Life Really Can Overcome Anything: How Bacteria Survive 250 Meters Under the Dead Sea

Scientists discovered new life in one of the most extreme environments in the world: Bacteria living on dead ancient microorganisms – which may have implications for finding life on Mars

The Dead Sea, from the Jordanian side
Manuel Cohen, AFP

New research has shown, yet again, the truth behind the old cliché that life overcomes all obstacles. Even under the most extreme conditions on Earth, and not just on its surface, organisms find a way to live, and even flourish.

A paper recently published in the scientific journal Geology, put out by the Geological Society of America, describes how micro-organisms can flourish in the most unlikely place: Deep below the Dead Sea, without any carbon or oxygen.

The study is based on findings from boreholes dug into the deep sediments underneath the Dead Sea, and discovered evidence of single celled organisms in all levels of the earth, going as far as down as 250 meters below seafloor. The organisms have found a way to live in micro-bubbles of water trapped in salt layer sediments that settled at the bottom of the Dead Sea.

In an extremely saline biosphere without readily available sources of oxygen and carbon organisms are able to survive because of their “necrophagic” behavior: They consume and recycle the remnants of microorganisms that lived in the Dead Sea and also settled out with the salt in the sediments.

“This is a sensational scientific discovery,” says Professor Moti Stein of the Institute of Earth Sciences at the Hebrew University of Jerusalem – who was not involved in the research. "This is one of the most fascinating pieces of geological research conducted in Israel in recent years,” he said.

The discovery offers new understandings on the development of early life on Earth – and about the possibility of finding similar microscopic life underneath the surface of Mars.

In addition to being the lowest place on earth, the Dead Sea has the world’s highest saline levels – 10 times saltier than the oceans. Despite the sea’s ominous name, a number of life forms have learned to adapt in these extreme conditions.

These microorganisms belong to the domain of Archaea – ancient single-celled organisms which were originally thought to be a type of bacteria, but differ from them. Archaea have been found in a number of extreme environments on Earth, though they can also be found in non-extreme environments. In the case of the Dead Sea, technically a salt lake, these organisms are known as halophiles, “salt-loving” in Greek.

Stein says that since the halophilic archaea were discovered, a number of research groups have begun reexamining the Dead Sea’s ecological system. The halophilic archaea have been found in the lake as well as in the fresh sediment layers underneath it. But until now, very little evidence had been found that such archaea – along with the heterotrophic halophilic bacteria – could survive in the harsh conditions deep below the surface.

Research into the Dead Sea lifeforms has been part of broader studies of the unique geological region, which examine various aspects of its history, such as the tectonic development of the lake basin, past earthquakes and the region’s climate and hydrological history.

Stein says that when the salts settle and sinks into the seafloor, the newly created layers trap tiny bubbles of water that preserve the composition of the seawater at the time they were trapped. As a result, boreholes drilled into the subsurface of the sea provide scientists with “time capsules” from the Dead Sea’s past, tens and hundreds of thousands of years ago. This allows scientists to reconstruct the climate during those prehistoric periods.

In this case, scientists from the department of Earth sciences at the University of Geneva, headed by Prof. Camille Thomas, used this method for a different purpose: To look for signs of life, while being extremely careful not to contaminate the samples with external substances.

Previous studies showed how archaea could live in the seafloor sediment layers, but the most recent research revealed a much more surprising result. This time, they found unique molecules used by the organisms to produce and store food in conditions with very limited sources of energy.

These molecules, known as wax esters, were found in layers 120,000 years old. The archaea are not capable of producing these molecules, and since it is incredibly unlikely that multicellular lifeforms could have survived under such extreme conditions, the scientists concluded that these molecular food storage reserves were created by bacteria.

Given that this biosphere, so far under the Dead Sea, was thought to be too extreme to support bacterial life, how did the bacteria manage to survive?

Recycling the dead

An analysis of the wax ester’s chemical composition suggests a possible solution. These bacteria recycled the “necromass,” the accumulation of materials from dead salt-loving archaea deep in the underground sediments, and made the food they needed to survive from the archaea’s cell membranes.

If this is true – as the scientists have theorized – the bacteria living deep under the Dead Sea have developed a necrophagic (from the Greek: eating the dead) lifestyle that enables them to create the carbon stores they need to survive. It even allows them to produce some water, slightly reducing the salt concentration in the bubbles they live in. This is the first time the researchers have managed to identify such a method of maintaining life in bacteria.

In a separate study on life underneath the Dead Sea conducted by Thomas and Yael Ebert, the manager of paleo-magnetic lab of the Institute of Earth Sciences at Hebrew University, examined the magnetic characteristics of the sediments, which were influenced by the minerals created as a result of the bacterial activity.

“The magnetic measurements provide evidence of the bacterial activity easily and with high resolution,” says Ebert. “This is an especially slow mechanism. Its pace explains why, until now, scientists have not been able to see microorganisms in the laboratory that exist in such extreme conditions as those found at the Dead Sea.”

The scientists don’t explicitly state in their paper that bacterial life continues deep under the sea, but the presence of the characteristic “rotten eggs” smell of the fresher soil samples provides a certain amount of evidence of bacterial life today. The hydrogen sulfide that causes the smell is often created by bacterial activity. However, hydrogen sulfide can also be made by inorganic geologic processes, so this should not be taken as unambiguous proof.

In any case, this research reveals a possibility that there is an enormous biomass that maintains life deep underground, in ways that were previously unknown to science.

The research’s implications in the search for life on Mars are no less interesting. In 2011, NASA’s Opportunity rover seemingly found a mineral called gypsum on Mars. This is the same mineral that Thomas found in the Dead Sea sediments. The presence of gypsum indicates that the red planet warmed up at some point in the past, and its lakes and oceans evaporated. If so, before the surface water disappeared completely, what remained would have been very similar to the composition of the Dead Sea. And maybe, similar biological processes took place there too.

To examine such possibilities, the European Space Agency is planning on landing the ExoMars explorer on the remains of one of Mars’ former ancient ocean in 2021. The rover is planned to study the soil cores from drilling into the dry ocean bed, in a much simpler version of the techniques used by Thomas and Ebert to study the magnetism of the Dead Sea.