Ultrastructure of the membranous layers enveloping the cell of the coccolithophoridEmiliania huxleyi

Abstract

Emiliania huxleyi is a marine unicellular alga that is covered by one or several layers of coccoliths (discs of crystalline calcium carbonate) or of organic scales. Immediately proximal to these layers are several membranous layers that surround the protoplast. In this study an ultrastructural analysis of this membranous envelope is presented using ultrathin-section and freeze-fracture electron microscopy. We found that the protoplast ofE. huxleyi is surrounded in a proximal-to-distal direction by a membrane studded with intramembranous particles (IMPs), an intermediate structure consisting of several smooth layers, and an outermost membrane studded with IMPs. On freeze-fracturing the intermediate structure reveals two or three closely apposed smooth fracture faces. It is possible that this structure consists of two or three membranes that are devoid of IMPs. However, the possibility is also discussed that this structure is composed of three or four lipid monolayers so closely stacked that water is hardly or not present between them.