After All, Only Millions?

An update on the census of species of Archaea and Bacteria published recently in mBio (P. D. Schloss, R. A. Girard, T. Martin, J. Edwards, and J. C. Thrash, mBio 7:e00201-16, 2016, http://dx.doi.org/10.1128/mBio.00201-16) showed again that, despite ever-increasing sequencing efforts, the PCR-based retrieval of 16S rRNA genes is approaching saturation. On average, 95% of the genes analyzed today are identical to those present in public databases, with rarefaction analysis indicating that about one-third of the bacterial and archaeal diversity has already been covered. Therefore, despite estimates of up to 10(12) microbial species, the option should be considered that the census of Archaea and Bacteria on planet Earth might yield only millions of species after all.