On Earth circa four billion years ago, life was hard. Frequent asteroid strikes turned parts of the planet into molten rock. Food and livable spaces were few and far between. What was a microbe to do to survive?
Some very early life could have made it by staying deep—living as far as six miles below the seafloor.
That’s the implication from a new study that found signs of microbes alive today below the deepest place on Earth, the vast underwater canyon called the Mariana Trench. (Also see pictures that reveal one of the last unexplored places on Earth near the Mariana Trench.)
The trench is part of a subduction zone, where the Pacific tectonic plate slips beneath the Philippine Sea plate. The surrounding seafloor is littered with hydrothermal vents and mud volcanoes, churning out ingredients from the deep Earth.
In the new study, published today in the Proceedings of the National Academy of Sciences, researchers sampled mineral-rich mud from the South Chamorro seamount, a mud volcano near the Mariana Trench fueled by the subduction zone below it. Though the team did not find intact microbes, they did discover tantalizing traces of organic material, which may add to evidence that life can survive in the most extreme of environments.
“This is another hint at a great, deep biosphere on our planet,” says study leader Oliver Plümper, a researcher at the Netherlands’ Utrecht University. “It could be huge or very small, but there is definitely something going on that we don’t understand yet.”
Life may be able to survive so deep because subduction zones are relatively cool; magma doesn’t hit the sinking crust until it reaches a lower point in the mantle. As such, Plümper extrapolated that the known temperature limit of life—around 250 degrees Fahrenheit—wouldn’t come until a depth of at least six miles below the ocean floor.
That could make these microbes the deepest life known on our planet, trumping microbes found in seafloor sediment as much as three miles down.
"I think the main take-home of this paper is how this has the potential to place life at some of the deepest environments on the planet,” says Matthew Schrenk, a geomicrobiologist at Michigan State University who studies the microbial ecosystems that live off serpentinization.
“If we’re looking for the depth limits of the biosphere, this could extend it by a lot.”
Plümper’s team examined organic material found in serpentine, a class of minerals formed when olivine in the upper mantle reacts with water pushed up from within the subduction zone. The combination produces hydrogen and methane gas, which microbes can use as food.
Known as serpentinization, this process creates habitats for microbes elsewhere, including at seafloor hydrothermal vents. (See "Deepest Volcanic Sea Vents Found; 'Like Another World.'")
Now, the team thinks they may have found waste produced by gas-munching microbes from even deeper realms. Lab tests found that the hydrocarbons and lipids from the mud volcanoes are highly similar to waste material produced by other bacteria. But the study team acknowledges that nothing is definitive for now.
“These organic molecules definitely hint toward life, but the source of that life, as the authors admit, is not clear yet,” says Frieder Klein, a researcher studying serpentinization at the Woods Hole Oceanographic Institute.
Outside sources of organics were a concern during the study. Among other checks, the minerals tested negative for carbonate, which would form if seawater from closer to the surface had been in contact with water within the subduction zone.
Klein called the paper’s findings “truly remarkable,” but noted there was still a chance the organic material could have come from another source, like the crust itself.
It is also possible that the organic material was produced without help from biology at all, in a natural version of the process humans use to make synthetic oil and fuel. However, this alternate possibility would still be an exciting one, according to the research team.
“If it can do that, it is amazing in itself,” says Plümper, who notes that the mud volcanoes where the serpentine formed are thought to have existed when life on Earth got its start. “Then we know that geologic process can create complex organic molecules.”
ALIENS IN THE DEEP?
Once scientists started looking for serpentinization in the 1960s, they started finding it everywhere—at the places where continents crash together and the molten margins where they form, at hydrothermal vents, and even within mountain ranges that were once deep rock and ancient seafloor.
Given how common it is on our own planet, serpentinization—and its potential to support extreme life-forms—has caught the attention of those looking for life on other worlds.
“There is a direct link between this process we study on Earth and the processes that are possibly happening elsewhere in solar system,” Klein says.
Two promising candidates are Jupiter’s moon Europa and Saturn’s moon Enceladus. Both are covered in ice but are thought to have briny liquid oceans extending deep below their surfaces.
Enceladus has also shown some hints of tectonic activity, which is needed to create the sort of subduction zones Plümper and his team studied, thought that speculation hasn’t yet been confirmed. (See "Vast Ocean Underlies Ice on Saturn's Moon Enceladus.")
“Wherever olivine occurs on a rocky planet, serpentinization probably occurs,” says Plümper. “In the absence of photosynthesis, it could provide some material to support life.”
However, astrobiologists hoping to visit other worlds in search of deep microbial life would face the same problem scientists like Plümper contend with here on Earth: Unable to reach the depths where this life might hide, scientists must interpret the hints coming out of geysers, rocks, and other samples extracted from the deep.
“I think of it kind of like a message in the bottle,” Plümper says of his deep-sea drilling samples. “We have this container coming up, and we are opening it up and trying to figure out what’s going on.”