How Low Can Life Go? New Study Suggests Six Miles Down

Evidence of life from below a mud volcano hints at life beneath the crust

A serpentinite sample Oliver Plümper/Utrecht University

There’s plenty of life on Earth's surface and deep in the oceans—so many species that researchers think they have barely scratched the surface naming all the plants and animals that make up the biosphere. Now, new research suggests there could be much more. As Chelsea Whyte reports for New Scientist new evidence hints at a “deep biosphere” locked miles below Earth’s surface.

The study, published in The Proceedings of the National Academies of Sciencedocuments the results of a deep-sea drilling expedition at the South Chamorro mud volcano. This massive underwater mountain sits near Challenger Deep in the Mariana Trench, the deepest spot in the world’s ocean, and is fueled by the tectonic activity that grinds below as the Pacific plate slips beneath the Philippine Sea plate. 

The researchers collected rock samples from between 46 and 360 feet below the surface, discovering evidence that they think hints to the possibility of life. Though the samples were from a shallow depth, the researchers believe that they likely originated much deeper in the earth and could have been belched up by the mud volcano. 

As Claudia Geib reports for National Geographic, the researchers identified fragments of serpentine, a type of mineral that forms deep in ocean subduction zones under high heat and pressure. But the process also produces hydrogen and methane, gases that microbes could munch on.

While the researchers did not find actual microbes in the 46 samples they collected, they did find traces of organic materials including amino acids and hydrocarbons. These compounds are microbial waste products, and their discovery hints at life down below.

But how deep?

"It is impossible to say from what depths the clasts come from exactly," Oliver Plümper, an author of the study from Utrecht University in the Netherlands, writes in an email to Researchers believe that the serpentine found in the clasts could form at depths of up to 12.5 miles down, where the subducting slab lies below the volcano. But there are many other factors that influence how deep life can form. One important one is temperature.

So Plümper and his team then tried to calculate how deep these microbes could survive based on their known temperature tolerance. Based on the known temperature limit for life, which Geib reports is some 250 degrees Fahrenheit, the researchers discovered that life could survive up to six miles below the planet’s surface.

“This is another hint at a great, deep biosphere on our planet,” Plümper tells Geib. “It could be huge or very small, but there is definitely something going on that we don’t understand yet.”

The organic elements found in the serpentine could also have been made by other processes, however, and not everyone is ready to say they represent life. “These organic molecules definitely hint toward life, but the source of that life, as the authors admit, is not clear yet,” Frieder Klein, who studies serpentinization at the Woods Hole Oceanographic Institute tells Geib. Still, he says, the findings are “truly remarkable.”

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