Surprising quantities of bacteria have been found living in clay-rich rocks under the Pacific seafloor. The discovery raises the possibility of equally resilient microbes living deep beneath the surface of Mars.
Ancient volcanic rocks beneath the Pacific seafloor provide a habitat for dense clusters of microbial communities, according to a new paper published this week in Communications Biology. The microbes were found as deep as 100 metres (330 feet) beneath the seafloor, huddling within the cracks of ancient, clay-rich volcanic rocks.
“Life will find a home in virtually every crevice that’s available,” explained Jennifer Biddle, an associate professor at the University of Delaware who was not involved with the new research, in a phone chat with Gizmodo. “Previous papers over the past 20 years have hinted at this possibility – that lots of microbes can exist within the cracks of ocean crusts – but this paper is nice in that it’s actually the first evidence of active microbes within these particular rocks, and not just their fossilized remains.”
A volcanic rock sample on a slide. Image: Caitlin Devor, University of Tokyo, CC BY 4.0
Subterranean microbes have been found before. Back in 2006, for example, scientists detected bacteria nearly 2 miles beneath Earth’s surface, and in 2018, scientists found photosynthesising bacteria thousands of feet below the surface of an abandoned mine in Spain. And late last month, a paper was published describing microbial communities living some 792 metres (2,600 feet) beneath the seafloor.
What makes the new discovery special is that the microbes were alive, and they were pulled from ancient volcanic rocks ranging in age from 13.5 million to 104 million years old. Whether active microorganisms might be found in these rocks was an unanswered question going into the research, but the new paper suggests they are thriving in their extreme habitat.
Magnified images of the rock samples, in which (b) is 1,000 times more magnified than (a). The green splotches to the right of each show the bacteria, as illuminated by fluorescent light. Grey areas are clay, and orange areas are solid basalt (volcanic) rock. Image: Suzuki et al. 2020, DOI: 10.1038/s42003-020-0860-1, CC BY 4.0
First author Yohey Suzuki from the University of Tokyo in Japan collected the samples back 2010. The researchers selected three different spots at the bottom the Pacific ocean between Tahiti and New Zealand. To rule out contaminating sources, the scientists selected areas far from hydrothermal vents.
A metal tube stretching 5.7 kilometres in length (3.5 miles) was lowered to the ocean floor, and a drill burrowed down an additional 125 metres (410 feet) into the seafloor. For the first 75 metres of the journey, the drill worked its way through thick mud, followed by another 40 metres (130 feet) through solid rock.
Once at the lab, the samples revealed traces of the bacteria inside the cracks of the volcanic rock. These tiny fractures, at less than 1 millimetre wide, formed when the hot lava was cooling.
Suzuki found the bacteria in surprising densities, with colonies reaching 10 billion bacterial cells for each cubic centimetre (0.06 cubic inches), which is roughly the same quantity found in the human gut. The seafloor itself yielded just 100 cells per cubic centimetre.
“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.”
The bacteria is likely drawn to the clay-rich minerals, which provide a suitable habitat for the aerobic microbes that feed off of oxygen and organic nutrients. And as Biddle told Gizmodo, “clay has a charged surface, making it easier for the bacteria to stick to.” Using DNA analysis, the researchers identified distinct but similar aerobic species of bacteria pulled from the different locations.
“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 in a press release. “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,” he added.
Suzuki and his colleagues will soon be collaborating with NASA as they work on a plan to return Martian rocks to Earth for analysis. The pending Perseverance rover, which launches later this summer (fingers crossed), is designed explicitly for this purpose. The rover will collect materials and deposit them on the Martian surface for a future sample return mission.
Suzuki says the clay minerals found beneath the Pacific seafloor could be similar to minerals on Mars.
“Minerals are like a fingerprint for what conditions were present when the clay formed,” he said. “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.”
Biddle, however, was circumspect about the new paper’s astrobiological implications.
“Can life be more robust? Sure, but I’d be cautious with these sorts of interpretations,” she told Gizmodo. “Mars might be missing a complete biogeochemical cycle for all elements required for life,” she explained, adding that the new paper is “not a slam dunk” for showing that life is certain to exist elsewhere in the solar system. “Other things need to be present to allow life to survive,” she said.
With a pending Mars sample return mission in the works, we could soon put this theory to the test. That said, we likely won’t have satisfactory answers until we can drill deep into the Martian crust, which Perseverance won’t be able to do. For now, we’ll have to be content with these fascinating speculations.
Featured image: NASA/JPL-Caltech