'Dark microbial matter' - uncultured microbes whose characteristics have never been described - could be dominating nearly all the environments on Earth, scientists say.
The study, published in the journal mSystems, is the first time to estimate the population of microbes that have not yet been grown in a lab culture.
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Researchers from the University of Tennessee in the US believe that as many as a quarter of the microbes on earth could come from the roughly 30 phyla -- a taxonomic classification between kingdoms and classes -- of microbes that have never been cultured.
To estimate how many uncultured microbes call this planet home, researchers collected every DNA sequence deposited in public databases by researchers all over the world, totalling 1.5 million, and compared them to 26,000 DNA sequences of microbes and bacteria that have already been cultured.
"We quickly hit a serious computational limitation, but lucky for us, we have a great collaboration with the Joint Institute for Computational Sciences at Oak Ridge National Laboratory," said Karen Lloyd, associate professor at University of Tennessee.
"They were able to provide the computing power we needed to get the job done," said Lloyd.
The study and characterisation of uncultured microbes can be a particularly valuable tool in specific fields -- such as in medicine, where scientists have described cases of culture-resistant pathogens.
"Uncultured microbes are so vastly different than cultured ones that they might be doing unusual things, like surviving on extremely low energy or growing extraordinarily slowly, in ways that are hard for us even to imagine," said Lloyd.
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"It is also possible that these microbes can't grow on their own in culture because they die if they are removed from their intricate relationships with each other or their particular environment," she said.
"Since these microbes provide many ecosystem services - such as helping crops grow and battling climate change - solving the considerable puzzle they've presented us is a crucial challenge for modern microbiology," said Lloyd.