Nematoda from the terrestrial deep subsurface of South Africa

In the past two decades, a variety of single-cell organisms have been found living within Earth's crust, forming what has become known as the deep subsurface biosphere. A new report describes the first multicellular creatures from this biosphere — nematode worms, including one new to science. Up to around half a millimetre in length, the worms feed on bacteria, reproduce without mating by parthenogenesis and tolerate the high temperatures found in their habitat, 3 kilometres deep in the Beatrix gold mine in South Africa. Similar organisms might be expected to occur in certain sub-seafloor settings, and the viability of multicellular life in such a hostile environment will also be of relevance to astrobiology. Since its discovery over two decades ago, the deep subsurface biosphere has been considered to be the realm of single-cell organisms, extending over three kilometres into the Earth’s crust and comprising a significant fraction of the global biosphere1,2,3,4. The constraints of temperature, energy, dioxygen and space seemed to preclude the possibility of more-complex, multicellular organisms from surviving at these depths. Here we report species of the phylum Nematoda that have been detected in or recovered from 0.9–3.6-kilometre-deep fracture water in the deep mines of South Africa but have not been detected in the mining water. These subsurface nematodes, including a new species, Halicephalobus mephisto, tolerate high temperature, reproduce asexually and preferentially feed upon subsurface bacteria. Carbon-14 data indicate that the fracture water in which the nematodes reside is 3,000–12,000-year-old palaeometeoric water. Our data suggest that nematodes should be found in other deep hypoxic settings where temperature permits, and that they may control the microbial population density by grazing on fracture surface biofilm patches. Our results expand the known metazoan biosphere and demonstrate that deep ecosystems are more complex than previously accepted. The discovery of multicellular life in the deep subsurface of the Earth also has important implications for the search for subsurface life on other planets in our Solar System.