Life in the abyss

A Jovial Look!

Colin Barras - Reed Business Information - UK

Deep life: biology's final frontier

From Methuselah microbes to animals that survive without any oxygen, strange underground organisms are redefining what it means to be alive.

Deep life: the hunt for the intraterrestrials

Deep life: the journey to the centre of the Earth

The strange creatures that live in the deepest abysses of the Earth's crust are rewriting our understanding of life, from its origins to its dying days

SOUTH AFRICA'S gold mines are crawling with demons, each sporting a whip-like tail and a voracious appetite. Not that theminers are worried. These demons are barely visible to the naked eye.

They are big news for people studying life on Earth, though. "The discovery floored me," says Tullis Onstott, a geologist at Princeton University, who discovered these nematode worms swimming in the water-filled fissures of the Beatrix gold mine in 2011.

The fact is, complex organisms just shouldn't be able to live so far beneath the Earth's surface. The nourishment and oxygen that animals need to survive are in short supply just tens of metres below ground, let alone 1.3 kilometres down. Noting that the worms shunned light like a mythical devil, Onstott's team named them Halicephalobus mephisto, after Mephistopheles, the personal demon of Dr Faustus.

Travelling even deeper into South Africa's crust, they found more surprises. On a trek down into TauTona, the country's deepest gold mine, they came across another species of nematode worm at 3.6 kilometres below ground?- making it the deepest land animal found to date (Nature, vol 474, p 79).

In fact we now know that the depths of Earth's crust harbour isolated ecosystems whose inhabitants defy many established biological rules. There are microbes that metabolise so slowly they may be millions of years old; bacteria that survive without benefiting from the sun's energy; and animals that do what no animal should?- live their entire lives without oxygen. This strange menagerie might give us insights into where life originated and where it is heading. It may even help our search for life on other worlds.

That would be an ironic twist. For most of the 20th century, few suspected that Earth's interior could harbour any life, let alone writhing worms or scuttling bugs. Biologists were looking for signs of life on Mars long before they turned their gaze downwards. "The prevailing view was that the deep Earth was sterile," says Barbara Sherwood Lollar, a geologist at the University of Toronto in Canada, who also studies South Africa's goldmines.

It took the nuclear arms race to overturn that orthodoxy. By the 1980s, the US had taken to burying sealed containers of radioactive waste below itsnuclear processing facilities, and the Department of Energy was concerned that deep microbes, if they existed, might eat through the seals. In 1987, to ease the fears, theDoE sponsored a team to hunt for life in boreholes below the Savannah river facility in South Carolina. To general astonishment, they found bacteria and single-celled organisms called archaea 500 metres below the surface.

It didn't take long to find out that deep life was not only possible, but extremely prevalent. In 1992, John Parkes, now at the University of Cardiff in the UK, found the sediment under the Sea of Japan to be teeming with life. Even at 500 metres below the sea floor, he found 11 million microbes in every cubic centimetre of dirt (Nature, vol 371, p 410).

The implications were extraordinary. Even when you consider that the heat emanating from the Earth's interior would kill anything deeper than 4 kilometres below the surface, there would be enough room to house a considerable portion of the planet's life. Estimates vary from less than 1 per cent of the world's biomass to 10 per cent, with a more thorough exploration of the Earth's crust needed to firm up that figure.

In the meantime, the focus has switched to answering some of the most pressing questions about the challenges facing organisms deep underground. Top of the list was the question of how they can feed in such barren regions. The microbes under the sea floor, for instance, must have originated on the seabed before being buried under sediment over thousands of years. Left with just small amounts of nutrients in the surrounding dirt, and without any new source of food, the microbes should have starved long ago. Indeed, given that the microbes are eerily still when viewed under a microscope, some sceptics argued that they were exquisitely preserved corpses of long-dead cells, rather than living organisms.

