MICROBIAL DIVERSITY: Domains and Kingdoms

With the discovery of the eukaryote nucleus, all living organisms were neatly divided into prokaryotes, which lacked a nucleus, and eukaryotes, which possessed it. As data derived directly from the genome became available, it was clear that prokaryotes were comprised of two groups, Eubacteria and Archaebacteria. These were subsequently renamed at the new taxonomic level of Domain as Bacteria and Archaea, with the eukaryotes named as the Eucarya Domain. The interrelationships of the three Domains are still subject to discussion and evaluation, as is their monophyly. Further data, drawn from various protein sequences, suggest conflicting schemes, and resolution may not be straightforward. Additionally, Bacteria and Archaea as well as Eucarya are largely based on organisms already in culture. Investigation of the potentially enormous quantity of uncultured organisms in nature is likely to have as broad-ranging implications as the exploration of new protein sequences.