Mummified Microbes

Scientists have found evidence that microbes can thrive deep below the seafloor—sustained by chemicals produced by reactions between seawater and rocks in Earth’s mantle.

 It’s difficult to gain direct access to the mantle, but a team led by Woods Hole Oceanographic Institution scientist Frieder Klein analyzed samples of ancient mantle rocks that had been thrust up from Earth’s interior toward the seafloor. The scientists found lipids, amino acids, and proteins of mummified microbes that were preserved and encased in tiny pockets in the rocks.

Fueling this life in the mantle—then and presumably still—is a confluence of chemical conditions that begins when seawater percolates through fractures in the seafloor. The water gets heated along the way. When it encounters a type of rock in Earth’s mantle called peridotite, it reacts with minerals in peridotite and generates hydrogen, methane, and other gases. The seawater is chemically altered into hot hydrothermal fluids, which rise buoyantly toward the surface. When hydrothermal fluids mix again with seawater, together they provide a banquet of energy- and nutrient-supplying chemical compounds for microbes such as bacteria and archaea.

“All the chemical ingredients necessary to support life and drive these ecosystems came from inorganic materials: seawater and rock,” Klein said. “Colonies of bacteria and archaea feeding off these chemicals became engulfed in the minerals in the fractured rock. This kept them completely isolated from the environment. The minerals proved to be the ultimate storage containers for these organisms, preserving their lipids and proteins for more than 100 million years.”

The mantle rocks analyzed by the scientists were extracted by a ship that drilled more than 2,265 feet below the seafloor off the coasts of Spain and Portugal, where 125 million years ago, large rifts began to split apart the massive supercontinent known as Pangaea and form the Atlantic Ocean. The rifting pulled mantle rocks up toward the seafloor, exposing them to seawater that drives hydrogen-generating reactions.

“Similar rock-powered ecosystems existed throughout most of Earth’s history to the present day, and they possibly exist on other water-bearing rocky planetary bodies, such as the icy moons of Jupiter,” Klein said.

The findings raise new ideas about how life on Earth and other planetary bodies originated—and could still exist today. By extracting DNA from ancient microbial samples, Klein said, scientists can also learn more about how life evolved on Earth. 

This research was published August 2015 in the Proceedings of the National Academy of Sciences by Frieder Klein, Susan Humphris, Weifu Guo, and William Orsi (WHOI), Esther Schwarzenbach (Virginia Tech), and Florence Schubotz (University of Bremen). It was funded by the Ocean Exploration Institute at WHOI.