As reported on Wired.
BY BRANDON KEIM
A sample of oceanic crust basalt (left) and a microscopic cross-section (right) denoting changes of concentration in sulfur, an element used by microbes there. Image: Spencer et al./Science
Deep beneath the ocean floor off the Pacific Northwest coast, scientists have described the existence of a potentially vast realm of life, one almost completely disconnected from the world above.
Persisting in microscopic cracks in the basalt rocks of Earth’s oceanic crust is a complex microbial ecosystem fueled entirely by chemical reactions with rocks and seawater, rather than sunlight or the organic byproducts of light-harvesting terrestrial and aquatic ecosystems.
Such modes of life, technically known as chemosynthetic, are not unprecedented, having also been found deep in mine shafts and around seafloor hydrothermal vents. Never before, though, have they been found on so vast a scale. In pure geographical area, these oceanic crust systems may contain the largest ecosystem on Earth.
“We know that Earth’s oceanic crust accounts for 60 percent of Earth’s surface, and on average is four miles thick,” said geomicrobiologist Mark Lever of Denmark’s Aarhuis University, part of a research team that describes the new systems March 14 in Science.
If what the researchers found resembles what’s found elsewhere below Earth’s oceans, continued Lever, “the largest ecosystem on Earth, by volume, is supported by chemosynthesis.”
The paper represents the culmination of findings that have gathered over the last two decades, starting in the 1990s with the discovery of strange microscopic holes in the basalt rocks that form much of Earth’s outer crust, floating above the planet’s viscous upper mantle and below seafloor sediments.
The holes looked as though they were made by bacterial activity, but there wasn’t supposed to be any life there. The crust isn’t just hot, deep, dark and dense, but mostly devoid of the organic compounds, supplied by plants and plankton and other sunlight-fueled organisms, on which life relies elsewhere.
In coming years, researchers noted that oceanic crusts, which form when rock heated by Earth’s core pours slowly through mid-ocean cracks between continental plates, differed greatly between the centers and edges. At the centers, near where they form, rocks are suffused with energy-rich compounds that support microbes. At the edges, where rocks arrive millions of years later, the chemicals are gone. It’s like they’ve been eaten.
Other researchers found DNA traces of microbes in the oceanic crusts, further making the case for life, but just what the microbes were doing remained uncertain.
“All these pieces of evidence have been coming together for over 15 years. It was time to put it all together,” said microbial ecologist Andreas Teske of the University of North Carolina, a co-author of the new study. “We now have the best available evidence that there is in fact microbial life in the cracks and fissures of deep ocean basalt. The question is, how far does it extend?”
Teske and Lever’s team collected samples of crust from the Juan de Fuca Plate, about 120 miles off the coast of Washington, drilling from boreholes made by other researchers some 1.5 miles below the ocean’s surface and beneath another 1,000 feet of sediment.
At that depth, there exists rock and water and carbon dioxide, and few if any traces of organic matter produced from sunlight in the illuminated surface world. The researchers put their rocks in a laboratory apparatus designed to simulate that environment, then spent the next seven years observing what happened.
They measured chemical ebbs and flows, slowly learning the system’s elemental cycles. Though the microbe populations didn’t grow at the densities necessary to convenient find them under a microscope, the researchers scoured their rocks for microbial DNA, identifying sequences that could be compared to known genes. Out of this emerged a picture of the oceanic crust community and how it lives.
Fundamental to the ecosystem is hydrogen, which in the absence of sunlight provides the energy on which all other biological processes rely. The hydrogen is released by reactions between iron- and sulfur-rich rocks and seawater, then used by microbes to fuel their conversion of carbon dioxide into organic matter.
That matter, along with metabolic byproducts like methane, would support other organisms, ultimately creating a web of life. That web is relatively simple compared to sunlight-based ecosystems, said Teske, and it’s unlikely that multicellular life will be found there, as it’s too hot and energy-poor compared to the places where higher life is found.
The work “confirms that there are subsurface environments that can support life without using oxygen,” said Martin Fisk, a biogeochemist at Oregon State University who also studies oceanic crust microbiology at the Juan de Fuca plate, but was not involved in the new research.
Biogeochemist Everett Shock of Arizona State University, also not involved in the research, isn’t yet ready to rule out multicellular life. “My bet is on fungi,” he said, “but there are other possibilities, including things that may be quite unfamiliar.”
Continued Shock, “As for invertebrate and vertebrates, much depends on their size and the sizes of interconnected pore spaces in the rocks. I’m not ready to rule out such possibilities. Our ignorance about these systems is staggering, and accessing them is not at all easy.”
Even if multicellular life isn’t to be found in oceanic crusts, the presence of any life there is still extraordinary. Lever emphasized just how disconnected it is from the rest of Earth’s life processes, a sort of “parallel universe” linked to ours only by seawater.
A map of Earth’s seafloor crust (color-coded by age; red is young, blue is old) conveys its vast size. Image: NASA/Wikimedia Commons
Despite that tenuous link, said Lever, it’s likely that over geological time “those processes happening in the crust have a profound chemical influence on the composition of our oceans and atmosphere.”
Another avenue of speculation involves the origins of life, which some scientists think might be traced to oceanic crusts. If simple interaction between seawater and rock provides the necessities, then Earth’s early environments were quite conducive to life.
“The emphasis on common processes is appealing,” Shock said. “It moves attention away from special circumstances, like spark discharges in implausible atmospheres, or conditions that may once have prevailed but no longer do.”
Lever mused on the possibility that primeval chemical systems with a tendency to replicate themselves, not quite alive yet something more than inanimate, might have accreted around hydrogen- and sulfur-generating processes in the oceanic crust.
“What’s proposed is that before there was life, there was this organic matter-producing chemical reaction going on,” said Lever. Life might have originated around the iron and sulfur compounds fueling that reaction, evolving to produce biomass and harvest energy.
Such ideas are speculative, emphasized Lever, and Teske preferred to think about the implications for life elsewhere. “What I find interesting here is not so much the origin of life, but the persistence of life,” he said.
“As long as there’s space for microbes, and biochemistry, life persists,” Teske continued. “Deep subsurfaces could be the best hiding place for life on other planets, where surface conditions are too harsh but the right chemical conditions are available below.”
Back on Earth, a more immediate implication of the findings is the possiblity that a large fraction of Earth’s life exists in oceanic crusts, not in ocean water or on land.
“We need to stretch our brains to consider that there should be much to discover, and much that will be unfamiliar,” said Shock.
Citation: “Evidence for Microbial Carbon and Sulfur Cycling in Deeply Buried Ridge Flank Basalt.” By Mark A. Lever, Olivier Rouxel, Jeffrey C. Alt, Nobumichi Shimizu, Shuhei Ono, Rosalind M. Coggon, Wayne C. Shanks III, Laura Lapham, Marcus Elvert, Xavier Prieto-Mollar, Kai-Uwe Hinrichs, Fumio Inagaki, Andreas Teske. Science, Vol. 339, 15 March 2013.