Scientists Estimate Earth Is Home to One Trillion Microbial Species

Lift your pinkies and summon your hairless cats, because it’s time to get a little Doctor Evil: researchers say Earth may currently support one TRILLION different microbe species. They published their report in the Proceedings of the National Academy of Sciences. 

This may seem like a strange pronouncement. After all, shouldn’t we already know how many species are sharing our home planet? Well, no. We can’t even agree on a standardized definition of the term “species.” And, well, microorganisms are very, very small. For most of human existence, we didn’t even know they were here.

So making an educated guess about the number of species has been kind of tricky. That hasn’t stopped scientists from doing it, of course, but they’ve always been limited by the technology available.

To create a more up-to-date estimate, scientists collected data from 20,376 reports on bacteria, archaea (single-celled organisms), and microscopic fungi, and another 14,862 on populations of trees, birds, and mammals. In total, the dataset incorporated information about more than 5.6 million species from 35,000 locations in all the world's oceans and on every continent but Antarctica. 

Kenneth Locey is an ecologist at Indiana University (IU) and co-author of the study. "A massive amount of data has been collected from these surveys," Locey said in a press statement. "Yet few have tried to pull together all the data to test big questions."

The laws of scaling are a set of scientific principles that explain how various natural elements like habitat area and population size are related. But scientists were previously unsure if biodiversity fit within those laws—that is, if it were scaleable.

The results of this study suggest that it is scaleable, and in a major way. “After analyzing a massive amount of data,” Locey said, “we observed simple but powerful trends in how biodiversity changes across scales of abundance."

Species abundance, they found, has a very strong relationship with the number of individuals. In other words, the more ants there are, the more ant species there are likely to be. Locey said biodiversity may be “the most expansive scaling law in biology.”

The researchers were then able to apply what they knew about existing species and biodiversity to build an estimate of the number of microbe species on Earth. That number? One trillion.

"Before high-throughput genetic sequencing,” said IU microbe diversity expert and co-author Jay Lennon, “scientists characterized diversity based on 100 individuals, when we know that a gram of soil contains up to a billion organisms, and the total number on Earth is more than 20 orders of magnitude greater.” 

But just because we know they’re out there doesn’t mean they’ll be easy to find or identify. “Of those species cataloged, only about 10,000 have ever been grown in a lab, and fewer than 100,000 have classified genetic sequences," Lennon said. "Our results show that this leaves 100,000 times more microorganisms awaiting discovery—and 100 million to be fully explored."