Yet that's not what Yuki Morono of the Japan Agency for Marine-Earth Science and Technology in Nankoku found in 2011. His team took cells from 460,000-year-old sediments found 220 metres below the Pacific Ocean seabed near Japan, and exposed them to a plentiful food supply labelled with stable isotopes of carbon and nitrogen. Two months later, Morono found traces of the isotopes in three-quarters of the cells (PNAS, vol 108, p18295). They were still alive?- although you could not tell from their behaviour.

"Their lives are so slow compared with ours,"says Morono. "It is really difficult to distinguish alive and dead cells." The key to their survival seems to be an incredibly slow metabolism, allowing the meagre food source to be rationed for thousands of years.

Methuselah microbes

If that lifestyle seems austere, it is nothing compared with the ecosystem discovered by Hans Ry at Aarhus University in Denmark and colleagues. Beneath the Pacific Ocean, they found active bacteria and archaea in sediments deposited 86 million years ago?- 20million years before the dinosaurs went extinct. The cells' reduced metabolism suggests each has been on a very strict diet for the entire time. Under such tight constraints, populations are very sparse, with a mere 1000cells occupying every cubic centimetre of sediment (Science, vol 336, p 922).

Evolution may work very differently in these isolated pockets of sediment. "If there is barely enough energy to meet the requirements of a single cell, it is suicide for that cell to divide," says Ry. Microbes in these ancient sediments might focus their efforts on repairing their own machinery rather than bothering with the activity that most other organisms live for: reproduction.

If these ideas are right, then some of these organisms could be among the oldest creatures on the planet. "Cells in these environments could be millions of years old for all we know," says Katrina Edwards at the University of Southern California.

As strange as they are, the Methuselah microbes living beneath the sea are pretty conventional compared with some of the organisms found below Earth's continents. Consider one species of bacteria living down South Africa's Mponeng gold mine, whose food chain begins with the radioactive decay of minerals in the surrounding rocks.

Just reaching these microbes can be physically exhausting, says Sherwood Lollar, who helped discover the bacteria. "You may go down in the cages with a mining shift crew at 7am, and not actually reach your site until 10.30am," she says. With temperatures and humidity almost unbearably high, the researchers have a few hours at most to collect samples from water-filled fissures in the mine's boreholes. "Then you turn around and make the trek back up."

At first sight, the crystalline rocks down there appear to be an even more desolate home than ocean sediment; formed deep in prehistory, they have received next to no organic matter, even in the distant past. It would seem impossible to find food down here, yet bacteria manage to eke out a meagre existence. Their secret? Uranium. As this element decays, the resulting radiation splits water molecules, releasing free hydrogen, through a process called radiolysis. The bacteria then combine the hydrogen with sulphate ions from the rock, producing enough energy to sustain life (Science, vol 314, p 479).

Powering their cells in this way, these bacteria are part of a select club of species that survive without any input from the sun (see "Where the sun don't shine", right). "I would say the energy sources are all independent from photosynthetic sources," says Li-Hung Lin at the National Taiwan University in Taipei, who led the team that discovered the bacteria.

While such discoveries extended the known boundaries of life on Earth, for a long time it seemed that deep-dwelling organisms would be limited to simple, single-celled life forms: bacteria, archaea and a few slightly more sophisticated fungi and amoebas. While they are all fascinating organisms in their own right, they are not very lively.

Then Onstott's demon worms showed that animals can live kilometres below the surface. They may be only half a millimetre long, but that still makes them hundreds of times bigger, and far more complex, than other deep dwellers. "The diversity in the crust is greater than I ever imagined," says Onstott.

The demon worms probably arrived in the mine relatively recently, though. Isotopic dating of the surrounding water suggests they reached the depths perhaps only 12,000 years ago, probably riding in groundwater that trickled into Earth's interior. Importantly, this water still contains oxygen from when it was last in contact with the atmosphere. Once that oxygen is used up, the worms will die, making it a fleeting stay in evolutionary terms.

But some animals have evolved to survive these suffocating conditions for the long haul,if one discovery from deep under the Mediterranean Sea is anything to go by. Found in 2010, these unusual Loricifera resemble tiny, dead houseplants?- complete with pots. The 250-micrometre-long animals each have avase-shaped armoured case, or lorica, and a straggly mess of tentacle-like projections emerging from its opening.

