The curious case of planctomycetes: a bacterium with a “nucleus” challenges eukaryotic evolutionary theory

When picturing a bacterial cell, one might envisage a cytoplasm with free floating ribosomes, plasmids and nucleoid, encapsulated in a peptidoglycan cell wall. Far different from the eukaryote, characterised by a membrane bound nucleus and organelles. However, defying bacterial stereotypes is the planctomycetes, which at first appears as a curious crossover between the two cell types. Possessing phenotypic features such as a lack of peptidoglycan cell wall and compartmentalization of the cell via internal membranes, including the formation of a membrane bound nucleoid. In addition, all species reproduce via budding, with the exception of one marine genera (Phycisphaera) still utilising binary fission. The Planctomycetes form a distinct phylum of the domain Bacteria and their unique cell biology challenges concepts of evolutionary history of eukaryotes and the bacterial cell plan.

Planctomycetes are a very diverse phylum, found in fresh and marine waters and sediments globally, from your garden pond to hydrothermal vents (surviving at 85°C!), existing as a range of nutritional types. Noteworthy are the anammox species, autotrophic anaerobes that oxidize ammonia to dinitrogen, contributing to 50% of all atmospheric N₂. In recent years full scale anammox plants utilize this species for nitrogen-rich-wastewater remediation. Further research of microbial usage in this way could lead to energy generating sewage treatment in the future.  

Arguably the most unusual species of this phylum is Gemmata obscuriglobus. Its nucleoid DNA is encased in a double membrane envelope surrounded by ribosomes, forming a nuclear body analogous to the eukaryotic nucleus. If that wasn’t enough, G. obscuriglobus can also “endocytose”, taking up environmental protein via the formation of internal vesicles, a process analogous to receptor- and clathrin mediated endocytosis of eukaryotes. This nutritional mode and membrane coat-like proteins have only been found in eukaryotes and members of the PVC superphylum (of which planctomycetes is a part).

This cross over of characteristics between the two domains can be explained by four different models:

1) Planctomycetes descended from a complex eukaryote like common ancestor that possessed membrane coat-like proteins (perhaps even from LUCA itself).

2) Planctomycetes evolved these characteristics and provided the gene transfer to a proto-eukaryote lineage which became the common ancestor to all modern eukaryotes.

3) These traits are simply a product of parallel convergent evolution resulting from similar adaptive needs. There is not homology only analogy in structure.

4) Ancient and extensive lateral gene-transfer from evolved eukaryotes occurred.

Regardless of which model (if any) is correct, this suggestion of autogenous development of internal membranes resulting in endomebrane systems and endocytosis like mechanism, discounts the need for symbiotic fusion between archaeal and bacterial cells to produce the modern eukaryote. Further research is needed to understand if planctomycetes possess homologues of genes associated with membrane formation in eukaryotes or if the functions of these eukaryotic genes are performed by functional analogues in planctomycetes.

One thing is for sure, future study of this species is needed to help understand our own evolutionary past.  

Reference paper:

Fuerst, J. and Sangulenko, E. (2011). Beyond the bacterium: planctomycetes challenge our concepts of microbial structure and function. Nature Reviews Microbiology, 9(6), pp.403-413.