It is now clear that the cellular construction of at least the eubacteria is more complex than realized, and includes organelle-like structures known as microcompartments, of which the best known are the carboxysomes.

Bacterial microcompartments

Although bacteria are routinely dismissed as primitive, this is an entirely mistaken point of view. Classical biology likes to contrast the prokaryotes with supposedly more “advanced” eukaryotes because the latter house such organelles as the chloroplasts and mitochondria, both of which are examples of endosymbiosis and the earlier capture respectively of some sort of cyanobacteria and α-proteobacteria (close to such rickettsialids as Typhus). It is now clear, however, that the cellular construction of at least the eubacteria is more complex than realized, and includes organelle-like structures known as microcompartments, of which the best known are the carboxysomes that are found in the cyanobacterial and chemoautotrophic bacteria. In point of fact there are at least 6 other types, including the so-called enterosomes in such pathogenic bacteria as Salmonella. The enterosomes of Salmonella are involved with detoxification, but in other instances the specific function is yet to be established. In general these microcompartments are built of a series of protein plates with hexagonal configuration that self-asssembles into a crystalline-like polyhedral structure. It is possible this had a single origin, but it is also conceivable that the same proteins have been repeatedly recruited. It is also quite possible that independently other types of microcompartments have evolved.

Carboxysomes, C4 photosynthesis and viruses

The carboxysomes provide excellent insights into evolutionary convergence. They evolved to overcome the problem of low concentrations of carbon dioxide, necessary for photosynthesis and belong to the general area of carbon dioxide concentrating mechanisms (CCMs), which are rampantly polyphyletic. The best known example is that of C4 photosynthesis, which itself has evolved many times in the flowering plants as well as at least twice in the algae. Interestingly the spur for the evolution of the carboxysomes was probably much the same as C4 photosynthesis, that is the declining levels of atmospheric carbon dioxide. Each carboxysome is packed with the Rubisco enzyme, and also carbonic anhydrase (again convergent), and evidently this greatly favours the process of photosynthesis although by no means all the processes are understood.

Another aspect of convergence that relates to carboxysomes is the striking similarity between the polyhedral structure of the carboxysome and the coat of a virus, each forming a tightly fitting array of protein plates, although those of the carboxysome are generally thinner probably because they are housed in the safety of the cell. This similarity is quite independent and shows how the same architectural principles must apply.