99.999% of Microbial Species on Earth Have Not Been Discovered

Microbes are so tiny that scientists sometimes forget about them.

Estimating the number of species on Earth is a major challenge for scientists. While past attempts to establish this number have likely done a good job at counting the larger species, they have either vastly under-sampled microbes, or ignored them altogether.

By using the largest available datasets and new ecological scaling laws, researchers reporting in Proceedings of the National Academy of Sciences have estimated that the Earth is inhabited by nearly 1 trillion microbial species. They also determined that 99.999 percent of these species have not yet been identified.

Microbes—which include bacteria, protists, and fungi—are found living in virtually all ecosystems, including some of the most extreme.

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"Until recently, we've lacked the tools to truly estimate the number of microbial species in the natural environment," Jay T. Lennon, associate professor at the Indiana University Bloomington College of Arts and Sciences and co-author of the study, said in a press release. "The advent of new genetic sequencing technology provides an unprecedentedly large pool of new information."

The past several years have seen an explosion in microbial sampling efforts, including the collection of human-related microorganisms by the National Institutes of Health's Human Microbiome Project; marine microorganisms by the Tara Oceans Expedition; and aquatic, terrestrial and host-related microorganisms by the Earth Microbiome Project.

"A massive amount of data has been collected from these new surveys," said Kenneth J. Locey, a postdoctoral fellow at the Indiana University Bloomington College of Arts and Sciences, whose work included programming required to compile the inventory. "Yet few have actually tried to pull together all the data to test big questions.

The researchers combined these newly expanded datasets with ecological models to obtain their rigorous estimate of the number of microbial species on Earth.

Microbes account for most of the diversity of life on the planet. New species are being identified all the time, but we are still barely making a dent in our efforts to catalog microbial diversity. For example, the Earth Microbiome Project—a global multidisciplinary project to identify microscopic organisms—has so far cataloged less than 10 million species.

"Of those cataloged species, only about 10,000 have ever been grown in a lab, and fewer than 100,000 have classified sequences," Lennon said. "Our results show that this leaves 100,000 times more microorganisms awaiting discovery—and 100 million to be fully explored. Microbial biodiversity, it appears, is greater than ever imagined”

This research was funded through the National Science Foundation’s Dimensions of Diversity program, which aims to fill the most substantial gaps in our understanding of the diversity of life on Earth by 2020.

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