I think most of us appreciate the power of high-throughput sequencing for uncovering the sheer diversity of microorganisms on Earth, from the depths of the oceans to the tops of mountains, and everywhere in between. But, how many microbes are out there? A new paper published in PNAS has provided an answer.

Locey and Lennon collated data from 20,376 sites of bacterial, archaeal, and microscopic fungal communities and 14,862 sites of tree, bird, and mammal communities, and used ecological scaling laws to investigate biodiversity metrics, such as the total number of species (N) and number of individuals belonging to the most abundant species (Nmax) in, and species richness of, large microbiomes.

As predicted, “greater N led to an increase in species richness, dominance, and rarity and a decrease in species evenness”. Interestingly, they found that Nmax scaled with N at similar and isometric rates, establishing a dominance scaling law that spans 30 (yes, 30!) orders of magnitude in N. N and Nmax are also used to extrapolate global species richness, predicting that Earth supports ~1 trillion microbial species.

A nice concluding sentence puts this number into context:

“~10^4 species have been cultured, less than 10^5 species are represented by classified sequences, and the entirety of the EMP [Earth Microbiome Project] has catalogued less than 10^7 species, 29% of which were only detected twice.”

Yikes, that's a lot of microorganisms still to find...