Nomenclature of prokaryotic ‘Candidatus’ taxa: establishing order in the current chaos

In the mid-1990s, the category ‘Candidatus’ was established for putative taxa of as yet uncultivated prokaryotes. The status of ‘Candidatus’ is not formally included in the rules of the International Code of Nomenclature of Prokaryotes. Thus, ‘Candidatus’ names do not have standing in the nomenclature. Curated annotated lists of ‘Candidatus’ names (not including phyla) have been published since 2020. By April 2021, about 2700 names of ‘Candidatus’ taxa had been published. The International Committee on Systematics of Prokaryotes recently rejected proposals to allow gene sequence data as nomenclatural types. An alternative code for naming uncultivated microorganisms (the ‘SeqCode’) is now being developed for naming the majority of prokaryotes that are as yet uncultivated. In the opinion of the author, there is no need for such a code, as the existing system, with nomenclature quality control also for ‘Candidatus’ names, fulfills the needs. Computer programs such as GAN which generates large numbers of correctly formed names from the short lists of Latin and Greek word elements and Protologger that produce descriptions directly from genome sequences will become important in the future for automated naming and description of large numbers of ‘Candidatus’ taxa from metagenomic and single cell genome data. However, the formation of interesting and meaningful names is encouraged whenever possible.