Scientists have discovered a group of microbes that live several kilometres under the surface of the Earth, need no light or oxygen and can only be seen in a microscope.
The researchers have also found how the microorganisms, named Hadesarchaea, first discovered in a South African gold mine at a depth of over three kilometres, are able to make a living in the absence of oxygen and light.
Microorganisms that live below the surface of the Earth remain one of the last great areas of exploration. Organisms that live there have not been grown in the laboratory and therefore their lifestyles are unknown.
Researchers found these microbes in vastly different aquatic and terrestrial environments; the deep mud of a temperate estuary in North Carolina and underneath hot springs at Yellowstone National Park in US.
"This new class of microbes are specialised for survival beneath the surface, so we called them "Hadesarchaea", after the ancient Greek god of the underworld," said lead author Brett Baker, assistant professor at the University of Texas.
As its name suggests, the Hadesarchaea belong to a relatively unknown group of microorganisms, the archaea.
Like bacteria, archaea are single-celled and microscopically small, but they differ more from each other than a human does from a tree.
Archaea were discovered only some 40 years ago. To date, they remain poorly studied in comparison to bacteria and more complex life forms, such as animals and plants.
"The discovery of the Hadesarchaea will help us increase our understanding of the biology and lifestyle of archaea that thrive in the deep biosphere," said Thijs Ettema, senior lecturer at Uppsala University.
In order to understand these elusive organisms, Baker and Ettema sequenced the genomes of several Hadesarchaea.
They were able to determine how these microbes should be classified and what physiologies they use to survive under these extreme conditions.
Hadesarchaea have the ability to live in areas devoid of oxygen and the scientists suggest that they are able to survive there by using carbon monoxide to gain energy.
The chemical pathways the Hadesarchaea cells use to metabolise carbon monoxide are unique to what has been seen before, researchers said.
"Before this essentially nothing was known about the Hadesarchaea's ecological role and what makes them so prominent throughout the world," said Jimmy Saw, researcher at Uppsala University.
"The new discovery expands our knowledge of how these organisms may have adapted to the extreme conditions of the deep biosphere," said Saw.
The study was published in the journal Nature Microbiology.
