If our knowledge of the number of unicellular and multicellular organisms found in the soil and in water bodies is incomplete, what we know about organisms thriving in subsurface environment is sketchy. The discovery of living multicellular nematodes or roundworms belonging to three different species, and the DNA of a fourth species, at depths ranging from 0.9 km to 3.6 km from the Earth's surface, has extended the known boundaries of Earth's biosphere. Before this discovery, scientists had harvested only unicellular bacteria from similar depths. One of the nematodes is a new species. It has been named Halicephalobus mephisto by the authors of a paper published recently in Nature (“Nematoda from the terrestrial deep subsurface of South Africa,” by G. Borgonie et al.) The half-a-millimetre-long worms were recovered from the fracture water in three deep gold mines in the Witwatersrand basin near Johannesburg. The fracture water, which is different from mining water, flows from deep cracks in the Earth's crust and so is free from any contamination. According to carbon-14 dating, the facture water is 3,000 to 12,000 years old.

Unicellular and multicellular extremophiles have been found from unexpected and highly inhospitable conditions, including volcanic vents in mid-oceanic ridges, and below Antarctic ice sheets at a depth of about 180 metres. Although roundworms are well known for their physiological tolerance — surviving in a state of suspended animation or anabiosis for prolonged periods of time and metabolising aerobically even in low-oxygen conditions — their subsurface presence has still taken scientists by surprise. In fact, researchers have difficulty in understanding how unicellular organisms thrive at such depths. The presence of nematodes at such depths demonstrates their ability to tolerate and survive in highly hostile conditions like reduced oxygen levels, high temperature, and scarce food supply. For instance, the hypoxic condition in the fracture water is as low as 1 per cent of the oxygen levels found in most oceans, and temperatures are as high as 41°C — much higher than what their terrestrial counterparts can tolerate. A fully functional ecosystem appears to have been in place in the fracture water. Sequencing the worms can provide answers to whether the worms had migrated to the fracture waters and adapted themselves to the harsh conditions. Science once again highlights the truth that the presence of unicellular as well as multicellular life in unthinkable environments is limited only by our imagination to explore — not by their ability to survive.

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