Classifying the uncultivated microbial majority: A place for metagenomic data in the Candidatus proposal

Microbial taxonomists have generally been reluctant to accept the valid publication of names of uncultured taxa given that only pure cultures allow for a thorough description of the genealogy, genetics and phenotype of the putative taxa to be classified. The classification of conspicuous uncultured organisms has been considered into the Candidatus provisional status, but this is only possible with organisms for which it is possible to retrieve basic data on phylogeny, morphology, ecology and some metabolic traits that unequivocally identify them. The current developments on modern sequencing techniques, and especially metagenomics, allow the recognition of discrete populations of DNA sequences in environmental samples, which can be considered to belong to individual closely related populations that may be identified as members of yet-to-be described species. The recognition of such populations of (meta)genomes allow the retrieval of valuable taxonomic information, i.e. genealogy, genome, phenotypic coherence with other populations, and ecological relevant traits. Such traits may be included in the Candidatus proposals of environmentally occurring, yet uncultured species not exhibiting exceptional morphologies, phenotypes or ecological relevancies.