Bacteria beneath the ocean floor inspire hopes for similar findings on Mars

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In a finding profoundly distinct to those covered by the news in these trying times, a group of researchers from Tokyo University have discovered a dense population of microbes in fractures found in sub-seafloor volcanic rock, a similar environment to areas of Mars. 

Bacterial density below the surface was eight orders of magnitude greater than on the seafloor

The aerobic bacteria were found in basaltic rock samples collected in 2010 from the Integrated Ocean Drilling Program, a study performed at three locations across the South Pacific Gyre. A 5.7 kilometre-long metal tube was extended to the seafloor, followed by a drill that cut 6 centimetre-wide cores from 125 metres below this point, past mud sediment and into solid rock. Cores from the three locations ranged between 13.5 to 104 million years old, and were found in areas absent of hydrothermal vents or sub-seafloor channels, suggesting the bacteria formed naturally inside the rock fissures.

The study, published in ‘Communications Biology’ on April 2nd, 2020, found that the bacterial community found in these rock cracks below the ocean floor is as dense as the human gut microbiome, with 10 billion bacteria per cubic centimetre. On the seafloor, only a couple of hundred metres above, the bacterial density is 100 cells per cubic centimetre; 100 million times lower than the volcanic rock.

Clay minerals are “like a magic material on Earth”

Lava from undersea volcanoes spews out at around 1,200Cº, cooling to form rocks filled with narrow cracks, often less than 1 millimetre across. On a timescale spanning millions of years, these cracks fill with clay minerals, described by Professor Yohey Suzuki, the first author of the research paper, as “like a magic material on Earth”. Bacteria that find themselves in the clay rapidly multiply, forming large colonies; in the words of Suzuki: “if you can find clay minerals, you can almost always find microbes living in them”. The study’s authors suggest that the clay mineral-filled cracks concentrate nutrients, providing an attractive spot for the bacteria to grow. This also supplies an explanation for bacterial density in these cracks being eight orders of magnitude greater than on the water-diluted seafloor.

Incredibly, this discovery was extremely close to being missed

Incredibly, this discovery was extremely close to being missed due to the methods used to find bacteria back when the cores were originally obtained. Researchers used to chip at the outer layers of rock and then grind the centre into a powder, from which cells could be counted. Discouragingly, this method yielded no findings of bacteria, even from the same core samples used in the 2020 study. As a last-ditch resort, Suzuki tried coating the rocks in epoxy prior to slicing so that they resisted crumbling when cut – a procedure inspired by pathologists producing ultrathin slices of body tissue samples. Suzuki’s thin sheets of rock were then stained for DNA, revealing the astounding numbers of bacteria in bright green surrounded by orange clay.

The “magical” clay minerals serving as a home for the multitudes of extremophile bacteria on Earth’s ocean floor are likely to be similar to minerals found in Martian rocks. “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.

This incredible finding is only the most recent in a string of similar discoveries, to the point where it seems like Mother Earth has practically been showing off with the insanely harsh environments we’ve discovered life to comfortably exist in. From tube worms on hydrothermal vents, to thermophilic bacteria in Yellowstone Park, even to tardigrades surviving the vacuum of space for ten days, life continues to challenge our expectations of life’s extreme boundaries. In Suzuki’s own words: “I am now almost over-expecting that I can find life on Mars”.

This may not be just a pipe dream. Suzuki’s research team is now collaborating with NASA’s Johnson Space Centre to implement similar methods of finding bacteria in rock cores collected from the Martian surface by rovers such as NASA’s ‘Perseverance’, set to launch in July, and the Rosalind Franklin rover, taking off in 2022. All in all, it seems like it’s only a matter of time before someone spots those tell-tale traces of life on Martian material, a discovery set to vastly alter our outlook on the Universe.

Image: Mariagat Mariagat via Flickr

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