HALICEPHALOBUS MEPHISTO: This newly discovered nematode thrives more than a kilometer beneath the surface, most likely by grazing on bacterial biofilms. Image: Property of the University Ghent, Belgium - Gaetan Borgonie
Deep in South Africa's gold mines water can be found in rock fractures, hosting bacteria that feed off the stone itself and form biofilms on the hard surfaces. Now new samples pulled from these sunless pools show that nematodes—roundworms of varying size that are essentially tubes with a digestive tract and thrive everywhere on the planet—likely graze on these bacterial films, surviving more than a kilometer underground. In fact, an entirely new species of nematode—dubbed Halicephalobus mephisto for a lifestyle reminiscent of Faust's underworld demon, Mephistopheles, or "he who loves not the light"—makes its home only in the deep subsurface, suggesting that life, even complex, multicellular life, may populate sulphate-loving ecosystems in the planet's unexplored depths.
"I had come to respect the very hardy nature of the animals—one species even survived the space shuttle Columbia break up in 2003," explains nematologist Gaetan Borgonie of Ghent University in Belgium. "When I looked at temperature, oxygen, pH and food requirements of nematodes on the surface and compared with values in the literature of mining results, from my point of view, they could survive there."
In fact, the team of researchers set out to find nematodes—or other complex organisms—given the quantity of bacteria and other microbes thriving in the subsurface. The team used submicron-size filters to capture any such organisms in deep borehole water. Samples from Beatrix, Driefontein and Tau Tona gold mines in South Africa yielded specimens or evidence of four different nematode taxa, including one already known from nearer the surface, Plectus aquatilis. No other more advanced organisms were found—neither funguses, amoebas nor any other multicellular life-forms—and each nematode was specific to a particular borehole, according to the report published in Nature on June 2. (Scientific American is part of Nature Publishing Group.)
Similar surveys of both the chemically treated water used in the mine as well as mine walls and surface soils yielded none of the same nematodes, arguing against contamination being responsible for the result. Radioactive carbon isotope dating also suggests the water from the boreholes had not seen the light of day in at least 2,900 years.
As for how the nematodes found themselves more than a kilometer beneath the surface, no one knows for sure. "Nematodes have an uncanny ability to follow bacteria," Borgonie notes, adding that the deep nematodes cultured in the lab preferred to eat subsurface bacteria rather than E. coli, like their counterparts nearer to the surface. "Another possibility is that mining activity does lower the water table and could 'drag' nematodes to the deep."
And there is plenty of bacteria in the deep to feed the nematodes for thousands of years, according to the population samples as well as rough estimates of how many nematodes might be down there. There are also oxygen levels high enough to support nematodes, even though dissolved O2 levels in the ancient water were as low as 13 micromoles. "Measurements have shown that terrestrial nematodes can survive indefinitely on 0.5 percent oxygen, well within the low oxygen we found in the subsurface," Borgonie notes. "No special adaptation was required for them to survive in the deep."
In fact, the nematodes' small size allows them to persist in the cramped spaces of the deep. "The width of a fracture at these great depths may be only several millimeters at the most, with lots of tight bottlenecks," explains geoscientist Tullis Onstott of Princeton University, who was also part of the research team. "We know that complex food webs exist in deep, sulfidic caves. Our findings are just looking at the roots of these systems—deeper, smaller, yet still with enough O2 for multicellular organisms to feed, reproduce and presumably evolve."
Most of the nematodes discovered reproduce asexually—either via eggs that need no fertilization or by containing both male and female organs in the same organism. The nematodes also had no trouble with the heat down low, as much as 48 degrees Celsius. "I think there is more down there than we were led to believe," Borgonie says, noting also that this argues life, even complex life, could be thriving beneath the surface of other worlds, like Mars. Similar mine sampling is now ongoing on other continents on this planet. "It was just a mind set that prevent[ed] us from having looked before," Borgonie adds. "I am sure there is more."