This electron microscopic image shows a micro-animal “tardigrade,” also known as a water bear, at the UNC in Chapel Hill, N.C. (AP/Bob Goldstein/Vicki Madden/UNC Chapel Hill)

Scientists agree that there are far more species on Earth than we’ve actually discovered — but exactly how many there are in total remains up for debate. Previous studies have placed it anywhere from 3 million to 100 million, and one much-touted 2011 paper estimated that there are 8.7 million species of eukaryotes — which include plants, animals, fungi and some (but not all) single-celled organisms — on the planet, with about 86 percent of them remaining undiscovered.

But now a new study, published Monday in Proceedings of the National Academy of Sciences, has blown all of those estimates out of the water. It suggests that there may be up to a trillion species of microbes, alone, on the planet — and the researchers say 99.999 percent of them have yet to be discovered. Unlike the previous research, this study includes all kinds of microscopic organisms, including bacteria — both those on land and those living in the ocean.

To estimate the number of species humans haven’t yet discovered, researchers have to rely on the patterns already observed in nature. Scientists know there are certain measurable trends that exist in natural ecosystems — for instance, bigger communities generally have room for more species. And when there are more species present, they tend to exist in different proportions, with some becoming more abundant than others.

These patterns tend to hold true for many different types of communities and are known as “scaling laws,” or laws that describe how different biological or ecological traits scale up as the number of organisms or space involved increases. Many scaling laws are fairly well-established in plant or animal communities — but they’ve been largely understudied when it comes to microbes. “These unexplored opportunities leave the understanding of biodiversity limited to the most conspicuous species of plants and animals,” the researchers wrote.

They’ve also noted that including all microbes in such studies “would aid the estimation of global species richness and reveal whether theories of biodiversity hold across all scales of abundance and whether so-called law-like patterns of biodiversity span the tree of life.”

So the researchers involved in this study — Kenneth Locey and Jay Lennon of Indiana University — decided to analyze the way different trends scale up in natural ecosystems, and to see if these trends hold true in microbial communities as well.

To do so, they first had to compile a huge amount of data collected by researchers, governments and even citizen scientists all over the globe — altogether amounting to a dataset including more than 5 million species observed at 35,000 different sites around the world. The researchers then analyzed the data to find out how various aspects of biodiversity changed as a community grew, focusing especially on the number and abundance of species present (and the number of individual organisms representing each species).

They found that across all communities — including both microscopic organisms and larger plants and animals — the same types of scaling laws applied. As the total number of organisms in a community increased, the number of species present grew and the ways the different species changed in abundance — some becoming more dominant, and some becoming more rare — were similar across all communities. In essence, the researchers had discovered what appeared to be a universal scaling law.  

By combining their scaling law with a well-established biodiversity model, the researchers were able to produce an estimate of the number of microbial species that should exist. In doing so, they concluded that Earth likely contains upwards of a trillion species of microbes alone.

In contrast, fewer than 10 million microbe species have ever been observed in nature (and some of these have only been documented once or twice) — meaning there may be 100,000 times more microbe species on the planet than have been found so far.  

Finding all of them would be a monumental task, the researchers note, pointing out that at the very least we’d need a major increase in microbial sampling efforts around the world.

“The profound magnitude of our prediction for Earth’s microbial diversity stresses the need for continued investigation,” they wrote. “We expect the dominance scaling law that we uncovered to be valuable in predicting richness, commonness, and rarity across all scales of abundance. To move forward, biologists will need to push beyond current computational limits and increase their investment in collaborative sampling efforts to catalog Earth’s microbial diversity.”

Even so, the study is a staggering reminder that life on Earth is far more abundant — and mysterious — than meets the eye.

Read more at Energy & Environment:

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