Hungry Worms From Hell

by Sara Reardon 

Demon worm. Halicephalobus mephisto, the first multicellular organism found far below Earth’s surface, is a ravenous bacteria eater.

Credit: Gaetan Borgonie/University Ghent, Belgium

Three kilometers beneath Earth’s crust, in a realm known as the subsurface, temperatures in the thick rock are well over 40°C, and the oxygen-poor water chokes out familiar life. Here, slow-growing bacteria and other microbes flourish in masses called biofilms. A ravenous predator also survives in these hellish conditions: a roundworm that researchers describe today as the first multicellular organism to be found in the deep subsurface of Earth.

Eleven years ago, while studying the microbes that live in water-filled rock fractures in South African gold mines, geomicrobiologist Tullis Onstott of Princeton University noticed wormlike organisms living in cultures taken from 1.3 kilometers below the surface. “Well, I’m not a worm kind of guy,” Onstott says. So he contacted Gaetan Borgonie, a nematologist at Ghent University in Belgium, who collected and filtered tens of thousands of liters of water samples from five mines in the area to find the rare creatures, which belong to the worm group called nematodes. In addition to some previously described nematodes, which scientists had never before seen living at this extreme depth, the researchers discovered a new species of nematode that subsists on microbes and requires only trace amounts of oxygen. They named the deep-living, heat-loving species Halicephalobus mephisto. The name refers to Faust’s devil, Mephistopheles, and means “he who loves not the light” in Greek.

In their paper, published today in Nature, the researchers describe H. mephisto as physically similar to surface nematodes, about half a millimeter long and having evolved to be fonder of devouring subsurface bacteria than of dining on the common surface bacterium Escherichia coli. The nematodes are extremely rare because there’s so little oxygen in their environment and they need to eat about 10,000 bacteria per day to survive.

The researchers carbon-dated dissolved organic molecules in water from the vents. They found that the water had been there for somewhere between 3000 and 12,000 years, suggesting that the drillers didn’t bring the oxygen (or the worms) down with them. David Smith, an oceanographer at the University of Rhode Island, in Narragansett, says that although contamination is always a “tricky issue,” the researchers show plenty of evidence of careful handling to support their conclusions. He calls the findings “eye opening” and expects they will encourage other researchers to start looking for multicellular creatures far beneath Earth’s surface.

Marine microbiologist Andreas Teske of the University of North Carolina, Chapel Hill, was not surprised that the first multicellular subsurface organism was a nematode; the worms are among the hardiest creatures on the planet. “If all other [nonmicrobial] life was extinguished, the survivors would be nematodes,” he says.

Onstott says that his group is now working with the J. Craig Venter Institute to sequence H. mephisto’s genome. They hope to discover how the worm can cope with high temperature and low oxygen. The molecular mechanisms, Onstott speculates, may be similar to those of other subsurface organisms such as yeast and fungi. But Teske, who says that there’s a “world of difference” between a predatory worm and a yeast cell, thinks there could be huge surprises.

The study is “another step forward in finding life in places you don’t expect it,” says Michael Meyer, an astrobiologist who is the lead scientist for NASA’s Mars Exploration Program in Washington, D.C. “The discovery of multicellular organisms in the Hadean subsurface world raises the unthinkable possibility there could be multicellular life on Mars.”

But Teske cautions against “wild speculation” about the impact of the study on the search for extraterrestrial life, because H. mephisto isn’t a completely isolated species but has cousins on the surface.

As for Onstott’s group, they are trying to expand the subsurface life panoply even more. “We have worms from hell; next we’ll be looking for viruses from hell,” he says.