Bacterium excluded from the Eukaryote Club
It’s something you learn in high school – there are two basic approaches to cellular life – prokaryotes (bacteria and archaea) and eukaryotes (the rest of us – aardvarks, amoebae, apricots, etc). Prokaryotes have an open-plan office, with all biological functions carried out in the one cellular space. Eukaryotes, however, have dedicated compartments for the chief executive(nucleus), finance (mitochondria), sales and marketing (golgi,endoplasmic reticulum), and so on.
But every now and then you get an upstart prokaryote that seems to have ideas above its station. One such is Gemmata obscuriglobus, an exemplar of a bunch of unusual bugs known as the PVC superphylum (for Planctomycetes, Verrucomicrobiae, Chlamydiae). Gemmata has a complex membrane structure, and previous studies of its 3D configuration have suggested that this bacterium has a compartmentalised cell, eukaryote-style, its genetic material encapsulated in a nucleus-like body.
With a nice combination of technical wizardry and sheer hard work, Rachel Santarella-Mellwig,Damien Devos and colleagues, authors of a paper just published in PLOS Biology, have managed to reconstruct the structure of the internal membranes of a typical Gemmata cell in spectacular detail. They embedded ten bugs in plastic, chopped them each into ten or so slices, and then took electron microscope snapshots of the Gemmata salami. In the absence of software that could do the job, they then manually tracked and assigned the membranes in each slice, building up a detailed 3D model of the membranes and other features.
The answer, perhaps sadly, is that Gemmatais actually rather a traditional bug, topologically speaking. The beautiful pictures and movies produced by the authors reveal a membrane system that, while extremely convoluted, doesn’t enclose any separate compartments. Gemmata is as prokaryotic as they come.