Planet Earth

Rock-Eating Slime


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Microbes are resilient creatures. Some inhabit boiling hot ocean vents. Others survive radiation doses that would kill a human. Now biologists have found some hardy bugs flourishing as much as 4,500 feet below Earth’s surface in basalt rock near the Columbia River in Washington State. These bacteria live completely independently of all products of the sunlit world, including oxygen; they apparently subsist on rocks and water alone. As a result they may tell us something about how organisms survived on Earth billions of years ago, before they learned how to make food through photosynthesis.

The basalt the bacteria call home is a layer cake of rock that formed 14 to 7 million years ago, when a rift opened in Earth’s crust, spewing tremendous amounts of molten lava. Since its formation, the basalt has been broken up and folded, and rainwater seeping into the fractures from far away has slowly created aquifers at great depths.

Microbiologist Todd Stevens and geochemist James McKinley at the Pacific Northwest National Laboratory in Washington were studying the area for the Department of Energy to monitor the spread of radioactive waste in groundwater beneath the nearby Hanford nuclear facility. When they sampled some deep aquifers, they were surprised to find a teeming population of bacteria.

Many of the microbes turned out to be methanogens--bugs that use hydrogen as an energy source to convert carbon into organic compounds, including methane as a by-product. Isotopic analysis of the aquifers showed that the bacteria were drawing their carbon from carbon dioxide dissolved in the water. The problem was, there seemed to be no available source of hydrogen. Most methanogens get it from the degradation of plant matter by other microbes--and thus ultimately from photosynthesis--but there wasn’t much plant matter to be found in an aquifer 4,500 feet down.

An explosion at another well site where they were doing research- -set off by some welders repairing a drill--gave Stevens and McKinley a clue to where the hydrogen was coming from. We found out that we were actually generating hydrogen by grinding up the rocks in the bottom of the well, explains Stevens. By drilling we were speeding up a natural reaction process. When the two researchers crushed samples of basalt and sealed them in an oxygen-proof container of water, they found that hydrogen was present. They then added bacteria from the aquifers. The bugs prospered.

How basalt and water mix to make hydrogen is a mystery, but Stevens and McKinley have some rudimentary ideas. Basalt has a large energy reserve because it contains reduced iron--iron that has a store of electrons to give away. After years of immersion, the iron in the basalt may surrender its electrons to the oxygen in water molecules, thereby becoming oxidized--and thereby liberating the hydrogen on which bacteria can thrive in the deep recesses of Earth. In the surface world there’s always some influence of photosynthesis, no matter where you look, says Stevens. This is one place where we think there’s no influence of photosynthesis.

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