Observations on the Fine Structure of Oedogonium IV. The Mature Pyrenoid of Oe. cardiacum

Abstract

The ultrastructure of mature pyrenoids is described for zoospores and actively growing vegetative cells of Oedogonium cardiacum. In all cases, the matrix of the mature pyrenoid is penetrated by an unusual system of ramified cytoplasmic channels which are lined by 2 membranes that are continuous with the 2 membranes of the chloroplast envelope. These channels represent invasions of extraplastidial cytoplasm into the chloroplast and pyrenoid matrix. Only 2 other algal genera, Platymonas and Prasinocladus, are presently known to form pyrenoids in which the matrix is similarly penetrated by channels lined with the invaginated chloroplast envelope. Other ultra-structural differences, however, readily distinguish the pyrenoids of these 2 genera from the pyrenoids of O. cardiacum.