Abstract
A heterotrophic organism 1–2 billion years ago enslaved a cyanobacterium to become the first photosynthetic eukaryote, and has diverged globally. The primary phototrophs, glaucophytes, are thought to retain ancestral features of the first photosynthetic eukaryote, but examining the protoplast ultrastructure has previously been problematic in the coccoid glaucophyte Glaucocystis due to its thick cell wall. Here, we examined the three-dimensional (3D) ultrastructure in two divergent species of Glaucocystis using ultra-high voltage electron microscopy. Three-dimensional modelling of Glaucocystis cells using electron tomography clearly showed that numerous, leaflet-like flattened vesicles are distributed throughout the protoplast periphery just underneath a single-layered plasma membrane. This 3D feature is essentially identical to that of another glaucophyte genus Cyanophora, as well as the secondary phototrophs in Alveolata. Thus, the common ancestor of glaucophytes and/or the first photosynthetic eukaryote may have shown similar 3D structures.
Highlights
The first photosynthetic eukaryote may have been a Cyanophora-like flagellate
Using ultra-high voltage electron microscopy (UHVEM) tomography, the 3D ultrastructural features of the plasma membrane and the flattened vesicles at the protoplast periphery of the two Glaucocystis species were visualised with high contrast (Figs 2 and 3 and Supplementary Movies 1–4)
We observed various regions of matured vegetative cells by UHVEM and tomography, as well as ultrathin section transmission electron microscopy (TEM) (Supplementary Fig. 2 and Supplementary Note 1); the peripheral 3D structures were essentially consistent within each species
Summary
As supported by ultrathin section transmission electron microscopy (TEM) and freeze-fracture TEM20–22, field emission scanning electron microscopy (FE-SEM) recently showed that the whole peripheral surface of naked vegetative cells in several species of Cyanophora is ornamented with angular fenestrations formed by ridges structured by overlapping, leaflet-like flattened vesicles underneath the plasma membrane[21,22]. This leaflet-like 3D morphology of the flattened vesicles has not been unambiguously demonstrated in other glaucophyte genera, possibly because FE-SEM cannot reveal surface ultrastructures of the periphery of the protoplast that is enclosed by a cell wall or extracellular matrix in these genera[7]. To examine the peripheral 3D ultrastructure of Glaucocystis protoplasts enclosed by a cell wall, we performed 3D-modelling based on the UHVEM tomography using high-pressure freezing (HPF) and freeze-substitution (FS) fixation of two divergent strains of Glaucocystis: “G. geitleri” SAG 229-1 and G. nostochinearum SAG 16.9822,27 (Fig. 1)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
