Scientists discover bacteria in solid rock deep beneath sea

Scientists have discovered single-celled creatures living deep beneath the seafloor -- in tiny cracks inside volcanic rocks millions of years old -- that they say can give us clues about life on Mars.

The study, published in the journal Communications Biology, estimates that the rock cracks are home to a community of bacteria as dense as that of the human gut, about 10 billion bacterial cells per cubic centimetre.

In contrast, the average density of bacteria living in mud sediment on the seafloor is estimated to be 100 cells per cubic centimetre, the researchers said.

"I am now almost over-expecting that I can find life on Mars. If not, it must be that life relies on some other process that Mars does not have, like plate tectonics," said Associate Professor Yohey Suzuki from the University of Tokyo in Japan, referring to the movement of land masses around Earth most notable for causing earthquakes.

"I thought it was a dream, seeing such rich microbial life in rocks," said Suzuki, recalling the first time he saw bacteria inside the undersea rock samples.

Undersea volcanoes spew out lava at approximately 1,200 degrees Celsius, which eventually cracks as it cools down and becomes rock, the researchers said.

The cracks are narrow, often less than one millimetre across, they said.

Over millions of years, those cracks fill up with clay minerals, the same clay used to make pottery. Somehow, bacteria find their way into those cracks and multiply.

"These cracks are a very friendly place for life. Clay minerals are like a magic material on Earth; if you can find clay minerals, you can almost always find microbes living in them," explained Suzuki.

The microbes identified in the cracks are aerobic bacteria, meaning they use a process similar to how human cells make energy, relying on oxygen and organic nutrients.

The clay minerals filling cracks in deep ocean rocks are likely similar to the minerals that may be in rocks now on the surface of Mars, the researchers said.

"Minerals are like a fingerprint for what conditions were present when the clay formed. Neutral to slightly alkaline levels, low temperature, moderate salinity, iron-rich environment, basalt rock -- all of these conditions are shared between the deep ocean and the surface of Mars," said Suzuki.

The team is beginning a collaboration with NASA's Johnson Space Center to design a plan to examine rocks collected from the Martian surface by rovers.

Ideas include keeping the samples locked in a titanium tube and using a computed tomography (CT) scanner, a type of 3D X-ray, to look for life inside clay mineral-filled cracks.

"This discovery of life where no one expected it in solid rock below the seafloor may be changing the game for the search for life in space," said Suzuki.