Microbes might find energy in surprisingly inhospitable places
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Rocks fractured by earthquakes could unlock a wide menu of chemical energy sources for microbes living deep underground – and similar processes could potentially support microbes within other planets.
“This opens up a whole new set of metabolisms,” says Kurt Konhauser at the University of Alberta in Canada.
All organisms on Earth use flowing electrons to power their lives. On the planet’s surface, plants harness sunlight to produce carbon-based sugars, which animals like us eat. Then electrons flow from the carbon we consume to the oxygen molecules we inhale. The chemical gradient between these carbon electron donors and oxygen electron acceptors, known as a redox pair, produces energy.
Below the planet’s surface, microbes also rely on such pairs for energy. But deep ecosystems lack access to the sun’s energy in any form, which means they can’t use the same carbon-oxygen pairs we do. “The problem with the deep subsurface has always been, where do these [chemical gradients] come from?” says Konhauser.
Hydrogen gas – generated underground by reactions between water and rock – is known to serve as a major source of electrons, much like carbon sugars do up above. This hydrogen comes from breaking down water into its components, which can occur when radioactive rocks splits water molecules or iron-rich rocks react with them. A smaller share of hydrogen is generated when earthquakes shear silicate rocks, exposing reactive surfaces capable of splitting water.
To make use of that hydrogen, however, microbes require electron acceptors to form complete redox pairs; hydrogen on its own isn’t worth much. “The food may be on the table, but if you haven’t got a fork, you’re not going to eat,” says Barbara Sherwood Lollar at the University of Toronto in Canada.
Konhauser, Sherwood Lollar and their colleagues used rock-crushing machines to test how the same reactions that generate hydrogen gas within faults might also generate complete redox pairs. They crushed quartz crystals, simulating the strain produced in different types of faults, then mixed the rock with water and various forms of iron, which is present in most rocks.
The crushed quartz reacted with water to generate large amounts of hydrogen in both its stable molecular configuration and more reactive forms. The researchers found many of these hydrogen radicals reacted with iron-containing fluids to generate a slew of compounds that could either donate or accept electrons, enough to form an assortment of redox pairs.
“More of the rocks become usable for energy,” says Konhauser. “These reactions… mediate many different types of chemical reactions, which means many different types of microbes can exist.” Other secondary reactions with nitrogen or sulphur could offer an even greater diversity of energy sources, he says.
“I was surprised by the numbers,” says Magdalena Osburn at Northwestern University in Illinois. “This is producing quite a lot of hydrogen. And also it produces this additional subsidiary chemistry.”
The researchers estimate earthquakes generate much less hydrogen than the other water-rock reactions in the planet’s crust. However, their findings suggest active faults could be local hotspots of microbial activity and diversity, says Sherwood Lollar.
And full-on earthquakes aren’t necessarily required. Similar reactions could also happen when rocks fracture in seismically quiet places, such as the interior of continents, or tectonically dead planets like Mars. “Even within those giant rock masses you do have pressure redistributions and shifts,” she says.
“I think it’s really exciting, pushing some sources that we knew about before a little farther,” says Karen Lloyd at the University of Southern California. The range of useable chemicals produced in real faults would likely be even more diverse. “This is probably happening under pressure, under different temperatures, over a very big spatial scale and with more diverse mineral formations,” she says.
Energy from infrequent events like earthquakes could also explain the lifestyles of what Lloyd calls aeonophiles, deep subsurface microbes that appear to live for extremely long periods of time. “If you can wait ten thousand years, there’s going to be a magnitude-9 earthquake and you’re going to get this massive rush of energy,” says Lloyd.
The findings are part of a general trend over the past two decades expanding our view of where and how organisms can survive underground, says Sherwood Lollar. Evidence the deep rocks of continents could support life “has massively opened up our concept of how habitable our planet is”, she says.
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