In Northern Ontario, researchers have found “geochemical fingerprints of life” and the energy sustaining it, in waters more than two kilometres below the surface of Earth. The discovery demonstrates how life can be sustained even in the seemingly inhospitable environments of the deep Earth crust.
Most life on Earth gets its energy — directly or indirectly — from the sun. But there are other options.
“All metabolism works through this kind of exchange of electrons. That’s how energy works. That’s how life works.”
“Microbial subsurface communities are often chemosynthetic, not photosynthetic,” says University Professor Barbara Sherwood Lollar in the Department of Earth Sciences at the Faculty of Arts & Science, University of Toronto. “In chemosynthesis, a molecule like hydrogen ‘donates’ electrons, and sulfate ‘accepts’ them. Basically all metabolism works through this kind of exchange of electrons. That’s how energy works. That’s how life works.”
The chemical reactions producing the electron donor in these deep waters had been identified several years ago, but the source of sulfate — the electron acceptor — had been elusive.
In a paper published this week in Nature Communications, Sherwood Lollar and her colleagues report that sulfate dissolved in these waters 2.4 km below the surface comes from oxidation of the sulfide minerals in the ancient rocks via chemicals produced when radiation breaks the water down into its constituent parts.
First author Long Li (now Canada Research Chair in Stable Isotope Geochemistry at the University of Alberta), worked with Sherwood Lollar at U of T as a postdoctoral fellow, and along with researchers from McGill University, studied the distribution pattern of multiple sulfur isotopes — that is, sulfur atoms that differ by the number of neutrons — in the dissolved sulfate in ancient subterranean waters near Timmins, Ontario.
Their earlier work had revealed that these waters contained hydrogen and sulfate — key components that make life possible without sunlight. The multiple sulfur isotope compositions in the sulfate show a unique pattern, only seen in rocks formed before oxygen appeared in Earth’s atmosphere about 2.4 billion years ago. By matching this isotopic feature in the dissolved sulfate with that of pyrite in the 2.7 billion-year-old rocks hosting the waters, the researchers demonstrated that the same pyrite and other sulfide ores that make these rocks ideal for economic mining of metals, produce the “fuel” for microbial metabolisms.
But there were other surprises in store. “When we look at the sulfate dissolved in these waters, we found it was more enriched in an isotope called sulphur 34 than expected,” Sherwood Lollar says. Living creatures and non-organic chemical reactions both affect these isotopic patterns, often in distinctive ways.
Geochemical fingerprints — not fossils — key to understanding ancient life
“People often think we study ancient life through fossils,” says Sherwood Lollar. “But the evidence that life arose on our planet 3.8 to 4 billion years ago comes not from fossils, which came much later in earth’s history, but from geochemical fingerprints.” Microbes leave behind geochemical isotopic signatures that allow researchers to detect their existence, even in the absence of fossil or biological data.
The authors tested models of both chemical and biological processes to try to explain the enrichments in sulphur 34. The results pointed to a biological process, and suggested that microbial communities must have colonized these rocks long ago.
“We looked carefully at chemical processes that might account for this pattern, but they just didn’t fit. That forces us to look at the other kind of process — a biological one, which fit very well,” she says “There must have been microbes in these waters on a geologically long timescale.”
Discovery speaks to the flexibility, variety and viability of life itself
For Sherwood Lollar, the discovery speaks to a larger idea about the flexibility, variety, and viability of life itself.
“You don’t look at the [interior] Canadian Shield and think it’s habitable. You don’t think of life living off it. But here are these rock-feeding microorganisms getting their hydrogen and sulfate from the rocks,” she says.
Other researchers are now working to find microbiological evidence to verify the conclusions derived from this team’s geochemical modeling.
Meanwhile, Sherwood Lollar says this discovery has implications not just for the study of life on this planet, but also for much further afield.
“The Canadian Shield is a very good analog for Mars,” she says. “Mars is primarily billion-year-old rock, and parts of it are mineralogically similar to the Shield. If we can find these kinds of life-sustaining waters in the subsurface of Earth, there’s every likelihood of finding similar waters in the subsurface of Mars.”
