New Estimate: There Are 1 Trillion Species of Microbes on Earth

There may be 1 trillion microbes on Earth, according to a new estimate. One gram of soil can contain 10,000 species, such as Actinomyces israelii, shown here. GrahamColm via Wikipedia

We scarcely know the life that exists on this planet. A new estimate suggests that there are 1 trillion species of microbes on Earth, and humans have only described less than one-thousandth of 1 percent of these (0.001 percent).

Jay Lennon and Kenneth Locey, researchers at Indiana University, came up with the estimate after combing through a massive amount of data, including samplings of microbial diversity from about 3,500 unique sites, ranging from human skin and coral reefs to animal intestines and mountain streams.

For plants and animals, scientists have found that they can reliably predict how many species they expect to find in a certain area, given details like the type of environment, how much food is around and the level of competition for essential resources. These reliable relationships are called scaling laws; Lennon and Locey wanted to see if one of these laws, governing the relationship between abundance and diversity, would also apply to the microbial world.

It turns out this law matched the data they found on microbes extremely well. Using this pattern, they were then able to extrapolate the total number of species they'd expect to see across the entire planet.

The estimate shows just how little we know about microbes. "We're far, far away from being anywhere near a complete census of microbial life," says Lennon, co-author of a study describing the findings, published May 2 in the journal Proceedings of the National Academy of Sciences.

The degree of microbial diversity is often difficult to comprehend. In one study, published in July 2015 in the journal Nature, for example, scientists found 35 phyla (plural of phylum, the second-largest taxonomic domain of life) of microbes in a single aquifer in Colorado. To put that in perspective, that is the same number of phyla in the entire animal kingdom. And of the approximately 6 million species of microbes identified, only about 10,000 have been grown in the lab. This means that there is a vast gap in our knowledge of microbes' genetics and properties—such as their size and shape, chemicals they produce and behavior—that can sometimes only be observed once they have been isolated in a lab.

James O'Dwyer, a researcher at University of Illinois, applauds the effort and says the extrapolation was worth doing, but that "there are a lot of ways it could go wrong, and the authors would probably agree with that." The relationship between abundance and diversity that apply at smaller scales may not hold true over the entire Earth, for example, and could lead the estimate to be off by a large amount. That said, these kinds of extrapolations are common in science, and always contain a certain degree of uncertainty. "It will be exciting to see if these predictions hold as more species are discovered," O'Dwyer adds.

"We could definitely say we're living in a microbial world," dominated by microbes that we barely know at all, Lennon says.