The Earth could contain nearly one trillion species, of which 99.999 percent are yet to be discovered, says a study based on the largest analysis of microbial data.
The findings suggest that only one-thousandth of one percent of all the species have been identified till now.
"Estimating the number of species on Earth is among the great challenges in biology," said one of the study authors Jay Lennon from Indiana University in Bloomington, Indiana.
The scientists combined microbial, plant and animal datasets from government, academic and citizen science sources, resulting in the largest compilation of its kind.
Altogether, these data represent more than 5.6 million microscopic and non-microscopic species from 35,000 locations across all the world's oceans and continents, except Antarctica.
"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," Lennon explained.
The estimate, based on universal scaling laws applied to large datasets, appeared in the journal Proceedings of the National Academy of Sciences.
The report's authors are Jay Lennon and Kenneth Locey of Indiana University in Bloomington, Indiana.
"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 a large pool of new information," Lennon added.
Microbial species are forms of life too small to be seen with the naked eye, including single-celled organisms such as bacteria and archaea, as well as certain fungi.
The study's results also suggest that identifying every microbial species on Earth presents a huge challenge.
"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," he added.
"Microbial biodiversity, it appears, is greater than we ever imagined," Lennon pointed out.
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