Further observations on the fine structure of Chrysochromulina chiton, with special reference to the pyrenoid

Abstract

Developments associated with the surface of the projecting pyrenoids of Chrysochromulina chiton have been traced in outline after glutaraldehyde fixation. In the simplest condition the pyrenoidal core is separated from the cytoplasm by two superposed pairs of membranes, the outer pair being continuous with a layer of endoplasmic reticulum spread over the surface of the subtending chloroplast. Various stages in the formation and removal of an additional membrane-bounded cap of translucent material are illustrated and interpreted in terms of local accumulation of a temporary storage metabolite in the path of translocation of materials from the chloroplast into the cell. A cap is thought to begin as a blister involving the innermost pair of membranes which become pushed apart as the transparent material accumulates between them. A bulge, involving the outer of the two separated membranes plus contents of the blister, could then push out between meshes in the overlying endoplasmic reticulum before spreading laterally. This process is not light-dependent. It is suggested that enzymes localized between the chloroplast membranes in this region are acting on a soluble translocation metabolite. At mitosis, and shortly after, the capping material may be absent.