Earth could contain nearly 1 trillion microbial species, with only one-thousandth of 1 percent now identified, a new study suggests.
To come up with the estimate, researchers combined microbial, plant, and animal community datasets from government, academic, and citizen science sources.
Altogether, these data represent more than 5.6 million microscopic and nonmicroscopic species from 35,000 locations from all the world’s oceans and continents, except Antarctica.
“Estimating the number of species on Earth is among the great challenges in biology,” says Jay T. Lennon, associate professor of biology at Indiana University and coauthor of the the study that is published in the Proceedings of the National Academy of Sciences.
“Our study combines the largest available datasets with ecological models and new ecological rules for how biodiversity relates to abundance. This gave us a new and rigorous estimate for the number of microbial species on Earth.
“Until recently, we’ve lacked the tools to truly estimate the number of microbial species in the natural environment. The advent of new genetic sequencing technology provides an unprecedentedly large pool of new information.”
Microbial species are all forms of life too small to be seen with the naked eye, including all single-celled organisms, such as bacteria and archaea, as well as certain fungi. Many earlier attempts to estimate the number of species on Earth simply ignored microorganisms or were informed by older datasets that were based on biased techniques or questionable extrapolations, Lennon says.
“Older estimates were based on efforts that dramatically under-sampled the diversity of microorganisms. Before high-throughput sequencing, scientists would characterize 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 over 20 orders of magnitude greater.”
‘Unimaginably huge challenge’
The realization that microorganisms were significantly under-sampled caused an explosion in new microbial sampling efforts over the past several years, 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.
These data sources—and many others—were compiled to create the inventory used in the current study, which pulls together 20,376 sampling efforts on bacteria, archaea and microscopic fungi and 14,862 sampling efforts on communities of trees, birds, and mammals.
“A massive amount of data has been collected from these new surveys,” says postdoctoral fellow Ken Locey, whose work included programming required to compile the inventory. “Yet few have actually tried to pull together all the data to test big questions.
“We suspected that aspects of biodiversity, like the number of species on Earth, would scale with the abundance of individual organisms,” he added. “After analyzing a massive amount of data, we observed simple but powerful trends in how biodiversity changes across scales of abundance. One of these trends is among the most expansive patterns in biology, holding across all magnitudes of abundance in nature.”
Scaling laws are known to accurately predict species numbers for plant and animal communities. For example, the number of species scales with the area of a landscape.
“Until now, we haven’t known whether aspects of biodiversity scale with something as simple as the abundance of organisms,” Locey says. “As it turns out, the relationships are not only simple but powerful, resulting in the estimate of upwards of 1 trillion species.”
Further, the study’s findings suggest that actually identifying every microbial species on Earth is an almost unimaginably huge challenge. To put the task in perspective, the Earth Microbiome Project—a global effort to identify microbial 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 says.
“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.”
The National Science Foundation and US Army Research Office funded the work.
Source: Indiana University