by Marco Allemann
Figure 1. The original interpretation of the G. obscuriglobus cellular organization. Thin section of cryosubstituted bud cell with large nuclear body bounded by a “nuclear envelope” (E) consisting of two membranes between which is a clear electron-transparent space. The nuclear body contains ribosome-like particles as well as a nucleoid. It is surrounded by cytoplasm that contains superficially similar ribosome-like particles and is bounded by a single intracytoplasmic membrane (ICM). Bar: 200 nm. Source.
Members of the bacterial phylum Planctomycetes (click here and here) inhabit a wide variety of environments throughout the world. What makes them special is that in the mind of some investigators they possess a mix of eukaryotic and prokaryotic structural attributes. Now that is something pretty unique and worth contemplating. This group of organisms has been previously described in this blog by previous graduate students. So this serves as an update, which is timely because there is critical news on the plancto front.
What Do You See Inside The Planctos?
Under the electron microscope, sectioned Planctomycetes cells reveal that their internal membrane is highly folded into apparent compartment, including a purported membrane-bound nuclear region. It has been suggested that this arrangement, almost unheard of among prokaryotes, is evidence for the planctos being a “missing link” between Bacteria and Eukaryotes. The linchpin for this notion that the planctos may be some kind of remnant of eukaryogenesis has been their “nuclear envelope,” which is thought to totally surround the nucleoid. There are yet other arguments in favor of this notion, e. g., they are able to endocytose and they divide by budding (although so do other bacteria, such as Caulobacter crescentus). In addition, those planctos capable of anammox (anaerobic ammonia oxidation coupled with nitrate reduction) contain an organelle, a membrane-bound structure called the anammoxosome. Yet, the presence of organelles is not unique to this group of bacteria. Of interest is also that planctos do not contain peptidoglycan in their cell wall, something limited to few bacteria. Surprisingly, they carry genes encoding for known outer membrane proteins and peptidoglycan synthesis, although in this regard they may not be all that unique either because chlamydiae and mycoplasmas also lack peptidoglycan, plus the chlamydiae also carry genes for its synthesis.