Hidden deep down under Danish waters lives a cornucopia of bacteria.

The deepest scientific drillings ever into the Danish seafloor have revealed that the unexplored subsurface is swarming with life.

“We’ve set the world record for the amount of bacteria found in a drilling,” says professor Bo Barker Jørgensen, director of the center for geomicrobiology at Aarhus University. “There are up to ten billion cells per cubic centimetre. No matter how deep we drilled, it’s bursting with life down there.”

Jørgensen led the large scientific expedition that for the past two years has drilled deep into the seabed below Kattegat and the Baltic Sea.

Organisms lived in isolation for millennia

A great number of the microorganisms on Earth live deep down in the subsurface. The exact number of microorganisms is not known but on the basis of drillings abroad a recent study estimates that up to 32 percent of the microorganisms on Earth live in the deep subsurface. Researchers have found traces of microorganisms in the seabed down to a depth of two kilometres. The microorganisms in the seabed include both bacteria and archaea, and there are even more viruses.

It’s still not known what type of life the researchers have found down there in the ocean’s underworld.

Jørgensen explains that researchers have counted the cells found in the samples but they have yet to examine what types of microorganisms they are.

“These are microorganisms that have lived in isolation from the rest of the world for millennia while the seabed settled on top of them. It’ll be incredibly exciting to examine them,” says Barker Jørgensen.

He expects analyses of the microorganisms’ genetic material to reveal lots of new and hitherto undiscovered species from the deep.

Archaea is one of the three domains that life is traditionally divided into. The two other domains are bacteria and eukaryotes (including humans, animals and fungi). The archaea are single cell organisms and differ from the bacteria on a number of points, including metabolism and cell membrane structure.

Many archaea live in extreme environments such as geysers with temperatures above boiling point, in very salty, acidic or alkaline environments. Researchers believe that a very large proportion of life in the subsurface consists of archaea.

“Similar drillings abroad have revealed unknown microorganisms – unknown in the sense that no organisms from the surface were closely related to them,” he says.

Both bacteria and archaea

So far it seems that the majority of life in the surface of the Danish seafloor consists of different species of bacteria.

However, when researchers drill deeper into the subsoil, they find an increasing number of archaea – an entirely different type of microorganisms that are significantly different from bacteria, viruses, animals and humans.

This is the first time researchers have ever drilled that deep into the Baltic subsurface and at the University of Copenhagen senior lecturer Mathias Middelboe is surprised how much life was discovered down there.

“It’s surprising that there’s such a high concentration of cells. The density they find is fairly high compared to what we’d normally expect,” he says.

Middelboe explains that scientific drillings in other areas have only revealed around 100 million cells per cubic centimetre -- which is far from the billions found in the new drillings.

“The Baltic expedition is very exciting and there really is basis for new exciting discoveries. A large part of the Earth’s microorganisms live deep down underneath the floor of the sea, and yet we know very little about their sources of nourishment or how many there are,” says Middelboe, who carries out research in deep microbial life. He did not participate in the Baltic expedition.

Fundamentally different forms of life

It’s only in the last couple of years that ocean drillings have made researchers realise that a large part of life on Earth is actually hidden underneath the seabed.

One of the most recent estimates indicates that as much as 32 per cent of all microorganisms on Earth live deep down under the oceans.

According to Jørgensen, the construction of the deep biosphere even seems to be fundamentally different to that of life on land.

“They live under extreme conditions and at the moment we actually don’t understand how it’s even possible for them to survive,” he says. “It contradicts a large part of the knowledge we thought we had about the limits of life.”

Living under starvation conditions

Jørgensen points out that life under the sea suffers from an extremely limited access to nourishment, no access to light and an extremely high pressure from the seabed and the ocean above.

In short: it’s a great mystery how the microorganisms manage to get the energy needed to live down there in the depths of our planet.

Researchers can only guess to how the organisms manage.

“We have now found astonishingly large communities of bacteria under the Baltic Sea, so it will be exciting to see what they live on,” says Jørgensen.

“We have knowledge of the deep microbial communities from a number of ocean drillings, but in those cases the drillings were carried out in open sea,” he says. “The Baltic Sea offers some fairly unique opportunities to see how these communities develop.”

Unique research opportunities in the Baltic

One of the major reasons for the unique research opportunities in the Baltic Sea is the dramatic change it has undergone.

In the course of the past 140,000 years, Scandinavia has been covered by a one kilometre thick layer of ice and the climate has changed from frozen to temperate. Meanwhile, the Baltic Sea has been filled with both freshwater and seawater and it has been oxidised and deoxidised.

Researchers identify these changes in the climate by looking at the deposits in the seabed.

“The exciting part is that as we examine the deposits, we see how long it’s been since the microorganisms were isolated from the rest of the world. In that way we can tell how they develop over time – for instance how they grow and how quickly,” says Jørgensen.

“One of the questions we would like to find the answer to is whether or not the microorganisms in the subsurface are descendants from the life that once was on the surface – that is whether they developed differently and under what circumstances they were buried,” he says. “Or perhaps they only depend on current conditions? Our drillings in the Baltic Sea offer a unique opportunity to find the answers to these questions.”

Can life in the deep be dangerous?

But couldn’t it be dangerous to bring the microbial life forms of the past back the surface?

“We have of course considered this but it isn’t a worry,” says Jørgensen.

“What is special about the microorganisms that grow down there is that they grow and procreate incredibly slowly. These are qualities that mean that they do not stand a chance up here on the surface. Especially not in any way that could constitute a health or safety risk,” he says.

Jørgensen points out that the oil companies are already drilling their way into the subsurface without it raising any concerns.

“They drill for oil in the same layers and the amount of material the oil industry extracts from the deep subsurface by far exceed the amounts we can access through our drillings,” he says. “The oil industry doesn’t regard the microorganisms from the deep as a threat to the marine environment at the surface,” says Jørgensen.

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Read the story in Danish on Videnskab.dk

Translated by: Iben Gøtzsche Thiele

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