Halicephalobus mephisto, a new species of bacteria feeding nematode discovered at -1.3 km in Beatrix gold mine, near Johannesburg. The species name was given in reference to the story of Faust where the Lord of the Underworld was called Mephisto.
Credit: University Ghent, Belgium / Gaetan Borgonie
CAMBRIDGE: A new species of nematode worm has been discovered living several kilometres beneath the surface of the Earth's crust, indicating that deep, underground ecosystems are more complex than previously thought.
These microscopic underworld worms are the first examples of life other than than single-celled organisms found in the deep subsurface biosphere, the region beneath the Earth's surface in which life exists.
The newly identified species has been named Halicephalobus mephisto, after Mephisto, Lord of the Underworld in the classic German legend of Faust, and according to the researchers its discovery could be an important step in finding extra-terrestrial life.
"There may be more under our feet than we accepted until now," said Gaetan Borgonie from the University of Ghent, Belgium, and lead author of the paper published in the current issue of Nature. "It opens thrilling possibilities for the search for life on other planets."
Thriving in the deep subsurface
It has long been known that single-celled organisms such as bacteria and yeast live beneath the Earth's surface down to more than 3 km, but it was thought that the difficult conditions of high temperatures, lack of oxygen and little space could not support anything more complex.
The research team took water that was between 3,000 and 12,000 years old from the rock face at boreholes in South African mines and filtered it to capture any resident nematodes.
The worms' characteristic asexual reproduction was then exploited to let the team look at them in more detail. "Once a worm was found," explained Borgonie, "we let it reproduce...and then did analysis [on] genetics and morphology."
Harsh conditions don't prevent complex life
Four nematode species were found in samples from three different mines, at depths ranging from 0.9 to 3.6 km. As well as three species that had previously been seen in soil or freshwater at the surface, an unknown species (Halicephalobus mephisto) was also found.
The team confirmed that the worms had not been introduced through the mining activity, and that the subsurface conditions could sustain them.
"The finding of multicellular organisms thriving in the deep subsurface is [a] significant find," said Borgonie. "It shows that harsh conditions do not prevent complex life."
The most inhospitable habitats
Andreas Teske, professor in marine sciences at the University of North Carolina, who studies deep subsurface microbiology, is unsure of how much this work reveals about multicellular life in the deep subsurface.
"Nematodes have a reputation as some of the toughest multicellular life forms that have colonised the most inhospitable habitats [at the surface]," he said. "This nematode still requires oxygen for respiration, like all multicellular eukaryotes, and oxygen is available in deep fracture water of these mines.
"I don't think these nematodes evolved in the subsurface; otherwise we should see deep radiations in the phylogenetic tree. Instead, the South African nematodes look like a recent divergence, an offshoot that branched off from the rest of the nematodes."
Nevertheless, Borgonie believes that the nematodes' ability to survive in difficult conditions indicates new possibilities for complex life elsewhere in the solar system.
"If life arose on Mars and it thrived underground," he said, "it may have continued to evolve to a higher degree of complexity [than] thought possible, even if the conditions on the surface are too harsh for it to survive there. It is something to take into account when searching for life on other planets or moons."
