Life underground

The strange beasts that live in solid rock deep underground

Drill down a mile or two into Earth's crust and you will find solid rock, unbearable heat and little oxygen. Yet it turns out there is also thriving animal life

Just how far down in the Earth's crust can animals survive? In the dark, hot depths of several South African gold mines, there live some tiny worms that may hold the key to answering that question.

These creatures are the deepest living animals that have ever been discovered.

No one knows how they got down there, but they could have been living in the mines for thousands of years. Their very existence suggests that complex life can survive far deeper in the Earth than was ever thought possible.

In the 1980s, scientists discovered that life can endure thousands of feet beneath the Earth’s surface..

Nematodes can transform into a special form called the dauer stage

However, most scientists believed that the bowels of the Earth could only house single-celled organisms like bacteria. In the deep Earth it is hot and dark, and there is very little oxygen. There is also not much food down there, so any animal making its home in the belly of the Earth would struggle to get a decent meal.

One scientist thought differently. Gaetan Borgonie was previously at the University of Ghent in Belgium and is now at Extreme Life Isyensya in Gentbrugge, Belgium. He was convinced that one animal could survive deep down in the Earth's crust: a nematode worm.

That is because nematodes are extremely hardy, able to cope with extreme heat, cold and dehydration. They have an ingenious trick that allows them to survive.

Nematodes can transform into a special form called the daier stage. In this form, they can survive harsh conditions for long periods before reawakening when the going gets good again.

Surely if any animal could live deep inside the Earth it would be this hardy worm?

This makes them similar to tardigrades: tiny creatures that can survive being boiled, frozen, crushed, dried out or blasted into space by going into a kind of stasis.

In the dauer stage, the nematode goes into stasis and its metabolism slows. The stage is triggered by a pheromone and occurs when there is a lack of food, high temperature, or overcrowding.

Nematodes in the dauer state are spectacularly resilient. When the space shuttle Columbia broke up after re-entering Easth’s atmosphere in 2003, there were nematodes on board: they survived both the disintegration and the fall back to Earth,.

Nematodes have populated almost all areas of the world. They can be found in hot springs, deserts, high up mountains and in the deepest oceans. They live in Antarctica, inside the bellies of other animals including humans, and even incide the plancentas of sperm whales. Surely if any animal could live deep inside the Earth it would be this hardy worm?

Borgonie decided to look for himself, but sceptical grant holders denied him funding. So he teamed up with Tullis Onstott of Princeton University in New Jersey, who also believed that nematodes could exist deep underground. The pair used money from their own savings to travel to South Africa.

Before this, nematodes were only known to live a few dozen feet underground

They chose South African mines because they are the deepest accessible places on Earth. Ore miners have drilled to depths of more than 2 miles (3.2km), so researchers can access Earth's hidden subterranean world.

Borgonie began searching the hot and humid mines for life. He took samples of water pouring from boreholes, which had previously been drilled into the rock by mine geologists to check for gas and water pockets ahead of mining operations. He then filtered the water, searching it for life.

When he looked under a microscope he found lots of little worms, the first animals discovered this deep underground. Before this, nematodes were only known to live a few dozen feet (tens of metres) underground.

This video shows the conditions in which the nematodes are living.

The next step was to show that the worms originated in the mine, and had not been accidentally brought in on miners' shoes or through other contamination. To do this, Borgonie spent a year testing 8,343 gallons (31,582 litres) of water used in mining operations. He also searched the soil near the boreholes.

Borgonie did not just find one species of worm: he found four, living in three separate mines

He found no worms in the mining water, demonstrating that the nematodes were not coming into the mine from above. The nematodes in the soil belonged to different species. He also demonstrated that the worms preferred to munch on the bacteria that live in the mine, rather than bacteria from outside, which suggested that the worms were well-established in the mine.

The team finally published their findings in 2011.

"When you go underground the pressure increases, the temperature increases, oxygen diminishes and food becomes scarcer," says Borgonie. "Therefore one assumed that looking for multicellular life was akin to bonkers."

Yet Borgonie did not just find one species of worm: he found four, living in three separate mines.Two species – Plectus aguatilis and an unknown nematode – were found in the Driefontein mine at a depth of 0.6 miles (0.9 km) at 24 °C.

They were topped by Halicephalobus mephisto, a new species of nematode. It was found living in the Beatrix gold mine 150 miles (240 km) south-west of Johannesburg. The worm was living 0.8 miles (1.3 km) underground. At such depths the temperatures reach 37 °C, hotter than most terrestrial nematodes can survive.

The worms feed on snotty layers of goo called biofilms

At the time, many media outlets described H. mephisto as the deepest-living animal known, or at least gave the impression that it was. But it was nothing of the sort.

The DNA of another unknown species was found in water 2.2 miles (3.6km) down in the TauTona mine, where temperatures are around 48 °C. This mystery species was the deepest-living animal ever. "Because H. mephisto got all the attention, it stuck that 1.3km was the deepest, but it isn't," says Borgonie.

What's more, according to Borgonie the nematodes are not simply eking out a marginal existence underground. They are thriving.The worms feed on snotty layers of goo called biofilms, which consist of millions of bacteria stuck together. Each bacterium is 10 billion times smaller than the worm. The gooey biofilms have attached themselves to the man-made boreholes. This suggests that, by mining the rocks for gold, humans have inadvertently created the perfect habitat for the worms.

P. aquatilis is a common freshwater nematode

The low oxygen levels do not bother the worms. Whilst humans need their air to contain 21% oxygen, nematodes are not so picky and can do just fine with as little as 0.5%.

The hot temperatures are also not a problem. Even the 48 °C heat of the TauTona mine is nothing to some species of nematodes, which have been found in hot springs reaching 61 °C.

It seems the worms had already evolved to withstand heat and crushing pressure, long before they entered the mines.
Reference:
http://www.bbc.com/earth/story/20151124-meet-the-strange-creatures-that-live-in-solid-rock-deep-underground