Microbes Deep Beneath The Ocean Floor

Radiolysis, anyone?:

Last month, a team of scientists in Japan reported life in basalts collected by a 2010 drilling expedition that had reached 120 meters into the crust beneath the ocean, to rocks that were from 33 million to 104 million years old. The age of the rocks alone was exciting, according to Lloyd, who did not participate in the study. “When it formed, dinosaurs were walking the Earth,” she says.

But even more tantalizing were the densities at which the microbes were growing in the rocks. The researchers, using a new technique to precisely enumerate the cells in their samples, found that the microbes were concentrated in a particular subset of the basalt’s mineral-filled fissures. There, they formed something resembling a biofilm, reaching counts of 10 billion cells per cubic centimeter. “[We] didn’t think that there [would be] so many cells in old, cold, hard rocks,” Yohey Suzuki, a geoscientist at the University of Tokyo and the lead author of the study, says.

The researchers were also able to map which types of minerals the cells did and did not associate with. They posit that the bacteria survived by living off of organic matter trapped in those minerals.

...They’ve also found evidence that those microbes persist by getting energy from an abiotic process called radiolysis, during which radiation released by the rocks reacts with water in the system to release hydrogen, which the cells can then use in various forms as fuel. That’s posed an intriguing question for scientists: Could radiolysis be an alternative process driving much of subsurface life?

Given that radiolysis occurs everywhere, “it could also be supporting the ocean deep life,” Orsi, from the University of Munich, says. “No one knows.”

...That is, it’s possible that life might have gotten its start on the surface of the Earth, where it found creative ways to survive and spread, including to deeper environments. But it’s also possible that life began underground, at some fortuitous juncture of rock and water—eventually also making its way to the surface and figuring out how to harness the sun’s energy. (On that note, photosynthesis-dependent surface life and radiolysis-dependent subsurface life have so far been found to have an ancient, shared ancestry—but some researchers are intrigued by the possibility that life could have evolved more than once on Earth, in a “second genesis.”)

That, in turn, has significant implications for the search for life on Mars, Saturn’s moon Enceladus, and exoplanets beyond our solar system. Given the prevalence of water and volcanic rock throughout the universe, “life could have started anywhere,” Fisk says.