Life in the Crust: The Earth’s Biggest Ecosystem and Its Implications
Deep in the crust, in rock under the seafloor, exists what scientists say could be the biggest ecosystem on Earth in terms of geographic area. In this complete darkness and intense pressure live organisms that we are only beginning to understand. Hydrogen, iron, sulfur, and carbon substitute for light and oxygen. Instead of photosynthesis, these organisms turn minerals into energy through chemosynthesis.
“Working on the seafloor is always so exciting because every time you’re on an expedition you’re bound to find something completely new: whole new organisms, whole new environments,” commented Martin Fisk, a biogeochemist at OSU. “There is most likely multicellular life. It’s possible that there are even some sort of roundworms or flatworms or very simple organisms that might be grazing on bacteria on the seafloor.”
Fisk has worked for a long time on identifying features left behind by bacteria that live in oceanic crust. The bacteria create tunnels as they extract nutrients from the rock. Some of these tracks have been found in the oldest rocks on Earth: 3.5 billion years old.
Megan Drinnan, a graduate student of geology at OSU, is studying the conditions in which these tunnels form. In the crust, one of the most important factors for the existence of life is warmth (but not too much of it). Organisms also need water, and an energy source such as that caused by seawater’s reaction with certain compounds. In volcanic glass where seawater has altered the structure of the rock, the microbes are almost always found.
“It’s definitely very common as long as you have volcanic glass with alteration,” said Drinnan.
Drinnan studies palagonite in particular. It’s an alteration of a volcanic glass similar to basalt, and a substance close to what is believed to constitute the rocky surface of Mars. Found approximately 100 to 1,000 meters beneath the seafloor, the samples she’s cultivated provide a window into a different universe.
“When I’m looking down the microscope, you can see demonstrations of behavior. They go near one thing or another, they’ll follow little pathways—it’s like you’re looking down into a little world,” she said. “It’s amazing to think it’s rock—they’re living off of volcanic rock.”
Another OSU graduate student, Amy Smith, is studying the organisms that live deep beneath the seafloor. So far, she’s found bacteria and fungi in samples from the Juan de Fuca Ridge, an underwater mountain range south of Vancouver Island. Smith is trying to grow her microbes on different minerals to see if they will release magnesium or iron, which would reveal whether they contribute to geochemical cycling of these elements.
“I’m also looking at whole microbial communities that colonize different minerals that are common in the crust,” she said.
Smith has found two of the three domains of life—bacteria and archaea—in her samples.
“We think there are some fungi down there,” she added. “A lot of people had been thinking it was contamination, but now we’re starting to believe that they’re just really there. It’s pretty common all over the world—it’s kind of cool that we found it down there, too.”
Smith noted that the rock under the seafloor isn’t as inhospitable as it first sounds. A blanketing layer of sediment helps retain geothermal heat, creating a warm environment where life can thrive.
Some of the conditions for life under the seafloor are also present on other planets. One of the most abundant minerals in the crust, olivine, is common on Mars.
“It’s a potential source of energy for life on other planets,” noted Smith. “A lot of the microbes we’re looking at grow on that mineral.”
Fisk and his students are waiting eagerly to hear the results of drilling into the surface of Mars using the Curiosity rover—a project in which Fisk is personally involved. Tests have also been conducted on Mars’ sand. Water, one requirement for life, existed in liquid form on Mars’ surface in the past. Scientists haven’t found organic matter yet, but, like on Earth, organisms could survive underneath the surface.
“I think that’s quite possible,” said Fisk. “Life on the surface here is protected by ozone that prevents a lot of high-energy radiation like ultraviolet from destroying the organic matter. Mars doesn’t have that and neither does Europa, so things on the surface would basically be exposed to high-energy radiation all the time. I think without a protective layer or water or oxygen, life’s going to have to be under the surface.”
Life could have evolved first on Earth or on Mars, and been transferred in the past from one planet to another. A second genesis is also possible, in which life evolved independently on both Earth and on Mars. Fisk ended with an even more mind-bending thought: What if life evolved twice on Earth, once on the surface and once under the seafloor?
“What if there is a second genesis on Earth and we just haven’t seen it yet?” he postulated. “It could be co-existing with the life that we know and we just haven’t been able to discover it yet.”
Discovering lifeforms that evolved independently on Earth would be staggering—the finding of alien life under our own seafloor.