Electron-opaque annular structure girdling the constricting isthmus of the dividing chloroplasts of Heterosigma akashiwo (Raphidophyceae, Chromophyta)

The plastokinesis (kinesis of chloroplasts) of a raphidophyte alga, Heterosigma akashiwo, was studied by electron microscopy using rapid freezing and freeze-substitution techniques. The chloroplasts are enveloped by two pairs of tightly appressed double membranes, the inner and the cytoplasmic outer pair. The inner pair constricts to divide in advance of the outer pair. By observation of serial sections an electron-opaque, annular structure (plastid-dividing ring) was observed at the isthmus of constricting chloroplasts, girdling the periplastidal outer surface of the inner pair of the four surrounding membranes. These observations suggest that the mechanisms underlying the constriction of the inner and outer pair may differ from each other. The localization of the annular structure (plastid-dividing ring) suggests that the inner pair of the surrounding membranes may be homologous to the double envelope membranes of the chloroplasts of Chlorophyta and Rhodophyta. In addition these findings provide a new evidence supporting the secondary endosymbiosis hypothesis for the origin of the chloroplasts in chromophyte algae.