It isn't their appearance that makes them a showstopper for biologists, though. Antonio Pusceddu at the Marche Polytechnic University in Ancona, Italy, and colleagues have found these Loricifera have evolved a unique method of metabolism that does not rely on oxygen, unlike that of all other animals. Indeed, their cells completely lack mitochondria, the organelles that power other animals. Instead, they generate energy from hydrogen sulphide using organelles called hydrogenosomes (BMC Biology, vol 8, p 30).

No oxygen, no problem

For William Martin at the University of Dusseldorf, Germany, the Loricifera are evidence that oxygen is not the key to complex animal life. However, he points out that their sluggish behaviour has caused some sceptics to question the find - just as some critics had believed the inactive underground microbes to be dead. "Some researchers would like to see independent corroboration that the   Loricifera are really alive," he says.

If that verification comes, it would raise hopes that deep life may be far more sophisticated than anyone had imagined. That would bode well for two new projects?- the Census of Deep Life and the Center for Dark Energy Biosphere Investigations?- that are setto catalogue underground life in the next few years.

Besides giving us a better understanding of life on Earth today, the results may also give us a glimpse back in time to our early origins. At the very least, South Africa's radiolysis-powered bacteria may give us a new viewpoint on the molecular machinery that allowed life to thrive before photosynthesis reshaped the planet. Some go even further, suggesting that life itself began deep underground.

"There were tumultuous geological processes going on at the time life appeared," says Sherwood Lollar. "There's a strong argument to consider that life arose in a warm little fracture where it might have been protected from heavy asteroid bombardments or the lethal ultraviolet light that bathed early Earth." It is by no means a mainstream theory: most believe hydrothermal vents in the ocean to have been the cradle of life.

But even if these fissures within the crust didn't witness the birth of the first life forms, they will almost certainly be the last refuge for organisms at the end of our planet's life. Last year, Jack O'Malley-James at the University of St Andrews, UK, and his colleagues modelled the likely fate of life on Earth as the ageing sun makes conditions increasingly hostile (see diagram, left). The model suggests that about a billion years from now the oceans will begin to evaporate, and the only life to survive will be microbes deep below the Earth's surface, where they might hold on for another billion years (International Journal of Astrobiology, doi.org/ktf). For all the grandeur of rainforests, savannahs and coral reefs, deep life is probably a more persistent feature of our planet.

The same may also be true of other worlds. "The results from the deep biosphere are completely changing the exploration strategy for life on other planets," says Sherwood Lollar. "The Viking expeditions to Mars in the 1970s were looking for life on the surface. Now we know that signs of life are much more likely to be found in the subsurface." And following the discovery of the Loricifera, there are renewed hopes for complex life forms.

For the moment, though, many eyes are gazing downwards. "This is the last unexplored part of our planet," says Pusceddu. "We can expect even more exciting discoveries of animals and unicellular organisms in future." It seems we have barely scratched the surface.

Loricifera are so sluggish some doubt they are even alive.

Where the sun don't shine

Thirty metres beneath the surface of southern Romania there is a secret crypt known as Movile cave that has been locked away from the rest of the Earth for millions of years. The shrimp-like crustaceans and spiders that live in its dark, damp recesses may resemble their cousins outside the cave, but their ecosystem is built on entirely different foundations.

While most life relies in some way on the sun's energy, the animals in Movile cave are almost completely locked away from its influence. Not only do they live in pitch blackness, but virtually none of the nutrients created on the surface filter through the rocks into the cave.

Instead, bacteria scavenge hydrogen sulphide released into the aquifer from the surrounding rocks, using it to produce life-giving energy. They are then the prey of many other creatures, creating a complex food web with no input from photosynthesis.

Such "chemosynthesis" has long been known to power life around hydrothermal vents in the ocean, where tectonic activity releases minerals from the rocks. But Movile cave shows that it is not limited to the deep sea. Similar mechanisms could also fuel complex ecosystems deep within the Earth's crust, hundreds of kilometres below our feet.