The radioactive splitting of water molecules could yield enough energy to fuel a large portion of the deep subsurface biome.
Scientists poke and prod at the fringes of habitability in pursuit of life’s limits. To that end, they have tunneled kilometers below Earth’s surface, drilling outward from the bottoms of mine shafts and sinking boreholes deep into ocean sediments. To their surprise, “life was everywhere that we looked,” said Tori Hoehler, a chemist and astrobiologist at NASA’s Ames Research Center. And it was present in staggering quantities: By various estimates, the inhabited subsurface realm has twice the volume of the oceans and holds on the order of 1030 cells, making it one of the biggest habitats on the planet, as well as one of the oldest and most diverse.
Researchers are still trying to understand how most of the life down there survives. Sunlight for photosynthesis cannot reach such depths, and the meager amount of organic carbon food that does is often quickly exhausted. Unlike communities of organisms that dwell near hydrothermal vents on the seafloor or within continental regions warmed by volcanic activity, ecosystems here generally can’t rely on the high-temperature processes that support some subsurface life independent of photosynthesis; these microbes must hang on in deep cold and darkness.
Two papers appearing in February by different research groups now seem to have solved some of this mystery for cells beneath the continents and in deep marine sediments. They find evidence that, much as the sun’s nuclear fusion reactions provide energy to the surface world, a different kind of nuclear process — radioactive decay — can sustain life deep below the surface. Radiation from unstable atoms in rocks can split water molecules into hydrogen and chemically reactive peroxides and radicals; some cells can use the hydrogen as fuel directly, while the remaining products turn minerals and other surrounding compounds into additional energy sources.
Although these radiolytic reactions yield energy far more slowly than the sun and underground thermal processes, the researchers have shown that they are fast enough to be key drivers of microbial activity in a broad range of settings — and that they are responsible for a diverse pool of organic molecules and other chemicals important to life. According to Jack Mustard, a planetary geologist at Brown University who was not involved in the new work, the radiolysis explanation has “opened up whole new vistas” into what life could look like, how it might have emerged on an early Earth, and where else in the universe it might one day be found.
Hydrogen Down Deep
Barbara Sherwood Lollar set off for university in 1981, four years after the discovery of life at the hydrothermal vents. As the child of two teachers who “fed me on a steady diet of Jules Verne,” she said, “all of this really spoke to the kid in me.” Not only was studying the deep subsurface a way to “understand a part of the planet that had never been seen before, a kind of life that we didn’t understand yet,” but it “clearly was going to trample [the] boundaries” between chemistry, biology, physics and geology, allowing scientists to combine those fields in new and intriguing ways.
Throughout Sherwood Lollar’s training in the 1980s and her early career as a geologist at the University of Toronto in the ’90s, more and more subterranean microbial communities were uncovered. The enigma of what supported this life prompted some researchers to propose that there might be “a deep hydrogen-triggered biosphere” full of cells using hydrogen gas as an energy source. (Microbes found in deep subsurface samples were often enriched with genes for enzymes that could derive energy from hydrogen.) Many geological processes could plausibly produce that hydrogen, but the best-studied ones occurred only at high temperatures and pressures. These included interactions between volcanic gases, the breakdown of particular minerals in the presence of water, and serpentinization — the chemical alteration of certain kinds of crustal rock through reactions with water.
By the early 2000s, Sherwood Lollar, Li-Hung Lin (now at National Taiwan University), Tullis Onstott of Princeton University and their colleagues were finding high concentrations of hydrogen — “in some cases, stunningly high,” Sherwood Lollar said — in water isolated from deep beneath the South African and Canadian crust. But serpentinization couldn’t explain it: The kinds of minerals needed often weren’t present. Nor did the other processes seem likely, because of the absence of recent volcanic activity and magma flows.
“So we began to look and expand our understanding of hydrogen-producing reactions and their relationship to the chemistry and mineralogy of the rocks in these places,” Sherwood Lollar said.
A clue came from their discovery that the water trapped in those rocky places held not just large amounts of hydrogen but also helium — an indicator that particles from the radioactive decay of elements like uranium and thorium were splitting water molecules. That process, water radiolysis, was first observed in Marie Curie’s laboratory at the beginning of the 20th century, when researchers realized that solutions of radium salts generated bubbles of hydrogen and oxygen. Curie called it “an electrolysis without electrodes.” (It took a few more years for scientists to realize that the oxygen came from hydrogen peroxide created during the process.)
Sherwood Lollar, Lin, Onstott and their collaborators proposed in 2006 that the microbial communities under South Africa and Canada derived the energy for their survival from hydrogen produced through radiolysis. So began their long quest to unpack how important radiolysis might be to life in natural settings.
‘A Completely Self-Sustained System’
For much of the next decade, the researchers obtained samples from deep aquifers at various mining sites and related the complex chemistries of the fluids to their geological surroundings. Some of the water trapped beneath the Canadian crust had been isolated from the surface for more than 1 billion years — perhaps even for 2 billion. Within that water were bacteria, still very much alive.
“That had to be a completely self-sustained system,” Mustard observed. By the process of elimination, radiolysis looked like a possible energy source, but could there be enough of it to support life?
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