Primitive bacteria exist in huge numbers deep in the Earth, living on hydrogen gas produced in rocks, a NASA scientist reports in the spring issue of the journal Astrobiology.
Recent studies suggest that the mass of bacteria existing below ground may be larger than the mass of all living things at the Earth’s surface, according to recent studies cited by the paper's lead author, Friedemann Freund, who works at NASA Ames Research Center in California's Silicon Valley. Similar hydrogen-consuming microbes may some day be discovered on Mars, raising new prospects for the possible existence of life beyond Earth, Freund added.
"The hydrogen that could feed bacteria in the depth of the Earth comes from a subtle chemical reaction that occurs within rocks that were once hot or even molten. In the top 20 kilometers (12.4 miles) of Earth's crust," Freund said, "the conditions are right to produce a nearly inexhaustible supply of hydrogen. In the top 5 to10 kilometers (about 3 to 6 miles) all fissures and cracks in the rocks are probably filled with water. Hydrogen molecules will seep out of the mineral grains, enter the intergranular space and saturate the water. Microorganisms that live in these water films can be expected to use this hydrogen as their vital energy source."
Many of the microorganisms in the ‘deep biosphere' do not live off the sunlight-derived energy that green plants trap during photosynthesis, but live on chemically derived energy sources such as hydrogen, according to Freund. "If deep microbial communities are to thrive over long periods of time, they need a steady supply of hydrogen," he said.
It has long been known that hydrogen gas is produced when water reaches freshly formed cracks in many common rocks, but Freund's paper describes a different hydrogen-producing reaction that occurs inside the minerals that make up such rocks. This reaction does not require rocks to crack — a necessarily episodic event. Instead, it occurs in the entire rock volume during its gradual cooling as continents slowly age over millions of years. Because the Earth's crust contains a huge quantity of rock, even a small amount of hydrogen produced in each small section of rock results in a large volume of gas.
To understand the details of this hydrogen-producing reaction, Freund said, requires some insight into the structure of minerals where silicon, oxygen and metals have combined to form a dense pack of atoms and ions. When these minerals crystallize at high temperatures, water is always present, and some water molecules are trapped in the atomic structure of the minerals, said Freund. These water molecules are ripped apart and change into hydroxyl anions, each of which is negatively charged and has one oxygen ion with a proton attached.
"During cooling, at temperatures below 400 to 500 degrees C (752 to 932 degrees F), a strange reaction takes place. Pairs of these hydroxyl anions rearrange their electrons in such a way that hydrogen gas molecules are formed," Freund said.
What is unusual and still not fully understood, said Freund, is that the electrons needed to make the hydrogen molecules are taken away from negatively charged oxygen anions. "Suddenly, some oxygen anions, which everybody thought only existed in a doubly charged negative state, convert to singly charged negative ions," he said. "These single negative oxygen anions join in pairs. In this form, they are innocuous and can stay inactive over geological times."
The hydrogen molecules, however, wander around inside the mineral structure and can squeeze into the narrow spaces between the mineral grains. If the intergranular space is filled with water, the hydrogen molecules will dissolve in the water. If microbes live in the intergranular water films, one can imagine, said Freund, that these bacteria extract the dissolved hydrogen from the water and use this hydrogen as an energy source, not unlike fish that extract oxygen dissolved in the water of rivers, lakes and the sea to respire.
"What is potentially important," Freund said, "is that, if and when microorganisms in the deep underground use this hydrogen dissolved in the intergranular water films, the rocks around them will replenish the hydrogen supply - indefinitely, over eons of time."
The paper by Freund and his coworkers also may help answer non-biological questions related to the commercial viability of tapping hydrogen reserves deep in the rocks and to questions of mine safety. For example, sometimes, during mining and drilling operations, enough hydrogen seeps out of wall rocks that explosive gas mixtures can be produced, according to some reports.
"Since old, old times, the mining industry has had its share of mine explosions in which hydrogen played a role," Freund said, "but hydrogen gas could also be used as an energy source and fuel in today’s or tomorrow’s society. For years, pipelines have been distributing hydrogen gas between different industrial partners in the Ruhr Valley in Germany, and the experts say it can be handled about as safely as natural gas."
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