED observation and calcite decoration on specimens, each ray apparently seems to be a single calcite crystal in all types of discoasters. The [0001] direction is likely perpendicular to the tangential plane of the central area and each ray joins together in the plane that is parallel to [0001]. The difference of crystallographic nature between nannoliths and heterococcoliths suggests that the phylogenetic origin and the evolutionary history of nannolith-producing phyioplankton are different from those of heterococcolith-bearing coccolithophores.