Abstract
Glaucophytes are one of the three major lineages of primary photosynthetic eukaryotes (Archaeplastida), together with red algae and Chloroplastida (green algae and embryophytes). Within Archaeplastida, glaucophytes are thought to retain ancestral features of the first photosynthetic eukaryote. The biflagellate glaucophyte genus Cyanophora has been widely studied as a model organism of primitive phototrophs, with released nuclear genome sequence, but no morphological study has clearly shown ultrastructural diversity to delimit glaucophyte species. Recently, we applied advanced electron microscopic (EM) methodologies for comparative morphology in a purpose of delineating the glaucophyte species. On the basis of ultra-high resolution field emission scanning EM (FE-SEM), we demonstrated that the cell surface of Cyanophora was ornamented with angular fenestrations framed by ridges. As supported by observation using several transmission EM methods, these ridges were formed by the edges of overlapping or attaching outermost plate vesicles that distributed throughout the cell periphery just underneath the cell membrane. Based on differences in the pattern of surface ornamentations detected by FE-SEM, Cyanophora biloba and C. sudae were distinguished from each other. FE-SEM, however, cannot be applied for the native protoplast surfaces that are enclosed by a thick cell wall or extracellular matrix as in the immotile glaucophyte genus Glaucocystis. Alternatively, we very recently used the advanced ultra-high voltage EM tomography to unveil the in situ peripheral ultrastructure of protoplasts in Glaucocystis and revealed the ultrastructural diversity in this genus. Furthermore, Glaucocystis cells have numerous, leaflet-like flattened vesicles distributed throughout the protoplast periphery just underneath the cell membrane, as in Cyanophora as well as some secondary photosynthetic eukaryotes. Thus, a similar 3D peripheral ultrastructure might have appeared in the common ancestor of glaucophytes and/or the first photosynthetic eukaryote.
